several embodiments of ablation systems useful for practicing an electrosurgical method corresponding to the present invention are shown in the drawings . in general , each of these embodiments utilizes a gas ionized at a first polarity and contained within a thin - wall dielectric enclosure that provides for capacitive coupling of rf current from the gas to a target tissue in contact with an electrode at a second polarity and spaced apart from , and exterior of , the dielectric enclosure . the system embodiments typically include an instrument with a working end including the thin - wall dielectric enclosure for containing an ionizable gas . current flow to the tissue initiates when sufficient voltage is applied to ionize the contained gas into a plasma and the contemporaneous capacitive coupling through the surrounding dielectric structure occurs . the invention thus provides a voltage - based electrosurgical effect that is capable of ablating tissue to a controlled depth of 1 mm to 5 mm or more very rapidly , wherein the depth of ablation is very uniform about the entire surface of the dielectric enclosure . the instrument working end and dielectric enclosure can take a variety of forms , including but not limited to an elongated shaft portion of a needle ablation device , a dielectric expandable structure , an articulating member , a deflectable member , or at least one engagement surface of an electrosurgical jaw structure . the system embodiments and methods can be used for interstitial tissue ablation , intraluminal tissue ablation or topical tissue ablation . the system embodiments described herein utilize a thin - wall dielectric structure or wall at an instrument working end that contains an electrically non - conductive gas as a dielectric . the thin - wall dielectric structure can be a polymer , ceramic or glass with a surface configured for contacting tissue . in one embodiment , an interior chamber within the interior of the thin - wall dielectric structure carries a circulating neutral gas or static neutral gas such as argon . an rf power source provides current that is coupled to the neutral gas flow or static gas volume by an electrode disposed within the interior of the working end . the gas flow or static gas contained within the dielectric enclosure is of the type that is non - conductive until it has been transformed to a conductive plasma by voltage breakdown . the threshold voltage for breakdown of the gas will vary with variations in several parameters , including the gas pressure , the gas flow rate , the type of gas , and the distance from the interior electrode across the interior chamber to the dielectric structure . as will be seen in some of the embodiments , the voltage and other operational parameters can be modulated during operation by feedback mechanisms . the gas , which is ionized by contact with a conductive electrode in the instrument working end , functions as a switching mechanism that only permits current flow into targeted tissue when the voltage across the combination of the gas , the dielectric structure and the contacted tissue reaches a predetermined threshold potential that causes capacitive coupling across the dielectric structure . by this means of permitting current flow only at a high threshold voltage that capacitively couples current to the tissue , the invention allows a substantially uniform tissue effect within all tissue in contact with the dielectric structure . further , the invention allows the ionized gas to be created contemporaneously with energy application to tissue by the conversion of a non - conductive gas to a plasma . in one embodiment of the apparatus , the ionized gas functions as an electrode and comprises a gas flow that can conduct current across an internal contained volume of the gas within a dielectric structure , typically from an electrode at an interior of a working end in contact with the gas flow . the gas flow is configured for the purpose of coupling energy to the dielectric structure uniformly across the surface of the dielectric structure , but that will only conduct such energy when the non - conductive gas media has been transformed to a conductive plasma by having been raised to a threshold voltage . plasma . in general , this disclosure may use the terms “ plasma ” and “ ionized gas ” interchangeably . a plasma consists of a state of matter in which electrons in a neutral gas are stripped or “ ionized ” from their molecules or atoms . such plasmas can be formed by application of an electric field or by high temperatures . in a neutral gas , electrical conductivity is non - existent or very low . neutral gases act as a dielectric or insulator until the electric field reaches a breakdown value , freeing the electrons from the atoms in an avalanche process thus forming a plasma . such a plasma provides mobile electrons and positive ions , an acts as a conductor which supports electric currents and can form spark or arc . due to their lower mass , the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions , and hence carry the bulk of the current . dielectric and dielectric loss . the term dielectric is used in its ordinary sense meaning a material that resists the flow of electric current , that is a non - conducting substance . an important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat . the lower the dielectric loss ( the proportion of energy lost as heat ), the more effective is a dielectric material . dielectric constant or relative permittivity . the dielectric constant ( k ) or relative static permittivity of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux , or stated alternatively is a number relating the ability of the material to carry alternating current to the ability of vacuum to carry alternating current . the capacitance created by the presence of a material is directly related to its dielectric constant . in general , a material or media having a high dielectric constant breaks down more easily when subjected to an intense electric field than do materials with low dielectric constants . for example , air or another neutral gas can have a low dielectric constant and when it undergoes dielectric breakdown , a condition in which the dielectric begins to conduct current , the breakdown is not permanent . when the excessive electric field is removed , the gas returns to its normal dielectric state . dielectric breakdown . the phenomenon called dielectric breakdown occurs when an electrostatic field applied to a material reaches a critical threshold and is sufficiently intense so that the material will suddenly conduct current . in a gas or liquid dielectric medium , this condition reverses itself if the voltage decreases below the critical point . in solid dielectrics , such a dielectric breakdown also can occur and couple energy through the material . as used herein , the term dielectric breakdown media refers to both solid and gas dielectrics that allow current flow across the media at a critical voltage . degree of ionization . degree of ionization describes a plasma &# 39 ; s proportion of atoms which have lost ( or gained ) electrons , and is controlled mostly by temperature . for example , it is possible for an electrical current to create a degree of ionization ranging from less than 0 . 001 % to more than 50 . 0 %. even a partially ionized gas in which as little as 0 . 1 % or 1 . 0 % of the particles are ionized can have the characteristics of a plasma , that is , it can strongly respond to magnetic fields and can be highly electrically conductive . for the purposes of this disclosure , a gas may begin to behave like conductive plasma when the degree of ionization reaches approximately 0 . 1 %, 0 . 5 % or 1 . 0 %. the temperature of a plasma volume also relates to the degree of ionization . in particular , plasma ionization can be determined by the electron temperature relative to the ionization energy . a plasma is sometimes referred to as being “ hot ” if it is nearly fully ionized , or “ cold ” or a “ technological plasma ” if only a small fraction ( for example , less than 5 % or less than 1 %) of the gas molecules are ionized . even in such a cold plasma , the electron temperature can still be several thousand degrees celsius . in the systems according to the present invention , the plasmas are cold in this sense because the percentage of ionized molecules is very low . another phrase used herein to describe a “ cold ” plasma is “ average mass temperature ” of the plasma which relates to the degree of ionization versus non - ionized gas and which averages the temperatures of the two gas volume components . for example , if 1 % of a gas volume is ionized with an electron temperature of 10 , 000 ° c ., and the remaining 99 % has a temperature of 150 ° c ., then the mass average temperature will be 149 . 5 ° c . it has been found that measuring the plasma temperature can be used to determine an approximate degree of ionization which can be used for feedback control of applied power , and as a safety mechanism for preventing unwanted high temperatures within a thin - wall dielectric structure . referring to fig1 , a first embodiment of a tissue ablation system 100 utilizing principles of the present invention is shown . the system 100 includes a probe 110 having a proximal handle 112 and an elongated shaft or extension member 114 that extends along axis 115 . the handle 110 is fabricated of an electrically insulative material such as a plastic , ceramic , glass or combination thereof . the extension member 114 has a proximal end 116 coupled to handle 112 . the extension member 114 extends to a distal working end 120 that includes a dielectric member or structure 122 that is configured for contacting tissue that is targeted for ablation . in the embodiment of fig1 , the working end 120 and dielectric structure 122 is elongated and cylindrical with a cross - section ranging from about 0 . 5 mm to 5 mm or more with a length ranging from about 1 mm to 50 mm . the cross - section of the working end 120 can be round , oval , polygonal , rectangular or any other cross - section . as can be seen in fig2 a - 2b , in one embodiment , the working end 120 has a sharp tip 124 for penetrating tissue to perform an ablation procedure , such as ablating a tumor indicated at 125 in a tissue volume 130 . in other embodiment , the distal tip of a working end 120 can be blunt . in yet other embodiment , the entire working end can have a guide channel therein for advancing the working end over a guide wire . now turning to fig3 , an enlarged view of the working end 120 of fig1 , 2 a and 2 b is shown . it can be seen that the dielectric structure 122 has a thin wall 132 that provides an enclosure about an interior chamber 135 that contains a gas media indicated at 140 in fig3 . in one embodiment , the dielectric structure 122 can comprise a ceramic ( e . g ., alumina ) that has a a dielectric constant ranging from about 3 to 4 . the thickness of wall 132 can range from 0 . 002 ″ to 0 . 10 ″ depending on the diameter , or more typically 0 . 005 ″ to 0 . 050 ″ in a diameter ranging from 1 to 4 mm . in other embodiment shown in fig4 a , the dielectric structure 122 can comprise a ceramic , glass or polymer in a molded form with strengthening support portions 142 or ribs that end axially , radially , helically or a combination thereof . the support portions 142 alternatively can comprise members that are independent of a thin - wall 132 of a dielectric material . in such an embodiment ( fig4 a ) as will be described below , the thin wall portions 144 of the dielectric structure 122 permit capacitive coupling of current to tissue while the support portions 142 provide structural strength for the thin - wall portions 144 . in another embodiment , a portion of which is shown in fig4 b , the dielectric structure 122 has support portions 142 in a waffle - like configuration wherein thin - wall portions 144 are supported by thicker wall support portions 142 . the waffle - like structure can be substantially planar , cylindrical or have any other suitable configuration for containing a gas dielectric in a chamber indicated at 135 on one side of the dielectric structure 122 . in another embodiment of fig5 a and 5b , the dielectric structure 122 can have support portions 142 comprising posts that support the thin - wall portions 144 over another supporting member 145 . the planar dielectric structure 122 can be used , for example , in planar jaw members for applying rf energy to seal tissue . in another example , fig5 b shows a blunt - tipped , cylindrical thin - wall 132 of a dielectric structure 122 supported by a core supporting member 145 . in the embodiment of fig5 b , the interior chamber 135 which can contain a plasma comprises a space between the thin wall portions 144 and the core support member 145 . referring again to fig3 , the extension member 114 is fabricated of an electrically non - conductive material such as polymer , ceramic , glass or a metal with an insulative coating . the dielectric structure 122 can be bonded to extension member 114 by glues , adhesives or the like to provide a sealed , fluid - tight interior chamber 135 . in one embodiment , a gas source 150 can comprise one or more compressed gas cartridges ( fig1 and 6 ). as will be described below ( fig6 ), the gas source is coupled to a microcontroller 155 that includes a gas circulation subcontroller 155 a which controls a pressure regulator 158 and also controls an optional negative pressure source 160 adapted for assisting in circulation of the gas . the rf and controller box 162 in fig1 can include a display 164 and input controls 165 for setting and controlling operational parameters such as treatment time intervals , gas flows , power levels etc . suitable gases for use in the system include argon , other noble gases and mixtures thereof referring to fig3 , the gas source 150 provides a flow of gas media 140 though a flexible conduit 166 to a first flow channel 170 in extension member 114 that communicates with at least one inflow port 172 interfacing with interior chamber 135 . the interior chamber 135 also interfaces with an outflow port 174 and second flow channel 180 in extension member 114 to thereby allow a circulating flow of gas media 140 within the interior of dielectric structure 122 . still referring to fig3 , a first polarity electrode 185 is disposed about flow channel 170 proximate to the inflow port 172 thus being in contact with a flow of gas media 140 . it should be appreciated that electrode 185 can be positioned in any more proximal location in channel 170 in contact with the gas flow , or the electrode 185 can be within interior chamber 135 of dielectric structure 122 . the electrode 185 is electrically coupled to a conductor or lead 187 that extends through the extension member and handle 112 and is coupled to a first pole of a high frequency rf generator 200 which is controlled by controller 155 and rf subcontroller 155 b . an opposing polarity electrode 205 is disposed on the exterior surface of extension member 114 and is electrically coupled by lead 207 to a second pole of rf generator 200 . the box diagrams of fig6 and 7 schematically depict the system , subsystems and components of one embodiment that is configured for delivering ablative electrosurgical energy to tissue . in the box diagram of fig6 , it can be seen that an rf power source 200 and circuit is controlled by rf subcontroller 155 b . feedback control subsystems ( described below ) based on systems and probe pressure feedback , probe temperature feedback , and / or gas flow rate feedback are also operatively coupled to controller 155 . the system can be actuated by footswitch 208 or another suitable switch . fig7 shows a schematic of the flow control components relating to the flow of gas media through the system and probe 110 . it can be seen that a pressurized gas source 150 in linked to a downstream pressure regulator 158 , an inflow proportional valve 210 , flow meter 212 and normally closed solenoid valve 220 . the valve 220 is actuated by the system operator which then allows a flow of gas media 140 to circulate through flexible conduit 166 and probe 110 . the gas outflow side of the system includes a normally open solenoid valve 225 , outflow proportional valve 226 and flowmeter 228 that communicate with negative pressure source 160 . the exhaust of the gas can be into the environment or into a containment system . a temperature sensor 230 ( e . g ., thermocouple ) is shown in fig7 for monitoring the temperature of outflow gases . fig8 and 9 a - 9 d schematically illustrate a method of the invention wherein ( i ) the dielectric structure 122 and ( ii ) the contained neutral gas volume 140 function contemporaneously to provide first and second dielectric media that cooperatively function as independent mechanisms to optimize very high voltage current delivery to engaged tissue volumes . the two dielectric components can be characterized as having complementary voltage thresholds levels at which only high voltage current can couple through a filament 235 of a plasma 240 within chamber 135 and capacitively couple through the thin - wall dielectric 132 to allow a current to further pass through a least resistive path 245 in the engaged tissue . in fig8 , the engaged tissue is assumed to be surrounding the dielectric structure 122 and is transparent . in the embodiment of fig8 , the electrode 185 also functions and a gas delivery sleeve wherein a neutral gas 140 can exit ports 250 in chamber 135 . the high voltage current paths 245 in tissue are effectively “ scanned ” across and about the inner surface 252 of the dielectric structure 122 and within the contacted tissue to cause a voltage - maximized form of electrosurgical ablation . fig8 provides a schematic view of what is meant by the term “ scanned ”, wherein high intensity electrical fields are produced in the interior chamber 135 of the dielectric structure 122 by capacitive coupling through the dielectric wall 132 until a voltage threshold is reached in the neutral gas media 140 to convert the gas into a plasma 240 ( see fig8 ) which in turn allows plasma filaments 235 to form within the chamber 135 which randomly jump or scan about the interior surface 248 of the dielectric wall . the random jump of plasma filaments 235 within the dielectric chamber 135 ( from electrode 185 to inner surface 248 of dielectric wall 132 ) occurs where there is a transient , reversible voltage breakdown in a localized portion 252 of the dielectric wall 132 , which is determined by a transient highest conduction path 240 in engaged tissue to the second polarity electrode 205 ( fig3 ). an instant after the flow of current through the plasma 240 and path 245 in tissue , the localized portion 252 dissipates the electrical field and another capacitive coupling occurs through another plasma filament 235 ′ and current path 245 ′ in tissue to cause electrosurgical ablation in another random , discrete location . fig9 a - 9d are enlarged schematic illustrations of the electrosurgical ablation method of fig8 that depict other aspects of the ablation method . in fig9 a , it can be seen that the system and method is generalized to show clearly the first and second dielectric current transmission mechanisms characterized by selected voltage parameters to cause an electron avalanche in the gas and capacitive coupling in the thin - wall enclosure to optimize and maximize a form of high voltage current delivery to an exemplary tissue 260 . as described previously , the voltage threshold or dielectric breakdown mechanisms occur within ( i ) the gas dielectric or neutral gas volume 140 that is contained within an interior chamber 135 of dielectric structure 122 and ( ii ) the non - gas dielectric or structure 122 shown as a plane in fig9 a - 9d . fig9 a illustrates the working end components and tissue 260 prior to the actuation and delivery of energy to the tissue . it can be seen that the gas media 140 is neutral and not yet ionized . the first polarity electrode 185 positioned in the interior chamber 135 in contact with neutral gas 140 is shown schematically . the second polarity electrode 205 in contact with tissue is also shown schematically , but the illustration represents another aspect of the invention in that the second electrode 205 can have a small surface area compared to the surface areas of return electrodes / ground pads as in conventional electrosurgical systems . it has been found that the capacitively coupled energy delivery mechanism of the invention does not cause tissue heating at or about the surface of the second polarity electrode 205 as would be expected in a conventional electrosurgical device . as will be described below , it is believed that the constant flux in voltage breakdown - initiated and capacitive coupling - initiated current paths in the tissue 260 greatly reduces heat built up at or about the return electrode 205 . fig9 b illustrates the working end components and tissue 260 at an instant in time immediately after the operator actuates the system and delivers power to the probe working end . several aspects of the voltage - initiated breakdown ablation method are represented in fig9 b , including ( i ) in one aspect of the instant in time , the neutral gas 140 is converted to a plasma 240 by potential between the first and second polarity electrodes 185 and 205 ; and contemporaneously ( ii ) current flow defines a least resistive path 245 in the tissue 260 ; ( iii ) a portion 252 of dielectric structure 122 adjacent current path 245 allows capacitive coupling to the tissue ; ( iv ) the plasma filament 235 arcs across a high intensity plasma stream 262 between electrode 185 and the portion 252 of the dielectric structure . in other words , when the a selected voltage potential is reached , the voltage breakdown of the gas 140 and capacitively coupling through the dielectric 122 causes a high voltage current to course through path 245 in the tissue 260 . an instant later , thermal diffusion indicated by arrows 265 causes thermal effects in a tissue volume 270 a outward from the transient current path 245 . the thermal effects in and about path 245 elevates tissue impedance , which thus causes the system to push a conductive path to another random location . fig9 c illustrates the working end components and tissue 260 an instant after that of fig9 b when continued voltage potential causes voltage breakdown in plasma filament 235 ′ together with capacitively coupling through dielectric 122 to provide another high voltage current to course through path 245 ′ after which heat diffusion 265 ′ causes thermal effects indicated at 270 b . the “ scanning ” aspect of the ablation method can be understood from fig9 a - 9b wherein the plasma filaments 235 , 235 ′ and current paths very rapidly jump or scan about the interior chamber 135 to thereby deliver current in a path of least resistance 245 , 245 ′ in the tissue 260 . now turning to fig9 d , another schematic is shown following an interval of energy delivery in which a multiplicity of current paths through the pre - existing plasma and dielectric 122 have provided thermal effects diffused throughout a multiplicity of regions indicated at 270 a - 270 f . by this method , it has been found that ablation depths of 3 mm to 6 mm can be accomplished very rapidly , in for example 30 seconds to 90 seconds dependent upon the selected voltage . in one aspect of the invention , fig1 is a circuit diagram representing the steps of the method of fig9 a - 9d which explains the discovery that return electrode 205 can have a small surface area and not be subject to significant heating . in fig1 , it can be seen that voltage potential can increase until a dielectric breakdown occurs in both the neutral gas 140 and the dielectric structure 122 which cause a high voltage current through path p 1 to electrode 205 , followed by that path impeding out , thus causing the current to shift to current path p 2 , then current path p 3 ad infinitum to current path indicated at pn . the tissue 260 in fig1 thus is shown as variable resistor in each current path as the current path is in continual flux based on the path increasing in resistance . fig1 a and 11b are enlarged schematic illustrations of the method of using the embodiment of fig3 to capacitively couple current to tissue with a gas dielectric 140 in interior chamber 135 ( i . e ., plasma indicated at 240 ). referring to fig1 a , the system is actuated , for example by a footswitch 208 ( fig1 ) coupled to rf power source 200 and controllers 155 a and 155 b which initiates a gas flow from source 150 to provide circulating flow through the first ( inflow ) channel 170 , interior chamber 135 and the second ( outflow ) channel 180 . for convenience , the embodiments utilizing such a circulating gas flow will be described herein as using one preferred gas , which is argon . in one embodiment , the gas flow rate can be in the range of 1 ml / sec to 50 ml / sec , more typically from 5 ml / sec to 30 ml / sec . in fig1 a , the working end 120 of the probe is introduced into tissue 260 , for example to ablate a tumor as in fig2 a - 2b . the dielectric structure 122 is positioned in a desired location to ablate tissue adjacent thereto . the actuation of the system contemporaneously applies rf energy to electrode 185 and the gas flow which instantly converts the non - conductive argon 140 to a plasma indicated at 240 in fig1 a . the threshold voltage at which the argon becomes conductive ( i . e ., converted in part into a plasma ) is dependent upon a number of factors controlled by the controller , including the pressure of the argon gas , the volume of interior chamber 135 , the flow rate of the gas 140 , the distance between electrode 185 and interior surfaces of the dielectric surface 122 , the dielectric constant of the dielectric structure 122 and the selected voltage applied by the rf power source 200 . it should be appreciated that the actuation of the system can cause gas flows for an interval of 0 . 1 to 5 seconds before the rf generator powers on to insure circulatory gas flows . fig1 a schematically depicts current indicated at 280 being capacitively coupled through the wall 132 of the dielectric structure 122 to tissue 260 , with the electric field lines indicating that high energy densities do not occur about electrode 205 . rather , as described above , the high resistance developed in tissue about the current path dielectric structure 122 causes rapidly changing current paths and ohmic heating . in one aspect of the invention , the capacitive coupling allows for rapid , uniform ablation of tissue adjacent the dielectric structure . fig1 b schematically depicts the tissue after the rf energy delivery is terminated resulting in the ablated tissue indicated at 285 . now turning to fig1 , an alternate working end 120 ′ is shown in a method of use . in this embodiment , the dielectric structure 122 is similar to that of fig3 except the working end 120 ′ includes a central support member 290 that extends from extension member 214 to a distal tip portion 292 . in this embodiment , the central support member 290 can comprise , or carry , a conductive electrode surface indicated at 295 to delivery energy to the gas 140 in interior chamber 135 for creating the plasma . the embodiment of fig1 also includes concentric gas inflow and outflow channels , 170 and 180 , wherein the first ( inflow ) channel 170 comprises a lumen in support member 290 that communicates with a plurality of flow outlets 250 in a distal portion of interior chamber 135 . the gas outflow port 174 is again disposed in a proximal portion of interior chamber 135 . the placement of gas inflow and outflow ports in opposing ends of interior chamber allows for effective gas circulation which assists in maintaining a predetermined plasma quality . in fig1 , the ablative currents and ohmic heating in tissue are indicated at 200 . in another aspect of the invention , fig1 illustrates that at least one temperature sensor , for example thermocouples 300 a and 300 b , are provided within or adjacent to interior chamber 135 to monitor the temperature of the plasma . the temperature sensors are coupled to controllers 155 a and 155 b to thus allow feedback control of operating parameters , such a rf power delivered , neutral gas inflow rate , and negative pressure that assists outflow . by measuring the mass average temperature of the media in chamber 135 , the degree of ionization of the ionized gas 240 can be determined . in one aspect of the invention , the measured temperature within chamber 135 during operation can provide feedback to gas circulation controller to thereby modulate the flow of neutral gas to maintain a degree of ionization between 0 . 01 % and 5 . 0 %. in another aspect of the invention , the measured temperature within chamber 135 during operation can provide feedback to modulate flow of neutral gas to maintain a temperature of less than 200 ° c ., 180 ° c ., 160 ° c ., 140 ° c ., 120 ° c ., or 100 ° c . in several embodiments of polymeric dielectric structures , it is important to maintain a cold or technological plasma to prevent damage to the dielectric . in another aspect of invention , the system operating parameters can be modulated to maintain the mass average temperature within a selected range , for example a 5 ° c . range , a 10 ° c . range or a 20 ° c . range about a selected temperature for the duration of a tissue treatment interval . in another aspect of invention , the system operating parameters can be modulated to maintain a degree of ionization with less than 5 % variability , less than 10 % variability or less than 20 % variability from a selected “ degree of ionization ” target value for a tissue treatment interval . while fig1 shows thermocouples within interior chamber 135 , another embodiment can position such temperature sensors at the exterior of the wall 132 of the dielectric structure to monitor the temperature of the wall . it also should be appreciated that multiple electrodes can be provided in the interior chamber to measure impedance of the gas media to provide an additional from of feedback signals . in another embodiment similar to fig1 , the working end or flow channel in communication with the interior chamber 135 can carry at least one pressure sensor ( not shown ) and pressure measurement can provide feedback signals for modulating at least one operational parameter such as rf power delivered , neutral gas inflow rate , negative pressure that assists outflow , degree of ionization of the plasma , or temperature of the plasma . in another aspect of invention , the system operating parameters can be modulated to maintain a pressure within chamber 135 less than 5 % variability , less than 10 % variability or less than 20 % variability from a selected target pressure over a tissue treatment interval . in general , fig1 represents the steps of a method corresponding to one aspect of the invention which comprises containing a non - conductive gas in an interior of an enclosure having a thin dielectric wall , engaging and external surface of the dielectric wall in contact with a target region of tissue , and applying a radiofrequency voltage across the gas and the dielectric wall wherein the voltage is sufficient to initiate a plasma in the gas and capacitively couple current in the gas plasma across the dielectric wall and into the engaged tissue . this method includes the use of a first polarity electrode in contact with the gas in the interior of the thin dielectric wall and a second polarity electrode in contact with the patient &# 39 ; s tissue . fig1 represents aspects of a related method corresponding to the invention which comprises positioning a dielectric structure on a tissue surface , containing a non - conductive , ionizable gas within the dielectric structure , and applying rf voltage actoss the gas and tissue to to ionize the gas and deliver current through the dielectric structure to the tissue to ohmically heat the tissue . in general , fig1 represents the steps of a method corresponding to another aspect of the invention which comprises providing an electrosurgical working end or applicator with a first gas dielectric and a second non - gas dielectric in a series circuit , engaging the non - gas dielectric with tissue , and applying sufficient rf voltage across the circuit to cause dielectric breakdown in the gas dielectric to thereby apply ablative energy to the tissue . the step of applying ablative energy includes capacitively coupling rf current to the tissue through the second non - gas dielectric media . fig1 represents steps of another aspect of the invention which comprises positioning a dielectric structure enclosing an interior chamber in contact with targeted tissue , providing a gas media in the interior chamber having a degree of ionization of at least 0 . 01 %, and applying rf current through the gas media to cause capacitive coupling of energy through the dielectric structure to modify the tissue . in this aspect of the invention , it should be appreciated that an ionized gas can be provided for inflow into chamber 135 , for example with a neutral gas converted to the ionized gas media prior to its flow into chamber 135 . the gas can be ionized in any portion of a gas inflow channel intermediate the gas source 150 and the interior chamber 135 by an rf power source , a photonic energy source or any other suitable electromagnetic energy source . fig1 represents the steps of another method of the invention which comprises positioning a dielectric structure enclosing a gas media in contact with targeted tissue , and applying rf current through the gas media and dielectric structure to apply energy to tissue , and sensing temperature and / or impedance of the ionized gas media to provide feedback signals to thereby modulate a system operational parameter , such as rf power delivered , neutral gas inflow rate , and / or negative pressure that assists gas outflows . now turning to fig1 a - 22 , other embodiments of electrosurgical working ends are shown that adapted to apply energy to tissue as described above , except that the dielectric structures have differing dielectric portions each having a different relative permittivity to thus cause differential effects ( greater or lesser capacitive coupling ) in tissue regions in contact with the different portions of the dielectric structure . in one probe embodiment 400 shown in fig1 a - 18b , a working end carries multiple tissue - penetrating elements 405 that are similar to the needle - like working end 120 of fig1 - 3 . the tissue - penetrating elements 405 can be extendable from a shaft 410 of an endoscopic instrument 412 by actuation of lever 414 . each tissue - penetrating elements 405 has a working end with a dielectric structure 422 as described above and one or more return electrodes indicated at 425 . as can be seen in fig1 , the tissue - penetrating elements 405 are adapted for penetrating tissue 260 , such as a liver , on either side of a target line 430 that is to be a resection line or plane . thus , the tissue - penetrating elements 405 can coagulate tissue on either side of line 430 , and thereafter the tissue can be cut and bleeding will be prevented or reduced . such an instrument 412 can be used in liver resections , lung resections and the like . fig2 illustrates a cross - section of the multiple tissue - penetrating elements 405 of fig1 in tissue wherein it can be seen that the wall 432 of the dielectric structure varies from a thin - wall portion 435 to a thicker wall portion 436 with each portion extending axially along the length of the dielectric structure . as can easily be understood , the thin - wall portion 435 allows a greater coupling of current to adjacent tissue 260 when compared to the thicker wall portion 436 . for this reason , the depth of ablated or cauterized tissue regions 440 will vary depending on whether it is adjacent to thin - wall portion 435 or the thicker wall portion 436 . thus , the instrument can control the depth of ablation by varying the volume resistivity of the dielectric wall . for example , the thin - wall portion 435 can have a volume resistivity in the range of 1 × 10 14 ohm / cm as described above which can then transition to thicker wall portion 438 having a volume resistivity of 1 . 5 ×, 2 × or 3 × triple the 1 × 10 14 ohm / cm range . as depicted in fig2 , the energy delivery converges to ablate or cauterize tissue regions 440 inwardly toward line 430 that is targeted for cutting . outwardly from line 430 there is less collateral damage due to reduced ohmic heating . fig2 illustrate a plurality of probes 450 a - 450 d that demonstrate a similar use of “ directional ” dielectric structures 422 for directional control of energy delivery to tissue , in this case to provide converging regions of ablation to ablate tumor 452 as in the working ends of the device of fig1 a - 20 . in this embodiment , it can be seen that the probe handles include an indicator mark 455 that indicates the orientation of the thin - wall portion 435 or thick wall portion 436 to thus selectively direct rf energy delivery . in another embodiment , it should be appreciated that the proximal and distal ends of a dielectric structure 422 can be marked with any suitable imageable marker , for example radiopaque markings . in another aspect of the invention shown in fig2 , any probe can carry at least one thermocouple , for example thermocouples 456 a and 456 b , at locations proximal and distal to the dielectric structure 422 to measure tissue temperatures to provide an endpoint for terminating the delivery of energy . the thermocouples provide signal to controllers 155 a and 155 b to terminate the ablation procedure . the ablation probes 450 a - 450 d can each carry a return electrode as in the working end of fig1 , or alternatively there can be a remote return electrode as indicated at 458 in fig2 . fig2 illustrates another embodiment of electrosurgical working end 460 wherein wall 432 of the dielectric structure 422 ′ varies from thin - wall portion 435 ′ to a proximal and distal thicker wall portions 436 ′ with each portion extending radially about the dielectric structure . as can easily be understood as shown in fig2 , the central thin - wall portion 435 ′ thus allows a greater coupling of current to adjacent tissue 260 to cause a deeper ablated tissue 440 ′ as compared to the thicker wall portions 436 ′ at the ends of the dielectric structure ( cf . ablated tissue in fig1 b ). in all other respects , the working end 460 operates as previously described embodiments . in the electrosurgical ablation working ends of fig1 - 21 above , the dielectric structures 422 and 422 ′ provide differential or energy transmissibility by means of varying the thickness of a dielectric such as silicone . a portion of an exemplary dielectric wall 470 with varying thickness portions 435 ′ and 436 ′ is shown in fig2 which represents the dielectric of fig2 . in other words , a varied thickness wall with a uniform dielectric constant or volume resistivity of the material can provide varied coupling of rf current to tissue about the surface of the dielectric . it should be appreciated that an objective of the invention is controlled depth of ablation which can be accomplished equally well by having a uniform thickness dielectric but varying the electrical properties of the material . fig2 illustrates a constant thickness dielectric wall 475 with first and second dielectric materials 477 and 480 that provides for higher capacitive coupling through material 480 . the number of layers of materials , or material portions , and their dielectric properties can range from two to ten or more . further , combinations of varying material thickness and dielectric properties can be utilized to control capacitive coupling of current through the dielectric . fig2 a - 25d illustrate another embodiment of electrosurgical system 500 and working end 520 and method of use that is similar to the device of fig1 except that the dielectric structure 522 of fig2 a - 25d is fabricated of a thin - wall dielectric that can be moved from a first non - expanded condition to an expanded condition . in fig2 a , the working end is shown with a distally - extended sheath 524 that can be of plastic or metal . a first step of a method thus comprises introducing the working end into tissue interstitially or into a body lumen with the sheath protecting the dielectric structure 522 . the dielectric structure 522 is then expanded by gas inflows which causes compression of surrounding tissue and increases the surface area of the thin dielectric wall in contact with tissue . as can be seen in fig2 a , the expandable dielectric 522 can be fabricated of a distensible or non - distensible material , such as a stretchable silicone or a braided , reinforced non - stretch silicone . the wall thickness of a silicone structure can range from 0 . 004 ″ to 0 . 030 ″, and more typically from 0 . 008 ″ to 0 . 015 ″ with an interior volume ranging from less that 5 ml to more than 100 ml . the dielectric structure can have any suitable shape such as cylindrical , axially tapered , or flattened with interior baffles or constraints . fig2 depicts a cross - section of the sheath 524 and a non - distensible expandable dielectric 522 with a method of folding the thin dielectric wall . fig2 b illustrates multiple subsequent steps of the method wherein sheath 524 is retracted and the physician actuates the gas source 150 and controller to expand the expandable dielectric structure 522 . the structure 522 or balloon can be expanded to any predetermined dimension or pressure in soft tissue or in any body lumen , cavity , space or passageway . radiopaque marks on the dielectric structure ( not shown ) can be viewed fluoroscopically to determine its expanded dimension and location . the gas circulation controller 155 a can circulate gas flow after a predetermined pressure is achieved and maintained . fig2 c depicts a subsequent step of the method in which the physician actuates the rf power source 200 and controller 155 b to develop high voltage potential between central support electrode 295 and return electrode 205 which , as described previously , can cause a voltage breakdown in the gas dielectric 140 ( fig2 b ) to create plasma 240 and contemporaneously capacitively couple current to tissue 260 as indicated by current flows 530 . fig2 d depicts the termination of rf energy delivery so that the voltage breakdown and resulting plasma is extinguished — leaving uniform ablated tissue 540 similar to that shown in fig1 b . in one embodiment , the dielectric structure 522 was made from nusil med - 6640 silicone material commercially available from nusil technology llc , 1050 cindy lane , carpinteria , calif . 93013 . the dielectric structure 522 was fabricated by dipping to provide a length of 6 cm and a uniform wall thickness of 0 . 008 ″ thereby providing a relative permittivity in the range of 3 to 4 . the structure ends were bonded to a shaft having a diameter of approximately 4 mm with the expanded structure having an internal volume of 4 . 0 cc &# 39 ; s . the gas used was argon , supplied in a pressurized cartridge available from leland limited , inc ., post office box 466 , south plainfield , n . j . 07080 . the argon was circulated at a flow rate ranging between 10 ml / sec and 30 ml / sec . pressure in the dielectric structure was maintained between 14 psia and 15 psia with zero or negative differential pressure between gas inflow source 150 and negative pressure ( outflow ) source 160 . the rf power source 200 had a frequency of 480 khz , and electrical power was provided within the range of 600 vrms to about 1200 vrms and about 0 . 2 amps to 0 . 4 amps and an effective power of 40 w to 80 w . fig2 and 28 illustrate another embodiment of electrosurgical system 600 that comprises a catheter having working end 610 for treating atrial fibrillation by means of ablation about pulmonary veins pv . various methods of using conventional rf catheters for such treatments are known . catheter 610 is configured with a guidewire channel 612 and can be navigated to a site shown in fig2 - 28 . the catheter working end 620 included an expandable dielectric structure 622 similar to that of fig2 a - 25d that can be expanded to apply pressure between the balloon wall and the tissue to thereafter create a circumferential lesion in a pulmonary vein pv . fig2 is a schematic illustration that again show gas source 150 and gas circulation controller 155 a that can expand chamber 635 in the thin - wall dielectric structure 622 to engage the wall of the pulmonary vein pv . in the embodiment of fig2 , it can be seen that the wall of dielectric 622 includes a first ( lesser ) energy - transmissible region 636 and a second ( greater ) energy - transmissible region 638 thus allowing a focused circumferential ablation — which corresponds to the configuration of dielectric wall shown in fig2 . thereafter , the rf power source 200 and controller 155 b can be actuated to convert the neutral gas flow to plasma 240 and contemporaneously ablate tissue indicated at 640 . in this embodiment , a first polarity electrode 645 is provided on the catheter shaft in chamber 635 that can cooperate with a second polarity electrode on the catheter shaft remote from balloon 622 or any other type of ground pad may be used ( not shown ). in all other respects , the method of the invention for ablation of cardiac tissue follows the steps described above . the balloon can have radiopaque markings , and the system can be operated by an algorithm to expand the dielectric structure 622 or balloon to a pre - determined pressure , then delivery rf energy and terminate delivery automatically . it should be appreciated that additional electrodes can be provided in the balloon surface ( not shown ) for mapping conduction in the cardiac tissue . while fig2 - 28 illustrate an expandable dielectric 622 for treating cardiac tissue , it should be appreciate that the scope of the invention includes using a similar apparatus and method to controllably apply ablative rf energy to any body lumen , vessel , body cavity or space such as in a stomach , gall bladder , esophagus , intestine , joint capsule , airway , sinus , a blood vessel , an arteriovascular malformation , heart , lung , uterus , vaginal canal , bladder or urethra . fig2 - 31 schematically illustrate another embodiment of electrosurgical system 700 and catheter having working end 710 for treating atrial fibrillation with linear lesions within a heart chamber to block aberrant conduction pathways . catheter 710 can have a guidewire channel ( not shown ) and can be navigated to perform an elongated ablation in a heart chamber as in fig2 . in this embodiment , the catheter working end 720 has a flexible shaft portion 721 that included an axially - extending thin - wall dielectric 722 in one surface for engaging tissue to provide a linear lesion as depicted in fig3 . the catheter shaft 721 is deflectable by means of a pull - wire 728 that can be actuated from a catheter handle . fig3 is another schematic illustration that shows the gas source 150 and gas circulation controller 155 a that can provide gas circulation within interior chamber 735 interior of the thin - wall dielectric 722 . the rf power source 200 is coupled to a lead 738 and elongated first polarity electrode 740 in the interior chamber 735 . the rf power source 200 and controller 155 b can be actuated to convert the neutral gas flow to a plasma and contemporaneously ablate tissue engaged by dielectric 722 as described above . the second polarity electrode can be provided on the catheter shaft remote from dielectric 722 or any type of ground pad may be used ( not shown ). in all other respects , the method of the invention for ablation of cardiac tissue follows the steps described above . the working end can have radiopaque markings , and the system can be operated in accordance with algorithms . it should be appreciated that additional electrodes can be provided in the catheter working end ( not shown ) for mapping conduction in the heart pre - and post ablation . fig3 illustrates another catheter working end 720 ′ that is similar to that of fig2 - 31 that is deflectable by a pull - wire 738 to provide all or part of circumferential lesion in a pulmonary vein ( see fig2 - 29 ). in this embodiment , the thin - wall dielectric 722 ′ extends around the exterior surface of the articulated working end . fig3 and 34 illustrate another embodiment of electrosurgical system 800 that comprises a catheter having working end 810 for treating an esophagus 811 , for example to ablate barrett &# 39 ; s esophagus , to apply energy to lower esophageal sphincter or for other disorders . the system operates as previously described in fig2 a - 28 in embodiments that have an expandable dielectric structure . in the dielectric structure 822 of fig3 - 34 , the expansion of the structure is provided by a skeletal support member such as an interior spring - like member , with an optional pull - cable actuation mechanism . as can be seen in fig3 , a helical support member 825 is provided that is capable of a contracted cross - section ( axially - stretched ) or an expanded cross - section in chamber 835 which is assisted by pulling central cable 828 in catheter shaft 830 . in this embodiment , the dielectric can again comprise a thin - wall silicon as described above . in this embodiment , it has been found that the support member 825 can be of a conductive metal and coupled to rf power source to function as a first polarity electrode 840 . the second polarity electrode ( not shown ) can be located elsewhere on the catheter is a location in contact with tissue , or a ground pad can be used . fig3 illustrate another embodiment of electrosurgical system 800 ′ that is similar to that of fig3 with a dielectric structure 822 that is supported in an expanded condition by a plurality of bowed - out skeletal support members 825 ′ that are assisted by pull - cable 828 . in this embodiment , the portion of the pull - cable within chamber 835 functions as a first polarity electrode 840 ′. in operation in any of the embodiments above , it has been found that the first polarity electrode can provide sufficient voltage to create a substantially uniform plasma in an interior chamber ( see fig2 , 8 , 11 a , 28 , 30 , 34 , 35 ) of a non - expandable or expandable dielectric when the surface of the electrode is less than 15 mm , less than 10 mm or less than 5 mm from the interior wall of the dielectric . this maximum dimension from the dielectric wall to the electrode 840 ′ is indicated at d in fig3 . in has also been found that , in operation , the first polarity electrode can provide voltage to create a substantially uniform plasma in an interior chamber of a non - expandable or expandable dielectric wall when the electrode contacts the surface of the dielectric 822 as in fig3 , but the electrode surface should engage less than about 10 % of the interior surface of the dielectric wall . if the first polarity electrode engages greater than about 10 % of the interior surface of the dielectric wall , then the “ flux ” of energy delivery through tissue as schematically depicted in fig8 will be reduced , a greater capacitive coupling may occur about the regions of the electrode ( s ) in contact with the wall which can reduce the uniformity of tissue ablation . fig3 illustrates another embodiment of electrosurgical system 900 wherein the working end 910 comprises first and second opposable jaws 912 a and 912 b that are adapted for clamping tissue for coagulation , sealing or welding tissue 914 . in one embodiment , both jaws have a tissue - engaging surface that comprises a dielectric structure 922 a , 922 b that is similar in function to all other such dielectric structures described above . fig3 is a schematic illustration that again shows gas source 150 and gas circulation controller 155 a that can deliver gas to chambers 935 a , 935 b in the jaws . the rf power source 200 and controller 155 b can be actuated to convert the neutral gas flows in the chambers 935 a , 935 b into plasma 240 and contemporaneously to apply energy to engaged tissue 914 . in this embodiment , the jaws carry first and second polarity electrodes 945 a and 945 b , respectively , to thus make jaw function by means of a contained ionized gas and capacitive coupling , which differs from previous embodiments . it should be appreciated that one jaw can comprise a single electrode surface , as opposed to the plasma - initiated capacitive coupling system of fig3 . the dielectric structure of fig3 are of the type described in fig4 b and 5a wherein the thin - wall dielectric material is supported by support columns , posts , channels of the like . fig3 and 38 illustrate another embodiment of electrosurgical system 1000 again includes a catheter or probe shaft 1002 extending to a working end 1010 that carries an expandable dielectric structure 1022 . in this embodiment , the dielectric structure 1022 includes a plurality of interior chambers , for example first and second chambers 1024 a and 1024 b . the expansion of the dielectric structure 1022 can be provided by skeletal support members such as interior spring - like members as described above or by expansion by fluid pressure of gas inflows or a combination thereof . each chamber is configured to carry a flexible interior electrode , with adjacent chambers having opposing polarity interior electrodes , such as electrodes 1040 a and 1040 b indicated at (+) and (−) polarities in fig3 and 38 , to allow another form a bi - polar ablation . in this embodiment , the electrodes and support members can comprise the same members . as can be seen in fig3 , the external wall of dielectric structure 1022 has thin wall portions 1032 a and 1032 b for capacitively coupling energy to tissue , and a thicker wall portion 1042 that insulates and separates the first and second chambers 1024 a and 1024 b . the flexible electrodes 1040 a and 1040 b are operatively coupled to rf power source 200 . the gas inflow source 150 and negative pressure source 160 are coupled to in inflow and outflow channels communicating with each interior chamber , 1024 a and 1024 b , independently . in the transverse sectional view of fig3 , the open terminations 1046 and 1048 of the inflow and outflow channels can be seen in each interior chamber , 1024 a and 1024 b . thus , each chamber is provided with a circulating gas flow ( indicated by arrows in fig3 ) similar to that described in previous embodiments with respect to single chamber working ends . fig3 is a schematic sectional view of the dielectric structure 1022 deployed in a targeted tissue 1050 . it can be understood that the system can be actuated to circulate gas in the chambers 1024 a and 1024 b which then is converted to a plasma 240 in each chamber as described previously . in this embodiment and method of use , the capacitive coupling occurs through the thin dielectric walls 1032 a and 1032 b in paths of current flow indicated at 280 in fig3 . whereas the previous embodiments illustrated a single chamber containing a plasma that capacitively coupled current to a non - gas electrode , the embodiment of fig3 and 38 depicts the use of at least two contained plasma electrodes and capacitive coupling therebetween . it should be appreciated that the number of adjacent chambers carrying opposing polarity electrodes can be utilized in a thin - wall dielectric structure , for example 2 to 10 or more , with the chambers having any suitable dimensions or orientations relative to one another . in another aspect of the invention , it has been found that the operating parameters of the system can be adjusted to provide cooperating mechanisms for causing thermal effects in tissue , that is , ( i ) joule heating of tissue as described above and ( ii ) passive heat conduction from the dielectric wall which is heated by the plasma ( see co - pending u . s . patent application ser . no . 12 / 944 , 466 ). in one embodiment , the system parameters that can be adjusted to minimize joule heating of tissue and to correspondingly maximize passive conductive heating of tissue . the operating parameters that can be adjusted consist of ( i ) the dielectric constant of the dielectric wall ; ( ii ) the surface area of dielectric wall in contact with tissue ; ( iii ) the thickness of the dielectric wall ; ( v ) the circulation of neutral gas through the interior chamber which controls plasma resistance ; ( vi ) the frequency of the rf current ; ( vii ) the pressure within the interior chamber ; and ( viii ) the field strength which relates to voltage and spacing between the electrode in the interior chamber and the dielectric wall . in this aspect of the invention , it has been found that a high plasma resistance , compared to the tissue resistance , can result in plasma filaments or streamers coupling energy to the focal points of the dielectric wall without coupling as much energy through the dielectric to cause joule heating of the tissue . in this aspect of the invention , utilizing a dielectric wall with a lower dielectric constant will provide for greater passive tissue heating from the heated of focal points on the dielectric and lesser joule heating along current paths in tissue . although particular embodiments of the present invention have been described above in detail , it will be understood that this description is merely for purposes of illustration and the above description of the invention is not exhaustive . specific features of the invention are shown in some drawings and not in others , and this is for convenience only and any feature may be combined with another in accordance with the invention . a number of variations and alternatives will be apparent to one having ordinary skills in the art . such alternatives and variations are intended to be included within the scope of the claims . particular features that are presented in dependent claims can be combined and fall within the scope of the invention . the invention also encompasses embodiments as if dependent claims were alternatively written in a multiple dependent claim format with reference to other independent claims .
tissue is treated using a radiofrequency power supply connected to an applicator having a chamber filled with an electrically non - conductive gas surrounded by a thin dielectric wall . a radiofrequency voltage is applied at a level sufficient to ionize the gas into a plasma and to capacitively couple the ionized plasma with the tissue to deliver radiofrequency current to ablate or otherwise treat the tissue .