Charles E. Crist

Ku‐band radiation produced by a relativistic backward wave oscillator

Reported in this paper are the results of an experiment to produce high‐power Ku‐band (12–18 GHz) microwave radiation from a backward wave oscillator (BWO) driven by a relativistic electron beam. Experimentally measured output power was about 250 MW at 12.5 GHz and 100 MW at 14 GHz. A description of the slow wave structure is given along with theoretical predictions of the vacuum waveguide dispersion relations. The diagnostics to determine the frequency and power of the device are described. Finally, comparisons between the experimentally measured frequency and power, and the analytic and numerical simulations of the BWO are made.

Magnetic Bending of Laser Guided Electron Beams

Experiments have been performed to study magnetic turning of laser-guided electron beams. This technique is of interest for beam transport in circular high-current electron beam accelerators such as Sandias recirculating linac. A 1-MeV, 2-kA, 50-ns, electron beam was turned through a 45° angle with > 90% current transport efficiency. The 45° bend was accomplished by switching the beam between two laser ionized guide channels which intersected in the center of a 680 Gauss turning magnet. The beam radius was observed to increase as a result of turning by the uniform field in agreement with single particle simulations. These simulations predict much smaller emittance growth for optimized sector magnet bending.

Laser Generation and Transport of a Relativistic Electron Beam

A foilless diode usually requires an externally applied magnetic field to control expansion and transport of a relativistic electron beam. A new foilless diode has been developed that does not require an external magnetic field. A low pressure organic gas is introduced into the diode and the transport region. A UV laser beam is injected through the transport region and is terminated at the cathode. The laser photoionizes the low pressure gas forming an ionized channel that captures the electron beam near the cathode. The electron beam is focused and guided by electrostatic attraction to the ionized channel. A 1.5-MeV, 20-kA electron beam has been generated and transported 1 m using this technique. The laser was replaced by an 800-V, 250-ma, low-energy electron beam which was used to guide the relativistic electron beam 4 m through a 90° bend.

Study of the gamma emission from the 31-yr isomer of 178Hf induced by x-ray irradiation

A sample containing 6.3×1014 nuclei of the 16+ isomer of 178Hf having a half-life of 31 yr and an excitation energy of 2.446 MeV was irradiated with x-ray pulses from a device operated at 15 mA to produce bremsstrahlung with an endpoint energy of 90 keV. The gamma spectra of the isomeric target were taken with a Ge detector. The intensity of the 325.5-keV (6+ → 4+) transition in the ground-state band of 178Hf was found to increase by about 2%. Such an enhanced decay of the 178Hf isomer is consistent with an integrated cross section value of 3×10−23 cm2 keV if resonance absorption occurs within energy ranges corresponding to the maxima of the x-ray flux, either near 20 keV or at the energies of the characteristic emission lines of W.

RADLAC II/SMILE performance with a magnetically insulated voltage adder

A 12.5-m-long self-magnetically insulated transmission line (SMILE) that sums the voltages of eight, 2-MV pulse forming lines was installed in the RADLAC-II linear induction accelerator. The magnetic insulation criteria were calculated using parapotential flow theory and found to agree with MAGIC simulations. High-quality annular beams with beta perpendicular to <or=0.1 and a radius r/sub b/<2 cm were measured for currents of 50-100 kA extracted from a magnetic immersed foilless diode. These parameters were achieved with 11-15-MV accelerating voltages and 6-16-kG diode magnetic field. The experimental results exceeded design expectations and are in good agreement with code simulations.<<ETX>>