Frank L. Krawczyk

Beam Line Design, Beam Alignment Procedure, and Initial Results for the

A gain experiment was performed at Los Alamos using a 120-keV 2-A cylindrical electron beam with a ridged waveguide slow-wave structure at 94 GHz, demonstrating 22 dB of amplification through a traveling-wave interaction. The structure was planar with a gap of 0.75 mm and a length of 5 cm. The 2-A electron beam was confined in a 3.2-kG axial magnetic field, with roughly a 0.5-mm diameter. The electron beam was aligned along the magnetic axis of the solenoid by scribing out its cyclotron motion on a novel optical diagnostic using a procedure that depends on varying the solenoidal field strength. The transport through the structure was verified by letting the beam drill holes in a series of thin metallic foils before insertion of the structure

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LANL has developed a new vane loaded waveguide RF structure for a sheet electron beam traveling wave tube (TWT). The goal was to create a new class of wideband RF structures that allow simple mechanical fabrication and have geometry suitable for interaction with sheet electron beams. We have concentrated on structures at 94 GHz. We have achieved 6% bandwidth and believe that 10% is possible. We have performed 3D electromagnetic simulations using the codes Microwave Studio and HFSS, and fabricated several aluminium cold models of RF structures at 10GHz to confirm the design. Agreement between the 10 GHz cold test data and computer simulations was excellent. An RF structure at 94GHz was fabricated using electrical discharge machining (EDM) with a 0.004 inch wire and cold tested.

-Band Gain Experiment at Los Alamos

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.

Wideband RF Structure for Millimeter Wave TWTs

The team responsible for the design of the Accelerator Production of Tritium (APT) superconducting (SC) radio frequency (RF) power coupler has developed two 700-MHz, helium gas-cooled power couplers. One has a fixed inner conductor and the other has an adjustable inner conductor (gamma prototype and alpha prototype). The power couplers will be performance tested in the near future. This paper discusses the mechanical design and fabrication techniques employed in the development of each power coupler. This includes material selection, copper coating, assembly sequences, and metal joining procedures, as well as the engineering analyses performed to determine the dynamic response of the inner conductors due to environmental excitations. A bellows is used in both prototype inner conductors in the area near the ceramic RF window, to compensate for thermal expansion and mechanical tolerance build-up. In addition, a bellows is used near the tip of the inner conductor of the alpha prototype for tuning the power coupler after it is installed on the accelerator. Extensive analytical work has been performed to determine the static loads transmitted by the bellows due to thermally induced expansion on the inner conductor and on the RF window. This paper also discusses this analysis, as well as the mechanical analysis performed to determine the final geometric shape of the bellows. Finally, a discussion of the electromagnetic analysis used to optimize the performance of the power couplers is included.

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

In this paper, we present the electromagnetic and structural design of a low-/spl beta/ superconducting spoke resonator for a beam-test in the Low Energy Demonstration Accelerator (LEDA). This test is part of the Advanced Accelerator Applications (AAA) project. Recently, the use of superconducting resonators for energies greater than 6.7 MeV has been approved. The beam test will use the lowest-/spl beta/ resonator from this accelerator design. The choices of the cavity dimensions are driven by its use immediately downstream of the LEDA Radio-Frequency Quadrupole (RFQ). The frequency is 350 MHz. The length corresponds to a geometric /spl beta/ (/spl beta//sub g/) of 0.175. Our design approach has been to carry out an integrated RF and mechanical design from the start. The final cavity is well understood in terms of RF and mechanical properties. The RF properties, like Q, R/Q, peak surface fields and acceleration efficiency are very reasonable for such a low-/spl beta/ structure. The design also includes power coupler, vacuum and pick-up ports and their influences. The mechanical design added tuning sensitivities, tuning forces, stiffening schemes and the understanding of stresses under various load conditions.

Development of the SCRF power coupler for the APT accelerator

Summary form only given. A sheet-beam traveling-wave amplifier has been proposed as a high-power generator for RF from 95 to 300 GHz, using a microfabricated RF slow-wave structure. The planar geometry of microfabrication technologies matches well with the nearly planar geometry of a sheet beam, and the greater allowable beam current leads to high-peak power (up to 500 kW at 95 GHz), high-average power (up to 5 kW), and wide bandwidths (up to 10%). Simulations have indicated gains in excess of 1 dB/mm, with extraction efficiencies greater than 20%.

Design of a low-/spl beta/, 2-gap spoke resonator for the AAA project

The Los Alamos Accelerator Code Group (LAACG) is a national resource for members of the accelerator community who use and/or develop software for the design and analysis of particle accelerators, beam transport systems, light sources, storage rings, and components of these systems. Below we describe the LAACGs activities in high performance computing, maintenance and enhancement of POISSON/SUPERFISH and related codes and the dissemination of information on the INTERNET.

MM‐Wave Source Development at Los Alamos

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.

The Los Alamos Accelerator Code Group

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.

ADTF SPOKE CAVITY CRYOMODULE CONCEPT

Funnels are a key component of accelerator structures proposed for transmutation technologies. In addition to conventional accelerator elements, specialized rf‐cavities are needed for these structures. Simulations were done to obtain their electromagnetic field distribution and to minimize the rf‐induced heat loads. Using these results a structural and thermal analysis of these cavities was performed to insure their reliability at high average power and to determine their cooling requirements. For one cavity the thermal expansion data in return was used to estimate the thermal detuning.