14198728
Two loops in eleven-dimensions
The two-loop Feynman diagram contribution to the four-graviton amplitude of eleven-dimensional supergravity compactified on a two-torus, T^2, is analyzed in detail. The Schwinger parameter integrations are re-expressed as integration over the moduli space of a second torus, \hat T^2, which enables the leading low-momentum contribution to be evaluated in terms of maps of \hat T^2 into T^2. The ultraviolet divergences associated with boundaries of moduli space are regularized in a manner that is consistent with the expected duality symmetries of string theory. This leads to an exact expression for terms of order D^4 R^4 in the effective M theory action (where R^4 denotes a contraction of four Weyl tensors), thereby extending earlier results for the R^4 term that were based on the one-loop eleven-dimensional amplitude. Precise agreement is found with terms in type IIA and IIB superstring theory that arise from the low energy expansion of the tree-level and one-loop string amplitudes and predictions are made for the coefficients of certain two-loop string theory terms as well as for an infinite set of D-instanton contributions. The contribution at the next order in the derivative expansion, D^6 R^4, is problematic, which may indicate that it mixes with higher-loop effects in eleven-dimensional supergravity.
120539667
Anomaly cancellations in supersymmetric D = 10 gauge theory and superstring theory☆
Abstract Supersymmetric ten-dimensional Yang-Mills theory coupled to N = 1, D = 10 supergravity has gauge and gravitational anomalies that can be partially cancelled by the addition of suitable local interactions. The remaining pieces of all the anomalies cancel if the gauge group is SO(32) or E 8 × E 8 . These cancellations are automatically incorporated in the type I superstring theory based on SO(32). A superstring theory for E 8 × E 8 has not yet been constructed.
121062482
N=4 Yang-Mills and N=8 Supergravity as Limits of String Theories
The formulation of supersymmetric string theories in ten dimensions is generalized to incorporate compactified dimensions. Expressions for the one-loop four-particle S -matrix elements of N = 4 Yang-Mills and N = 8 supergravity in four dimensions are obtained by studying the string-theory loop amplitudes in the limit that the radii of the compactified dimensions and the Regge slope parameter simultaneously approach zero. If certain patterns that emerge should persist in the higher orders of perturbation theory, then N = 4 Yang-Mills in four dimensions would be ultraviolet finite to all orders, whereas N = 8 supergravity in four dimensions would have ultraviolet divergences starting at three loops.
121755839
PHASE CONTOURS AND BOOTSTRAPS. I.
123230985
Off-shell states in the dual model
Abstract We present a class of Lorentz invariant, gauge invariant off-shell amplitudes for the ordinary Veneziano and Ramond-Neveu-Schwarz dual models. They correspond to interactions with the string which are local in space-time.
9607753
Anomalous creation of branes
In certain circumstances when two branes pass through each other a third brane is produced stretching between them. We explain this phenomenon by the use of chains of dualities and the inflow of charge that is required for the absence of chiral gauge anomalies when pairs of D-branes intersect.
108681531
A 2-MeV microwave thermionic gun
A high-gradient, S-band microwave gun with a thermionic cathode is being developed in a collaborative effort by AET, Varian, and SSRL. A prototype design using an upgraded Varian dispenser cathode mounted with thermal isolation directly in the first half-cell of a 1-1/2 cell, side-coupled, standing-wave cavity has been fabricated and is being tested. Optimization of the cavity shape and beam formation was done using SUPERFISH, MASK, and PARMELA. An overview of design details, as well as the status of in-progress beam tests, will be presented. 9 refs., 6 figs.
122870434
A comparison of ASTROMAG coils made with aluminum and copper based superconductor
The use of an aluminum matrix superconductor in the coils for the ASTROMAG magnet will increase the integrated field for conducting particle astrophysics experiments in space as compared to equal mass coils made with a copper matrix superconductor. The increased ability to detect charged particles can be achieved without decreasing the current margin of the superconductor in the coils. The use of a low-resistivity aluminum matrix conductor increases the energy needed to initiate a quench by two orders of magnitude. The current decay time constant during a quench is substantially increased. As a result, the quench energy dumped into the helium tank is reduced (the ASTROMAG coils are thermally decoupled from the helium tank), and the forces on the shield and shells due to eddy currents will be lower. A description is also given of the problems associated with the use of an aluminum matrix superconductor in the coils.
36975121
The Role of Quench-Back in the Passive Quench Protection of Long Solenoids With Coil Sub-Division
This paper describes how a passive quench protection system can be applied to long superconducting solenoid magnets. When a solenoid coil is long compared to its thickness, the magnet quench process will be dominated by the time needed for quench propagation along the magnet length. Quench-back will permit a long magnet to quench more rapidly in a passive way. Quench-back from a conductive (low resistivity) mandrel is essential for spreading the quench along the length of a magnet. The mandrel must be inductively coupled to the magnet circuit that is being quenched. Current induced in the mandrel by di/dt in the magnet produces heat in the mandrel, which in turn causes the superconducting coil wound on the mandrel to quench. Sub-division is often employed to reduce the voltages to ground within the coil. This paper explores when it is possible for quench-back to be employed for passive quench protection. The role of sub-division of the coil is discussed for long magnets.
122003703
Quench back in thin superconducting solenoid magnets
Abstract Superconducting magnets with well coupled, low resistance, secondary circuits have been observed to become fully normal faster than quench propagation in the coil would permit. This process is referred to as ‘quench back’. Quench back observed at the Lawrence Berkeley Laboratory (LBL) was caused by heating the secondary circuit from the current induced from the primary circuit as normal region in the superconducting coil propagated. This paper develops the theory for thermal quench back in thin solenoid magnets and compares this theory with measurements made in two one-meter diameter superconducting solenoid magnets.
59453973
A superconducting focusing solenoid for the neutrino factory linear accelerator
A Superconducting Focusing Solenoid for the Neutrino Factory Linear Accelerator * M. A. Green, V. Lebedev, and B. P Strauss The 400-m long linear accelerator is divided into three sections. Upstream from the linear accelerator is a matching section that provides the first 60 MeV of muon acceleration. The end of the matching section consists of a pair of low stray field solenoids with adjustable currents for tuning. The first section has 4 short modules that are 5-meters long. Each short module has two cells of RF cavities. The second section has 16 medium length modules that are 8-meters long. Each intermediate module has two cells of RF cavities arranged in groups of two with a 1-meter space between groups. The short and intermediate modules have a 1-meter long focusing solenoid with a 1.25-meter long straight section at each end. The third acceleration section has 19 long modules that are 13-meters long. The long module has eight superconducting RF cells arranged in groups of two with a 1-meter space between cell groups. The focusing solenoid is 1.5 meters long with a 1.25-meter long space separating it from the adjacent RF cavity cells. The three types of acceleration modules used for the linear accelerator are illustrated in Figure 1. Unlike focusing quadrupoles, focusing solenoids can produce a stray field that exists some distance from the magnet. The superconducting RF cavities are sensitive to magnetic fields even at the level of 0.0001 T (versus 0.043 T for the smallest single solenoid in Table 1), so a key parameter in the focusing solenoid design is getting rid of the solenoid stray field in the RF cavities. The methods one can use to eliminate the stray field in the RF cavities are:1) The focusing solenoid should produce zero net magnetic moment[5,6]. This means that the coil that produces the field is bucked by a coil that is larger in diameter. 2) The field from the bucking coils should be distributed in the same way as the solenoid field. This suggests that the bucking solenoid be around the focusing solenoid so that the return flux from the focusing solenoid is returned between the focusing solenoid and the bucking solenoid[7]. 3) The nested solenoids should be surrounded by iron except where the muon beam passes through the solenoid. 4) An iron flux shield should be installed between the focusing solenoid and the RF cavities. 5) If the stray field is still not low enough, the RF cavities can be covered with a type 2 superconducting shield. This shield will trap earth’s magnetic field, but it will shield out the remaining stray flux from a nearby solenoid provided the cavities are cold (below 5 K) when the solenoid is turned on. An iron shielded actively shielded solenoid appears to be a minimum requirement in order to reduce the stray field to an acceptable level. Abstract--The proposed linear Accelerator that accelerates muons from 190 MeV to 2.45 GeV will use superconducting solenoids for focusing the muon beam. The accelerator will use superconducting RF cavities. These cavities are very sensitive to stay magnetic field from the focusing magnets. Superconducting solenoids can produce large stray fields. This report describes the 201.25 MHz acceleration system for the neutrino factory. This report also describes a focusing solenoid that delivers almost no stray field to a neighboring superconducting RF cavity. Index Terms—Superconducting Focusing Solenoid, Active Shielding, Superconducting RF Cavities I. I NTRODUCTION The proposed neutrino factory generates neutrinos from muon decay in a muon storage ring[1,2]. The muons are created from the decay of pions produced from the collision of protons on a fixed target. The pions are captured by a 20 T solenoid. The pions then decay to muons as they pass down a long channel. The muons are phase-rotated, bunched, and cooled before they can be accelerated and stored in the storage ring[3]. The first stage of acceleration is a linear accelerator that uses superconducting RF cavities[4]. Muon focusing during this acceleration occurs within superconducting solenoids. After the muons are accelerated to 2350 MeV, they enter a re- circulating linac that accelerates them to their final energy of 20 GeV. The 20 GeV muons are then stored in storage ring where they decay to two neutrinos and an electron or positron (depending on the charge of the originating muon type). II. F OCUSING S OLENOIDS FOR THE L INEAR A CCELERATOR The proposed linear accelerator that accelerates the muons from 190 MeV to 2350 MeV extends nearly 400 meters. The Linear accelerator will use high gradient superconducting RF cavities to accelerate the beam. These cavities will not only increase the acceleration gradient, but they will also save electric power. Between the RF cavities there will be superconducting solenoids that provide beam focusing. Received 26 September 2001 * This work was supported by the Office of Science, United States Department of Energy under DOE contract number DE-AC03-76SF00098 M. A. Green, is from the Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA;V. Lebedev, is from the Thomas Jefferson National Laboratory, Newport News VA 23606, USA; B. P Strauss is from the U S Department of Energy, Germantown MD 20874, USA