Electrostatic accelerators in general, and Pelletron accelerators in particular, and are well known. They have been installed in a number of research establishments throughout the world. Currently, Pelletron electrostatic accelerators are manufactured only by National Electrostatic Corporation (NEC) of Middleton, Wis., USA, and the major manufacturer of Van de Graaf accelerators is High Voltage Engineering Corporation (HVEC), Massachusetts, USA. The Australian National University has a Pelletron accelerator and because the present invention was developed as a consequence of an appreciation of deficiencies in the voltage grading used in that accelerator, this description of the present invention will tend to emphasise its use in a Pelletron accelerator.
In a Pelletron accelerator, a high voltage terminal, which is charged by the movement of charge-carrying pellets mounted in a conveyor type of arrangement, is supported within a large metal high pressure vessel by a plurality of supporting columns. Each supporting column extends from a wall of the pressure vessel (which is at ground potential) to the high voltage terminal and comprises a large number of insulating discs, separated by thinner metallic discs. The vessel is filled with an insulating gas--sulphur hexafluoride--to a pressure of about 6 atmospheres. To ensure that there is no inadvertent sparking within the vessel while the high voltage terminal is being charged (typically to 14 MV) or is being maintained at its charged level, there must be an equal electrical stress on each insulator. To achieve such an equal electrical stress, it is necessary to provide a uniform voltage gradient along the supporting columns.
The high pressure vessel also contains the tube of the accelerator, which is constructed from a large number of annular insulating discs, separated by thinner, annular, metallic discs or flanges. It is normally a requirement that the voltage gradient along the tube of the accelerator should be uniform, although in some cases it is desirable to be able to vary the voltage gradient (for example, to achieve particular beam optics).
Traditionally, the voltage gradient has been controlled by a corona discharge device connected between adjacent metallic discs of the columns and the tube. The corona discharge devices are relatively inexpensive and are robust, but there are some disadvantages associated with the corona voltage grading system.
The major disadvantage of the corona devices is that the voltage across the corona point-to-plane gap depends upon the separation distance, point sharpness, the presence of foreign material and the insulating gas pressure. When corona point discharge devices have been tested after their installation, using a spacer jig, more than 10 per cent of them have shown variations from the mean, in the voltage drop across the devices, of more than 10 per cent. In non-standard installations, such as between accelerator tube flanges, it has been established that more than 50 per cent of the corona point devices exhibit a variation from the mean voltage drop across the devices of more than 10 per cent. Thus over-stressed gaps in the components of the accelerator tube are quite common.
Another major disadvantage in the use of corona devices is that the corona discharge system causes some breakdown of the insulating SF.sub.6 gas, and the breakdown products are corrosive. The resultant chemical attack can be reduced to an acceptable level by efficient chemical scrubbing, but scrubbing incurs a significant capital cost to perform.
Most corona discharge devices in electrostatic accelerators need to be replaced at intervals of about two years. Although the replacement of the components in the accelerator involves a cost of only a few thousand dollars, the accelerator has to be shut down for at least two weeks while the replacement is effected. About 60 per cent of the shut down time is directly assignable to corona point replacement. It has been estimated that the true cost of replacement of the corona point devices is approximately $110,000 per year.
The deliberate variation from linear voltage gradient along an accelerator tube, mentioned briefly above, is very awkward to effect with corona point discharge devices.
To overcome the above-noted disadvantages associated with the use of corona point discharge devices, it has been proposed to use resistors in place of the corona discharge devices. There are no SF.sub.6 breakdown products when resistors are used, and varying the voltage gradient along an accelerator tube is a trivial operation using resistors. In fact, in some of the accelerators where this change has been made, the benefits have far outweighed the initial high cost of installing resistors in place of the corona point discharge devices. For example, at Daresbury, in the United Kingdom, an electrostatic accelerator incorporating a resistor system has been operated for 10 years without lost time due to resistor failure.
Unfortunately, some problems are experienced when resistors are used in an accelerator. High voltage resistors, by their nature, tend to be long devices. For example, in the HVEC accelerators, the resistors are 50 cm long and grade about 50 KV per gap between conducting plates in a support column or tube. Multiple element resistors in both in-line and serpentine configuration have also been used, but they also span about 50 cm. Such long resistors tend to act as aerials, and the induced charge developed by each "aerial" during a major accelerator discharge can substantially increase the electrical stress imposed upon the resistors.
The obvious solution of using shorter (and therefore thicker) resistors is impractical because, to fit into the space available in most electrostatic accelerators, the resistors must fit within a cylindrical housing having an outer diameter of about 16 mm. Marginally thicker resistors can be used in those accelerators having a higher electrode spacing (pitch) of 25 mm.
A recent development, by the French company Vivirad, is to mount a pair of resistors on each column or tube electrode, in a side-by-side arrangement, with the resistors connected in series by a spring extending between their ends which are remote from the column or tube.
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