D. C. Sutton

Operating Experience with MUSL-2

The second 6 pass microtron using a superconducting linac has been completed and has operated on 24 hour schedules for nuclear physics for about 5000 hours during the past year. The Q of the linac has remained at 3x109 and the C.W. energy gain has remained at 2.3 MeV/m as in its initial tests in 1975. Single pass C.W. currents up to 20 ¿A with energies up to 14.6 MeV have been available for resonance fluorescence experiments. Six pass beams with energies up to 67 MeV have been available to other experimental areas but useful currents have been limited to 0.3 ¿A by the excitation of transverse beam blowup modes around 2.3 GHz. The multiple pass currents, however, have been more than sufficient for all tagged photon experiments. Work is proceeding to replace our MUSL-2 linac with another Stanford linac in which the loading of the 2.3 GHz modes is increased by a factor of 100 or more by hybrid electric-magnetic loading probes. Plans to reach higher energies by using MUSL-2 as an injector into a second microtron continue to be attractive.

Microtron Using a Superconducting Electron Linac

The superconducting linac of the 6 pass microtron (MUSL-1) described previously has been operated at 4.2 K for about 3 years with an energy gain of 2 MeV/meter. The duty factor has been limited to about 50% by thermal effects. A new helium liquefier supplies more than the 13 liters per hour required to operate the linac continuously at its maximum duty factor. The system can supply 5 ¿a of electrons with energies up to 19 MeV and has been operated on a 24 hour per day schedule for nuclear physics experiments. A larger system (MUSL-2) utilizing a surplus 3 MeV Van de Graaff as an injector and a 6 meter superconducting accelerator section made for us at Stanford University is being assembled in another area. Although the installation is not yet complete microwave tests indicate that the section can be operated continuously with an energy gain of 13 MeV with an input power of about 10 watts. For the initial operation of MUSL-2 the 6 pass hardware from MUSL-1 will be used to recirculate the electron beam through the new linac section to a final energy of 60 MeV.

Design of a 600 MeV Superconducting Microtron

The design of a 600 MeV racetrack microtron proposed for the University of Illinois consists of a 30 MeV superconducting linac operating at 1.3 GHz between two uniform field bending magnets about 6.4 meters apart. There will be 19 parallel return paths separated by 14.7 cm. Vertical focusing is provided on each return orbit by a quadrupole pair close to each magnet. For quadrupoles 20 cm long and separated by 10 cm, the gradients required to advance the vertical oscillation phase by 90° per revolution increase monotonically from 150 to 350 gauss per centimeter. Only very weak horizontal focusing powers are needed to keep the return beams within 2 mm of linac axis. The linac itself provides some focusing but two weak quadrupole singlets on the linac axis, one near each magnet, provide additional focusing. Electron trajectories for all traversals through the linac and the magnets have been calculated in detail, by a step by step procedure where required, and are presented here. These results confirm earlier, less complete, calculations indicating that the microtron design parameters are not critical and that an output energy resolution of one part in 104 is easily obtainable.

Status of Musl-2, the Second Microtron Using a Superconducting Linac

A second racetrack microtron, MUSL-2, is being assembled in the area previously occupied by the 300 MeV betatron. It uses a Van de Graaff to inject electrons at about 2 MeV, a 6 meter, 1.3 GHz superconducting linac made for us at the Stanford High Energy Physics Laboratory as the accelerating section and the magnets from MUSL-1 for recirculation1. A digital control console has been installed to operate the linac and the injection and recirculation systems. The CTI 1400 helium liquefier together with the low pressure heat exchanger from MUSL-1 maintains the linac at about 2 K. Beams of 10 microamperes with energies up to 14 MeV with a resolution of 0.2% are being used for nuclear experiments. Continuous beams up to 72 MeV will be available after the installation of the 6 pass system is completed. Present plans for moving toward higher energies involve the use of MUSL-2 to inject electrons with energies up to 72 MeV into a second superconducting linac in a microtron operating at ¿=1. With return orbits separated by 7.35 cm relatively small magnets in the second system can accommodate 18 additional passes. At 12 MeV per pass electrons would reach a final energy of 288 MeV.

Plans for a Two Stage 450 MeV Cascade Microtron

We report some alternative plans for obtaining continuous electron beams with energies of about 450 MeV by using two microtrons in series. The first stage could be the accelerator we are now operating, which is a six transversal Microtron Using a Superconducting Linac, MUSL-2. The second stage would include another linac which can sustain an r.f. field continuously and a higher energy recirculation system. The recirculation that is best understood and seems to have the least distortion is the conventional microtron with a pair of 180° bending magnets such as are used in MUSL-2. The alternative recirculation system uses four 90° bending magnets, and could be extended to higher energy more easily.