J.S. Sokolowski

The ALPI project at the Laboratori Nazionali di Legnaro

Abstract The project of a superconducting linac acting as booster of the LNL-XTU tandem is briefly discussed. Experience with lead plated quarter wave resonators is discussed together with the machine configuration.

Dispersion interference in the pulsed-wire measurement method

Abstract The magnetic profile of the wiggler to be used in the planned Weizmann Institute FEL has been measured using the pulsed-wire method. The main transverse deflection pattern caused by an electrical current pulse in a wire placed along the wiggler was sometimes accompanied by minor faster and slower parasitic components. These components interfered with the main profile, resulting in distorted mapping of the wiggler magnetic field. Their periodical structure being very close to the main pattern could not be easily resolved by applying a numerical Fourier transform. A strong correlation between the wire tension and the amplitude of the parasitic patterns was found. Significant damping of these oscillations was achieved by applying high enough tension to the wire (close the yield point), allowing to disregard their contribution to the measurement accuracy.

A proposal for a tandem accelerator FEL experiment

Abstract A proposal for an FEL experiment, based on a tandem electrostatic accelerator, is presented. The proposed FEL is designed to operate at harmonic frequencies in a quasi-cw mode. An internal wiggler scheme is planned to be used, based upon a strong planar wiggler ( a w > 1), positioned in the high voltage terminal of a HVEC model EN tandem accelerator. The electron beam enters the wiggler at the natural injection angle ( a w / γ ). This approach provides a relatively simple and compact FEL system that will be used as a research tool for studying harmonic operation of FELs and investigating post-saturation laser dynamics and efficiency enhancement. The exceptional electron beam quality and long pulse characteristics of electrostatic accelerators give a special advantage to such an experimental scheme as a means for exploring the ultimate performance parameters of FELs.

Superconducting booster module using quarter wave resonators

Abstract A superconducting booster module has been recently installed at the Weizmann Institute of Science. The module consists of four β0 = 0.095 quarter wave resonators housed in a cylindrical vertical cryostat. Additional components of the system are a chopper buncher, control electronics and a helium refrigerator. The booster resonators have been operated continuously at a field gradient of 3 MV/m, providing a total accelerating voltage of 2 MeV/charge. Oxygen and carbon beams have been accelerated, and the module is being used in nuclear physics experiments.

The bulk niobium resonators program at LNL

Abstract The encouraging results of the 160 MHz niobium quarter wave resonator (QWR) have prompted us to design, construct and test a complete set of resonators which could be used in the three consecutive sections (low-β, intermediate and high-β) of a superconducting heavy ion booster. They seem to be ideally suited for industrial production, since the majority of parts are identical for all three types of resonators. They are made of bulk niobium, and are heat treated under high vacuum conditions; this treatment improves the niobium RRR ratio. The direct contact with liquid helium of most niobium surfaces provides the resonator with excellent cooling. The 160 MHz resonator of β = 0.112 planned for the intermediate-beta section of the linac gave an accelerating field of 5 MV/m at 10 W power dissipation and its low power Q was 2.4 × 10 8 . The 80 MHz QWR intended for the low-beta section was constructed and heat treated; the first rf test, performed on the resonator damaged by a serious vacuum accident, looked promising, thus we have repaired the mechanical damage and the resonator will be tested again as soon as the surface treatment is performed. We have recently completed construction of the high-beta 240 MHz QWR; it will be heat treated and measured and we expect to present its rf test results in the near future.

Development of a tandem electrostatic accelerator quasi-cw FEL

Abstract The EN tandem electrostatic accelerator at the Weizmann Institute of Science has been converted into an electron accelerator with beam power recovery. We report on the design and performance of the accelerator as well as on a new approach to stable, long-pulse operation of this class of machines. The long-pulse model of operation offers interesting possibilities for the operation of free electron lasers, in particular the study of high-coherence, single-mode operation.

Design of a tandem accelerator free electron laser

Abstract The motivation for using a tandem electrostatic accelerator as an electron accelerator for a free electron laser (FEL) is presented. The adaptation of the HVEC EN tandem at the Weizmann Institute for this purpose, electron beam optics and nonlinear FEL computation relevant for this FEL realization are described. In the tandem configuration the terminal is held at a positive potential. The electron beam is accelerated from ground potential to the terminal in one beam tube and then decelerated down the other beam tube. The FEL wiggler and cavity are at the terminal. Due to the beam energy recovery this scheme produces a high power beam at the terminal with a small investment in electrical power.

Performance tests of a superconducting quarter wave resonator

Abstract The performance of a superconducting quarter wave resonator, suitable for heavy-ion linac use, has been tested. The resonator was built using the lead-plated copper scheme. Measured performance established the anticipated advantages of this structure in such an application. The Q value of 2 × 10 8 is independent of the average accelerating gradient up to 3 MV/m where the onset of field emission dominates the losses. Excellent short term frequency stability accounts for the fact that the resonator can be phase-locked with as little as 2 W of tuning power at 2.5 MV/m.

Current leads for HERA

Abstract Three prototypes of current leads (6500 A, 10 × 100 A and 4 × 100 A) for the HERA accelerator were designed, built and tested. The measured total cooling load of these current leads in the refrigerator mode of operation was 3.2, 3.9 and 2.4 W/kA, respectively. Subsequently, these current leads were produced in quantity by a commercial outfit and they are now in the process of assembly within the HERA cryogenic systems in DESY. The essential technical details and experimental results are described.

Superconducting linear accelerator cryostat

Abstract A large vertical cryostat for a superconducting linear accelerator using quarter wave resonators has been developed. The essential technical details, operational experience and performance are described.