A. M. Cook

Pressure dependence of plasma structure in microwave gas breakdown at 110 GHz

Recent studies of 110 GHz microwave discharges in air at atmospheric pressure have demonstrated formation of a large array of quarter-wavelength-spaced plasma filaments. Here we present measurements showing that as pressure is decreased from atmosphere to a few torr, the discharge transitions from a well-defined array to a smeared-out array and finally to a diffuse plasma. Despite the distinct nature of breakdown phenomena at high microwave frequencies, the pressure dependence of the breakdown threshold field is seen to follow a Paschen-type curve. Data for air and argon at 110 GHz are compared with previous low-frequency data.

Observation of plasma array dynamics in 110 GHz millimeter-wave air breakdown

We present dynamical measurements of self-organizing arrays of plasma structures in air induced by a 110 GHz millimeter-wave beam with linear or circular polarization. The formation of the individual plasmas and the growth of the array pattern are studied using a fast-gated (5–10 ns) intensified camera. We measure the time-dependent speed at which the array pattern propagates in discrete steps toward the millimeter-wave source, observing a peak speed greater than 100 km/s. We observe the expansion of an initially spherical plasma into a disk or an elongated filament, depending on the polarization of the incident beam. The results show good agreement with one-dimensional ionization-diffusion theory and two-dimensional simulations.

Measurements of electron avalanche formation time in W-band microwave air breakdown

We present measurements of formation times of electron avalanche ionization discharges induced by a focused 110 GHz millimeter-wave beam in atmospheric air. Discharges take place in a free volume of gas, with no nearby surfaces or objects. When the incident field amplitude is near the breakdown threshold for pulsed conditions, measured formation times are ∼0.1–2 μs over the pressure range 5–700 Torr. Combined with electric field breakdown threshold measurements, the formation time data shows the agreement of 110 GHz air breakdown with the similarity laws of gas discharges.We present measurements of formation times of electron avalanche ionization discharges induced by a focused 110 GHz millimeter-wave beam in atmospheric air. Discharges take place in a free volume of gas, with no nearby surfaces or objects. When the incident field amplitude is near the breakdown threshold for pulsed conditions, measured formation times are ∼0.1–2 μs over the pressure range 5–700 Torr. Combined with electric field breakdown threshold measurements, the formation time data shows the agreement of 110 GHz air breakdown with the similarity laws of gas discharges.

Microscopic fused silica capillary nozzles as supersonic molecular beam sources

Use of fused silica microcapillary tubing for supersonic molecular beam nozzles is reported. Speed ratio measurements were made with nozzles of various diameters and sidewall profiles, demonstrating that these nozzles perform as well as the best conventional nozzles. Commercial microcapillary nozzles therefore offer an effective and inexpensive alternative to conventional metal nozzles.

Millimeter wave scattering and diffraction in 110 GHz air breakdown plasma

We present measurements of the scattering, reflection, absorption, and transmission of a 1.5 MW, 110 GHz quasioptical gyrotron beam by a self-induced air breakdown plasma. The breakdown forms a periodic array of plasma filaments, oriented parallel to the incident electric field polarization that propagates toward the microwave source. For incident intensity of 3 MW/cm2, calorimetric measurements show that as much as 45% of the full beam power is absorbed by the plasma, averaged over the pulse, 1% is reflected backward, and the remainder is transmitted and also scattered into a wide angular spread. We observe that approximately 10 times more power is scattered in the direction perpendicular to the filaments than parallel. The far-field angular distribution of transmitted power exhibits a diffraction pattern that changes throughout the 2-μs life of the plasma.

Experimental results with the SPARC emittance-meter

The SPARC project foresees the realization of a high brightness photo-injector to produce a 150-200 MeV electron beam to drive a SASE-FEL in the visible light. As a first stage of the commissioning a complete characterization of the photoinjector has been accomplished with a detailed study of the emittance compensation process downstream the gun-solenoid system with a novel beam diagnostic device, called emittance meter. In this paper we report the results obtained so far including the first experimental observation of the double emittance minimum effect on which is based the optimised matching with the SPARC linac.

Experimental Testing of Dynamically Optimized Photoelectron Beams

We discuss the design of and initial results from an experiment in space‐charge dominated beam dynamics which explores a new regime of high‐brightness electron beam generation at the SPARC photoinjector. The scheme under study employs the tendency of intense electron beams to rearrange to produce uniform density, giving a nearly ideal beam from the viewpoint of space charge‐induced emittance. The experiments are aimed at testing the marriage of this idea with a related concept, emittance compensation. We show that this new regime of operating photoinjector may be the preferred method of obtaining highest brightness beams with lower energy spread. We discuss the design of the experiment, including developing of a novel time‐dependent, aerogel‐based imaging system. This system has been installed at SPARC, and first evidence for nearly uniformly filled ellipsoidal charge distributions recorded.

Chicane radiation measurements with a compressed electron beam at the BNL ATF

The radiation emitted from a chicane compressor has been studied at the Brookhaven national laboratory (BNL) accelerator test facility (ATF). Coherent edge radiation (CER) is emitted from a compressed electron beam as it traverses sharp edge regions of a magnet. The compression is accompanied by strong self-fields, which are manifested as distortions in the momentum space called beam bifurcation. Recent measurements indicate that the bunch length is approximately 150 fs rms. The emitted THz chicane radiation displays strong signatures of CER. This paper reports on the experimental characterization and subsequent analysis of the chicane radiation measurements at the BNL ATF with a discussion of diagnostics development and implementation. The characterization includes spectral analysis, far-field intensity distribution, and polarization effects. Experimental data is benchmarked to a custom developed start-to-end simulation suite.

MITIGATION OF RF GUN BREAKDOWN BY REMOVAL OF TUNING RODS IN HIGH FIELD REGIONS

The π-mode resonant frequency of the 1.6 cell SLAC/BNL/UCLA style RF photoinjector electron gun is conventionally tuned using cylindrical copper tuning pieces that extend into the full-cell cavity through holes in the side of the gun. This design begins to fail in many versions of this popular gun design at higher voltage levels, when the cavity undergoes electric breakdown in the vicinity of the tuners. In order to remove the tuners from the region of high electric field, mitigating this problem, one must change the full cell geometry significantly. We have investigated a method for accomplishing this, in which we stretch the gun structure to tune the resonant frequency up by over 2 MHz. We constructed a device to perform this stretching and tested the modified photoinjector in an RF test bed. We succeeded in putting approximately 8.4 MW of RF power into the gun, an improvement over the 4 MW routinely achieved with a similar gun using conventional tuning methods installed at the UCLA Neptune laboratory. Recent results in testing this gun with a magnesium cathode insert are reported as well.

Experimental investigation of air breakdown utilizing a 1.5-MW, 110 GHz gyrotron

We present experimental results from air breakdown utilizing a 1.5 MW, 110 GHz 3 µs pulse length gyrotron beam in atmospheric pressure air. The beam is focused to a peak intensity of 5 MW/cm2 and the plasma formed is a two-dimensional array of filaments oriented along the electric field lines with spacing one quarter of the microwave wavelength (∼0.68 mm) that propagate back toward the microwave source.1 The effect of beam polarization on air breakdown structures is examined with a slow and fast gating camera. The periodic filament arrays that are repeatedly observed with the linearly polarized beam disappear when the gyrotron beam is given a circular polarization. This discovery fits with the explanation that array development arises from the result of diffraction of the beam on plasma filaments, and filament formation therefore requires the beam to have linear polarization. A fast gating, high-resolution spectrometer and a broadband spectrometer are used to study breakdown plasma temperature and electron density. Furthermore, diodes are used to measure power reflection and transmission through the plasma. Breakdown field/intensity threshold, power transmission/reflection, and plasma temperature and density measurements are all important in predicting the transmission of high-power millimeter-waves through atmospheric air at various altitudes.