D. L. Schrage

Results of two LANL /spl beta/ = 0.175, 350-MHz, 2-gap spoke cavities

Two /spl beta/ = 0.175, 350 MHz, 2-gap superconducting (SC) spoke cavities were fabricated in industry under the Advanced Accelerator Applications (AAA) project for the transmutation of nuclear waste. These cavities are promising candidates for the accelerating structures between a RFQ and the elliptical SC cavities for proton and heavy ion linacs. Since their delivery in July 2002, they have been tested in terms of mechanical properties, low-temperature performance, i.e., Q/sub 0/-E/sub acc/ curves at 4 K and 2 K, surface resistance dependence on temperature and for multipacting (MP). The two cavities achieved accelerating fields of 13.5 MV/m and 13.0 MV/m as compared to the required field of 7.5 MV/m with enough margin for the quality factor. These cavities seem to need more time to condition away MP than elliptical cavities, but MP does not occur once the cavity is conditioned and kept at 4 K. The length of the 103 mm-diameter nominal coupler port was found to be too short for the penetrating field.

Linear accelerator for tritium production

For many years now, Los Alamos National Laboratory has been working to develop a conceptual design of a facility for accelerator production of tritium (APT). The APT accelerator will produce high energy protons which will bombard a heavy metal target, resulting in the production of large numbers of spallation neutrons. These neutrons will be captured by a low‐Z target to produce tritium. This paper describes the latest design of a room‐temperature, 1.0 GeV, 100 mA, cw proton accelerator for tritium production. The potential advantages of using superconducting cavities in the high‐energy section of the linac are also discussed and a comparison is made with the baseline room‐temperature accelerator.

ADTF SPOKE CAVITY CRYOMODULE CONCEPT

The Accelerator Driven Test Facility (ADTF) is being developed as a reactor concepts test bed for transmutation of nuclear waste. A 13.3 mA continuous-wave (CW) proton beam will be accelerated to 600 MeV and impinged on a spallation target. The subsequent neutron shower is used to create a nuclear reaction within a subcritical assembly of waste material that reduces the waste half-life from the order of 10 5 years to 10 2 years. Additionally, significant energy is produced that can be used to generate electrical power. The ADTF proton accelerator consists of room-temperature (RT) structures that accelerate the beam to 6.7-MeV and superconducting (SC) elements that boost the beams energy to 600-MeV. Traditional SC elliptical cavities experience structural difficulties at low energies due to their geometry. Therefore, stiff-structured SC spoke cavities have been adopted for the energy range between 6.7 and 109 MeV. Elliptical cavities are used at the higher energies. This paper describes a multi-spoke-cavity cryomodule concept for ADTF.

CW RF Cavity Design for High-Average-Current Photoinjector for High Power FEL

This project is a coordinated effort among NAVSEA, LANL and AES, to develop a key enabling technology for high-power FEL called for by the High Energy Laser Joint Technology Office: a high current RF photoinjector capable of producing continuous average current greater than 100 mA. The specific aim is a π-mode, normal-conducting RF photoinjector, 5 nC of bunch charge, 100 mA of current (at 21.88 MHz bunch repetition rate) and emittance less than 10 mm-mrad. This level of performance will enable robust 100-kW-class FEL operation with electron beam energy <100 MeV, thereby reducing the size and cost of the FEL. This design is scalable to the MW power level by increasing the electron bunch repetition rate from 21.88 MHz (the 32nd sub-harmonic of 700 MHz) to a higher value. The major challenges are emittance control and high heat flux within the CW 700 MHz RF cavities. Preliminary results of RF cavity designs and cooling schemes are presented, including both high-velocity water and liquid-nitrogen cooling options. PACS codes: 41.60. Cr, 29.17+w, 29.27. Bd, 41.75. Fr

High-power electron beam injectors for 100 kW free-electron lasers

A key technology issue on the path to high-power FEL operation is the demonstration of reliable, high-brightness, high-power injector operation. We describe two ongoing programs to produce 100 mA injectors as drivers for 100 kW free-electron lasers. In one approach, in collaboration with the Thomas Jefferson National Accelerator Facility, we are fabricating a 750 MHz superconducting RF cryomodule that will be integrated with a room-temperature DC photocathode gun and tested at the Laboratory. In the other approach, in collaboration with Los Alamos National Laboratory, a high-current 700 MHz, normal-conducting, RF photo-injector is being designed and will undergo thermal management testing at the Laboratory. We describe the design, the projected performance and the status of both injectors.

RF surface resistance of copper-on-beryllium at cryogenic temperatures measured by a 22-GHz demountable cavity

A 22-GHz demountable cavity on the cold head of a compact refrigerator system was used to measure the RF performance of several copper-plated Beryllium samples. The cavity inner surface was treated by chemical polishing and heat treatment, as well as an OFE copper coupon to provide a baseline for comparison. The measured surface resistance was reasonable and repeatable during either cooling or warming. Materials tested included four grades of Beryllium, OFE copper, alumina-dispersion strengthened copper (Glidcop/spl reg/), and Cu-plated versions of all of the above. Two coupons, Cu-plated on Beryllium O-30 and 1-70, offered comparable surface resistance to pure OFE copper or Cu-plated Glidcop. The RF surface resistance of Cu-on-Beryllium samples at cryogenic temperatures is reported together with that of other reference materials.

Photoinjector RF cavity design for high power CW FEL

The project is under way to develop a key enabling technology for high-power CW FEL: an RF photoinjector capable of producing continuous average current greater than 100 mA. The specific aim is a /spl pi/-mode, normal-conducting RP photoinjector, 3 nC of bunch charge, 100 mA of current (at 33.3-MHz bunch repetition rate) and emittance less than 10 mm-mrad. This level of performance will enable robust 100-kW-class FEL operation with electron beam energy <100 MeV, thereby reducing the size and cost of the FEL. This design is scalable to the MW power level by increasing the electron bunch repetition rate to a higher value. The major challenges are emittance control and high heat flux within the CW 700-MHz RF cavities. Results of RF cavity design and cooling schemes are presented, including both high-velocity water and liquid-nitrogen cooling options.

Q disease on 350-MHz spoke cavities

Q disease, i.e., an increase of RF surface resistance due to hydride precipitation, has been investigated with 350-MHz spoke cavities. This phenomenon was studied extensively in the early 1990s with cavities at frequencies >1 GHz. This is possibly due to the fact that the lower-frequency cavities were believed to show insignificant effect. However, early 500-MHz KEK elliptical cavities and JAERI 130-MHz quarter wave resonators have shown significant Q degradation, suggesting that this disease can be a serious problem with lower-frequency cavities as well. Since there were no quantitative data with 350-MHz cavities, we decided to measure our two spoke cavities. Our spoke cavities were made of RRR/spl sim/250 niobium and were chemically polished /spl sim/150 microns. A few series of systematic tests have shown that our spoke cavities do not show any Q/sub 0/ degradation after up to /spl sim/24 hours of holding the cavity at 100 K. However, it starts showing degradation if it is held for a longer time and the additional loss due to the Q disease increases linearly. It was also found that our spoke cavity recovers from Q disease if it is warmed up to 150 K or higher for 12 hours.

Fabrication of the MEBT chopper system for the Spallation Neutron Source

Los Alamos completed design, fabrication, procurement, and initial testing (without beam) of the SNS medium-energy beam-transport (MEBT) chopper, including the meander-line traveling-wave structure and the electrical-pulser system. This report reviews the design parameters and discusses the fabrication process for the chopper structures, including measurements of the impedance and rise time. (The MEBT vacuum system and chopper-target beam stop were developed at and reported by LBNL.) We discuss the specifications for the pulse generator and its fabrication and testing at Directed Energy, Inc. of Ft. Collins, CO. Experimental tests of the chopper system are currently being performed at the SNS site at ORNL and will be reported separately.

Results of Q Disease Tests With 350‐MHz Spoke Cavities

Spoke cavities have been developed at LANL for an accelerator‐driven nuclear waste transmutation system. One of the most important issues for this development is how we can build and operate the accelerator at minimum costs. It would save a significant amount of money if we do not need to heat treat the cavity at high temperatures to avoid Q disease. This motivated us to check to see if Q disease occurs with 350‐MHz spoke cavities. We have tested 3 cavities, ANL, LANL/EZ02 and LANL/EZ01 so far. The ANL cavity was made of RRR∼150 and the LANL cavities were made of RRR∼250 niobium. The ANL cavity was chemically polished 98 microns at LANL with a standard buffered chemical polishing (BCP) solution, i.e., HF:HNO3:H3PO4=1:1:2 by volume, at 14 – 18 °C. We did not see any Q degradation after holding the cavity at 100 – 102 K for 13 hours or at 100 – 142 K for 86 hours. This cavity was unintentionally baked at >200 °C under poor vacuum, which may have caused thicker oxide layer that prevent the Q disease from occ...