S. C. Schaub

Electron density and gas density measurements in a millimeter-wave discharge

Electron density and neutral gas density have been measured in a non-equilibrium air breakdown plasma using optical emission spectroscopy and two-dimensional laser interferometry, respectively. A plasma was created with a focused high frequency microwave beam in air. Experiments were run with 110 GHz and 124.5 GHz microwaves at powers up to 1.2 MW. Microwave pulses were 3 μs long at 110 GHz and 2.2 μs long at 124.5 GHz. Electron density was measured over a pressure range of 25 to 700 Torr as the input microwave power was varied. Electron density was found to be close to the critical density, where the collisional plasma frequency is equal to the microwave frequency, over the pressure range studied and to vary weakly with input power. Neutral gas density was measured over a pressure range from 150 to 750 Torr at power levels high above the threshold for initiating breakdown. The two-dimensional structure of the neutral gas density was resolved. Intense, localized heating was found to occur hundreds of nano...

Experimental Results for a Pulsed 110/124.5-GHz Megawatt Gyrotron

We report experimental results on a two frequency gyrotron, operating in the TE22,6 mode at 110 GHz and the TE24,7 mode at 124.5 GHz. The gyrotron uses the same electron gun as a previous single frequency 110-GHz gyrotron, with a new cavity and internal mode converter designed for optimized performance at the two frequencies. For a 98 kV, 42-A electron beam operating in 3-μs pulses, an output power of 1.25 MW was obtained at 110 GHz (30% efficiency) and 1.0 MW at 124.5 GHz (24% efficiency). The highest power obtained was 1.4 MW with a 96 kV, 45-A beam (32% efficiency) at 110 GHz. In both modes, mode competition was minimal around the high-power operating point and operation was extremely stable. The output power and efficiency in the TE24,7 mode were limited by the electron beam quality. At both frequencies, excellent Gaussian beam content was found: 1) 99% for the TE22,6 mode and 2) 97% for the TE24,7 mode. Both output beams had waist radii of 2.65 cm, in very good agreement with theory.

Prototyping high-gradient mm-wave accelerating structures

We present single-cell accelerating structures designed for high-gradient testing at 110 GHz. The purpose of this work is to study the basic physics of ultrahigh vacuum RF breakdown in high-gradient RF accelerators. The accelerating structures are π-mode standing-wave cavities fed with a TM 01 circular waveguide. The structures are fabricated using precision milling out of two metal blocks, and the blocks are joined with diffusion bonding and brazing. The impact of fabrication and joining techniques on the cell geometry and RF performance will be discussed. First prototypes had a measured Q 0 of 2800, approaching the theoretical design value of 3300. The geometry of these accelerating structures are as close as practical to singlecell standing-wave X-band accelerating structures more than 40 of which were tested at SLAC. This wealth of X-band data will serve as a baseline for these 110 GHz tests. Furthermore, the structures will be powered with short pulses from a MW gyrotron oscillator. RF power of 1 MW may allow an accelerating gradient of 400 MeV/m to be reached.

High power test of an internal coupler to corrugated waveguide for high power gyrotrons

High power gyrotrons utilize high order TE cavity modes that must be converted to a Gaussian beam that is matched to a corrugated transmission line that supports the HE11 mode for low-loss, efficient transmission to their loads. In the standard configuration, the corrugated waveguide is external to the gyrotron. We report hot-tests results of a novel design that converts the TE mode to the HE11 mode inside the gyrotron.

Spatially and temporally resolved characterization of an air breakdown plasma using A 110 GHz, 1.4 MW gyrotron

Summary form only given. We present new results on air breakdown using a 1.4 MW, 110 GHz gyrotron operating in 3 microsecond pulses. The linearly polarized beam is focused to a 3.2 mm diameter spot size. The resulting breakdown plasma spontaneously forms a two-dimensional array of filaments or streamers, oriented along electric field lines, that propagate toward the source1. Making use of a fast gating ICCD, we have documented the formation and dynamics of the array with 2 nanosecond time resolution. In addition, a two-wavelength laser interferometer, operating at 532 and 635 nm, has been used to make spatially and temporally resolved electron density measurements of the plasma array, again with nanosecond time resolution. Measurements are made as a function of incident microwave power and ambient air pressure. Microwave power is varied from the breakdown threshold up to 1.4 MW. Ambient air pressure is varied between 25 and 700 Torr.

Experimental characterization of air breakdown plasma utilizing a 1.5MW, 110GHz gyrotron

Summary form only given. We present the latest results of the MIT microwave-frequency air breakdown experiment. The experiment utilizes a 1.5 MW, 110 GHz gyrotron producing 3 μs pulses. The beam is focused to a peak intensity of 5 MW/cm2. At atmospheric pressure, the plasma forms a two-dimensional array of filaments oriented along the electric field lines1, 2. The filaments smoothly transition to a largely structureless plasma at low pressures, e.g. 100 Torr. Recent experimental results include spectroscopic and interferometric measurements of electron number density and plasma temperature. Measurements make use of a high resolution spectrometer and a two-wavelength interferometer operating at 632.8 nm and 532 nm. Electron density and plasma temperature measurements are essential for comparison with numerical simulations of breakdown plasma formation. The status of the experiment will be presented as well as recent experimental results and plans for future work.