H. J. Halama

Search for photon regeneration in a magnetic field

We have searched for the regeneration of photons propagating in a transverse magnetic field. Such an effect would reveal the existence of light scalar or pseudoscalar particles such as the axion that couple to two photons. We obtain for this coupling the limitgaγγ<(1.3×106 GeV)−1, provided the axion massma≲10−3 eV.

Measurement of the magnetic birefringence of neon gas

We constructed a sensitive ellipsometer to study extremely small ellipticities that are acquired by laser light when it traverses a multipass optical cavity in a 9-m-long superconducting magnet. This setup was used to measure the Cotton–Mouton constant (CCM) of neon at 514.5 nm. We find for 760 Torr (1 atm) and 25°C that CCM(Ne) = (5.5 ± 0.3) × 10−20 G−2 cm−1.

Investigation of Superconducting Niobium Cavities at S-Band

This paper describes the investigation of superconducting Nb cavities at S-band frequencies. The TE011 and TM011 cavities were machined by Varian Associates from solid billets of electron-beam-melted niobium and chemically polished to remove all work-damaged material. At low power levels, the TE011 cavity had a measured Qo(4.2°K) ? 324 × 106 and a residual Qres ? 6.97 × 109, whereas the TM011 cavity had a Qo(4.2°K) ? 89 × 106 and a residual Qres ? 425 × 106 . These results correspond to a surface resistance of R(4.2°K)/f2 = 0.29 ??/GHz2. High level measurements were limited at a critical RF field Hacc = 15.5 G in the TM011 mode and Hacc = 290 G in the TE011 mode.

Anomalous rf magnetoresistance in copper at 4 K

We have measured the effect of a magnetic field on the surface resistance of polycrystalline Cu at f=1.2 GHz and at 4.4 K; under these conditions the surface resistance is well into the anomalous skin effect regime but has not reached its limiting value. We find that the transverse and longitudinal magnetoresistance are an order of magnitude smaller than the dc magnetoresistance and depend quadratically on the field. At low fields we observe a decrease in surface resistance with increasing field which can be interpreted as a size effect of the rf surface current, but is also typical of superconductors.

Investigation of a superconducting niobium X‐band deflecting cavity

A 8.665‐GHz deflecting cavity, machined from solid niobium, electron beam welded, chemically polished, anodized and heat treated, yielded a peak magnetic field of 740 G corresponding to a peak electric field of 25 MV/m and a deflecting field of 6.9 MV/m. The maximum Q value was 4.5 × 109. Based on these results, design parameters and suggestions for the fabrication of an X‐band rf beam separator are given.

Superconducting Niobium S-Band Cavities

Recent studies of long-pulse separated beams indicate the need for the development of superconducting rf separators for counter experiments. To achieve this goal we investigated microwave resonators at S-band frequencies manufactured by the three following methods: 1) machining from solid electron-beam-melted niobium, 2) electroforming in molten fused salts by the Linde Company, 3) machining hydroformed cups of commercial grade niobium. Machined parts were electron-beam welded to form microwave cavities. All cavities were heat treated in either the Oak Ridge, Stanford or SLAC high-vacuum, high-temperature furnaces and chemically polished. We investigated and compared both surface resistance and peak rf magnetic field of the above cavities after every step. We measured an improvement factor of ~ 106 at B? = 330 G in a TE011 cavity. The highest rf magnetic field achieved was 464 G in the TM010 mode which corresponds to E of ~17 MV/m. In addition, we were looking for peaks in the microwave absorbtion caused by rf magnetic field induced surface states similar to those reported by various researchers on highly polished crystals in low dc magnetic fields. No such anomalies were found.

Superconducting Niobium Deflectors

The study of S-Band superconducting niobium cavities is being carried out for application to long-pulse rf beam separators in the momentum range of the AGS, whereas, X-Band frequencies are being considered for the NAL accelerator. Results for a 5-cell S-band prototype are: a peak field of 410 G, corresponding to an equivalent deflecting field of over 2 MV/m, and a loss improvement factor of 2 × 105. The fabrication and post-fabrication treatments are described. Perturbation measurements to determine deflector parameters are summarized. A new type of mode stabilizer is suggested. A 7-cell X-Band deflector is under construction.

Design Study of a Superconducting Radiofrequency Beam Separator

A design study of a superconducting RF beam separator suitable for counter beams is presented. A simple two-deflector separation scheme permits one-contaminant rejection in momentum ranges from 8.5 to 19 GeV/c for P and ? beams and from 4.5 to 10 GeV/c for K beams. The operating wavelength ? = 24 cm was obtained by optimizing the transmission of wanted particles. An interdeflector spacing of L = 33 m follows. The geometrical configuration of the lead-coated iris-loaded structure is selected for highest bandwidth which is obtained when the ratio of aperture radius and wavelength is close to 0.2. The maximum deflecting field in the 3 m long resonant ring deflector is limited to about 7 MV/m. The superconducting structure is expected to have a shunt impedance of R ? 1012 ?/m and an unloaded quality factor of Qo ? 1.5 × 109. The ensuing RF power requirements are discussed.

Characteristics of the Isabelle Vacuum System

We discuss the complete vacuum system of ISABELLE, emphasizing those design characteristics dictated by high vacuum, the avoidance of beam current loss, and the reduction of background. The experimental and theoretical justifications for our current choices are presented.

Comparison of Cold and Warm Vacuum Systems for Intersecting Storage Rings

In storage rings employing superconducting magnets, the use of a cold bore as a cryopump appears, at first glance, as simple and economical. Since the selection of a cold or warm vacuum system has far-reaching implications on the basic design, we consider each system in some detail. The theoretical and practical limitations imposed on the maximum beam current by the gas desorption from the chamber walls are discussed. A realistic design of a cold vacuum chamber is developed and then compared with the proposed warm ISABELLE vacuum system. The comparison shows that the warm approach is preferable.