Alan H. Shapiro

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.

Transverse impedance measurements of prototype cavities for a dual-axis radiographic hydrotest (DARHT) facility

The authors describe the results of wire measurements of several prototype DARHT cavities. The measurements reported cover MOD0 with ferrites removed, measurements with drive rods in place with various terminations, and measurements with compensation cans. Measured impedances are compared with AMDS code predictions. The effects of various rod terminations and compensation resistors on cavity impedances are shown.<<ETX>>

Subpicosecond, ultra-bright electron injector

We have designed and are building a subpicosecond electron injector. The injector is based on an 8 MeV photoinjector, used previously at Los Alamos in the APEX experiment. The nominal design includes magnetically compressing a 20 ps long, 3 nC bunch to a FWHM bunch length of 2/3 ps (peak current in excess of 3 kA) using a four dipole chicane buncher. The geometrical averaged transverse normalized transverse emittance after compression is about 15 /spl pi/ mm mrad.

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.

A new temperature and X-ray mapping system for 700-MHz 5-cell superconducting cavities

A new system to map temperature and X-ray radiation around the external surface of 700-MHz 5-cell superconducting cavities has been developed. It consists of an aluminum cylinder that is equipped with six modules of sensors. Eighty-one carbon resistors (temperature sensors) and seventy-one PIN diodes (X-ray sensors) are attached. This cylinder surrounds the 5-cell cavity and rotates about the cavity axis in about 6 minutes. A new feature, compared to the ones developed in the past, is its brush-contact mechanism on the outer surface of the aluminum cylinder, which enables the sensor array to rotate continuously in the same direction during the test. Although the present mechanism allows only one direction of rotation, it does not seem to be difficult to modify for both directions if electrical connections work in this manner. This paper describes the details of the structure and associated mechanisms as well as future schedule and plans of operation.

Neutron spectrometer automation at the Lujan Center

The Lujan Center at the Los Alamos National Laboratorys Neutron Science Center (LANSCE) is a spallation neutron source where research in materials and biological sciences is conducted on time-of-flight neutron scattering spectrometers on eleven beam lines. Execution of an experiment on a neutron spectrometer involves 1) control of the sample environment equipment, 2) measurement of the scattered neutrons, and 3) operation of the beam line itself. This paper describes the automation and coordination of these functions that is essential to effective conduct of experiments.

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.

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...

Test results for a heat-treated 4-cell 805-MHz superconducting cavity

Assessing superconducting technology for potential upgrades to existing proton accelerators as well as applications to future high-current machines necessitates developing expertise in the processing and handling of multicell cavities at useful frequencies. In order to address some of these technological issues, Los Alamos has purchased a 4-cell 805-MHz superconducting cavity from Siemens AG. The individual cavity cells were double-sided titanium heat-treated after equatorial welding, then the irises were welded to complete the cavity assembly. The resulting high RRR (550-730) in the cells enables stable operation at higher cavity field levels than are possible with lower RRR material. Additionally, the high thermal conductivity of the material is conducive to RF and high peak power processing. The cavity was also cleaned at Los Alamos with high-pressure water rinsing. Results from the initial cavity tests, utilizing various processing techniques, are presented.

Superconducting cavity development at Los Alamos National Laboratory

Single-cell, 3 GHz cavities are being tested to refine cavity processing and assembling procedures. Certain of these results indicated a cavity contamination problem, especially after high-field operation. In this instance, the observed behavior of poorly performing cavities indicated that indium contamination may be responsible. The standard chemical polishing treatment was modified to include a 10 minute pretreatment with pure concentrated nitric acid before the standard 2 minute dip in 1, 1, 1. Preliminary results from five cavity tests indicate that this modification may decrease the number of cavity tests that fall in the lower lobe of the performance distribution.<<ETX>>