[en] Initial measurements on a millimeter-wave collective free-electron laser (FEL) experiment have demonstrated very high-power superradiant emission in the range 60-100 GHz. This emission corresponds to an instantaneous conversion efficiency of electron beam kinetic energy into millimeter-wave radiation of 2.5%, an order of magnitude improvement over that seen in previous experiments. Computer simulations and experimental measurements have shown the beam quality to be sufficient to sustain a collective free-electron-laser interaction. In addition, the experiment has shown a regular parametric dependence of emission amplitude on guide and pump magnetic fields, both above and below gyroresonance, that had not been previously reported. This behavior agrees well with predictions based on single particle orbits and collective interaction theory. Initial spectral measurements have produced emission spectra that agree well with predictions. Coupling has been observed to the two lowest order modes of the overmoded circular waveguide. Moderate linewidths in the range of 6% to 15% have been measured, demonstrating the broad gain bandwidth of the interaction. Also, the predicted simple broadband tuning of the FEL interaction has been observed for the first time, with tuning demonstrated over a 50% range of frequencies through variation of the axial electron velocity by means of changing the strength of the wiggler field

[en] We report on the most recent results from a series of high power tests being carried out on rf-driven dielectric loaded accelerating (DLA) structures. The purpose of these tests is to determine the viability of the DLA as a traveling-wave accelerator and is a collaborative effort between Argonne National Laboratory (ANL), Naval Research Laboratory (NRL), and Stanford Linear Accelerator Center (SLAC). In this paper, we report on the recent high power tests of a fused quartz-based DLA structure that was carried out at incident powers of up to 12 MW at NRL and 37 MW at SLAC. We also report on test results of a TiN coated quartz structure, that exhibits good multipactor suppression

[en] The millimeter-wave free-electron laser is a relativistic fast-wave device that offers the possibility of reaching extremely high powers (10/sup 9/ W) at high efficiencies. Operating as an amplifier in the collective regime, this device offers the capability of very high gain with a broad instantaneous gain-bandwidth, is broadly tunable, and can have intrinsic efficiencies of order 10%. Previous results have been reported for this device operating in a superradiant configuration, that is as an amplifier of spontaneous emission. In that configuration, it has produced a record power of 75 MW at a frequency of 70 GHz with 6% experimental efficiency in a 15 nsec pulse, and has demonstrated tunability of this emission at high power over the entire range 25-100 GHz. In this mode of operation, the free-electron laser has also demonstrated the unique capability to produce air breakdown in full atmospheric pressure air with intense millimeter-wave emission

[en] Future applications of millimeter-waves may require significantly higher powers (>100 MW) than are available from the long-pulse thermionic gyrotrons that are presently available or under development. Scaling studies suggest that these power levels should be accessible to gyrotrons employing relativistic (0.5-1 MeV), multi-kA electron beams, such as can be generated for short pulse lengths (≤100 nsec) using pulseline accelerators with plasma-induced field-emission cathodes. To explore this potential, the authors have assembled a new gyrotron experiment based on a compact Febetron pulser. Initial experiments using a 350keV, 700A electron beam with a ratio of transverse to longitudinal velocity of ≅0.75 have produced ≅20MW at 35GHz at 8.5% efficiency in a TE/sub 62/ mode, in good agreement with the predictions of theory for the experimental parameters. Substantially higher powers and efficiencies are predicted for a new experimental configuration, which will operate at a higher voltage with improved beam parameters. In this new experiment, a 600 keV, multi-kA electron beam will be produced with low initial transverse energy. Transverse kinetic energy will then be added to the beam either by resonant pumping, via a magnetic wiggler, or by transit through a nonadiabatic magnetic ''bump.'' Finally, the beam will be adiabatically compressed to the desired radius in the gyrotron cavity with a final α≅1. The authors plan to report on the latest results from this new experimental configuration

[en] The magnicon is a scanning-beam microwave amplifier that is under consideration as an alternative to klystrons for powering future high-gradient electron linear accelerators. This paper reports the initial high-power operation of a frequency-doubling magnicon amplifier at 11.120 GHz. The deflection cavities operate at 5.560 GHz. The device is operating in a single-pulse mode at 650 kV and ∼225 A, using a 5.5-mm diameter beam from a plasma cathode, at a magnetic field of 6.7--8.2 kG. In order to overcome a gain saturation problem in the deflection cavities caused by plasma loading, the penultimate deflection cavity is operated very close to self-oscillation. The typical output pulselength is 100 ns full width at half maximum (FWHM), and is limited by RF breakdown of the penultimate cavity. Based on the measured far-field antenna pattern and absolute calibration of all microwave components, the measured output power is 14 MW (±3 dB), corresponding to an efficiency of ∼10%

[en] High power tests are currently being conducted on RF-driven dielectric-loaded accelerating (DLA) structures to determine their viability as traveling-wave accelerators. These tests are a collaborative effort between Argonne National Laboratory (ANL) and the Naval Research Laboratory (NRL). In a previous high power test, single-surface multipactor was reported to be capable of absorbing more than half of the RF power incident on an alumina-based DLA structure. In this paper, we report on the most recent set of high power tests that are attempting to further understand multipactor and eventually suppress it. Several methods were employed to suppress multipactor including: the use of a magnetic field; a TIN surface coating; and a different dielectric material (Magnesium-Calcium-Titanate based). The effectiveness of these three methods are presented and discussed in the paper

[en] A K/sub α/-band gyrotron oscillator powered by a compact pulse-line accelerator has been operated using oscillator cavities with and without axial slots. The use of axial slots has been shown to suppress low starting current ''whispering-gallery'' modes, in particular, modes of the TE/sub m2/ type, allowing stable operation in a linearly polarized TE/sub 13/ mode. A peak power of 35 MW has been observed at 6-percent efficiency

[en] We report on recent progress in a program to develop an RF-driven Dielectric-Loaded Accelerating (DLA) structure, capable of supporting high gradient acceleration. Previous high power tests revealed that the earlier DLA structures suffered from multipactor and arcing at the dielectric joint. A few new DLA structures have been designed to alleviate this limitation including the coaxial coupler based DLA structure and the clamped DLA structure. These structures were recently fabricated and high power tested at the NRL X-band Magnicon facility. Results show the multipactor can be reduced by the TiN coating on the dielectric surface. Gradient of 15 MV/m has also been tested without dielectric breakdown in the test of the clamped DLA structure. Detailed results are reported, and future plans discussed.

[en] A fundamental mode TE^degrees₁₁₁ two-cavity intense-beam gyroklystron amplifier experiment, operating at an accelerating voltage of 1 MV, is reported. The two cavities that were tested are designed to serve as bunching cavities for a high-power output cavity. The two-cavity amplifier has demonstrated a linear grain of 15 dB and an unsaturated output power of ∼40 kW, with the intracavity gain and power ∼4 dB higher. The frequency of the second cavity has been found to track the frequency of the driven cavity over a range of 300 MHz around a center frequency of 35 GHz. Stable amplifier operation was achieved with beam currents as large as 150 A and a velocity pitch ratio (ν_{perpendicular}/ν_parallel) of 0.36. The stable operating range was limited by spurious oscillation in the TE degrees 112 mode. Theoretical calculations indicate that higher gains might be attainable if this mode could be suppressed

[en] This paper presents a progress report on a joint project by the Naval Research Laboratory (NRL) and Argonne National Laboratory (ANL), in collaboration with the Stanford Linear Accelerator Center (SLAC), to develop a dielectric-loaded test accelerator in the magnicon facility at NRL. The accelerator will be powered by an experimental 11.424-GHz magnicon amplifier that presently produces 25 MW of output power in a ∼250-ns pulse at up to 10 Hz. The accelerator will include a 5-MeV electron injector originally developed at the Tsinghua University in Beijing, China, and can incorporate DLA structures up to 0.5 m in length. The DLA structures are being developed by ANL, and shorter test structures fabricated from a variety of dielectric materials have undergone testing at NRL at gradients up to ∼8 MV/m. SLAC has developed components to distribute the power from the two magnicon output arms to the injector and to the DLA accelerating structure with separate control of the power ratio and relative phase. RWBruce Associates, Inc., working with NRL, has investigated means to join short ceramic sections into a continuous accelerator tube by a brazing process using an intense 83-GHz beam. The installation and testing of the first dielectric-loaded test accelerator, including injector, DLA test structure, and spectrometer, should take place within the next year