Observation of large arrays of plasma filaments in air breakdown by 1.5-MW 110-GHz gyrotron pulses

We report the observation of two-dimensional plasma filamentary arrays with more than 100 elements generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW, 110-GHz gyrotron operating in 3-mu s pulses. Each element is a plasma filament elongated in the electric field direction and regularly spaced about one-quarter wavelength apart in the plane perpendicular to the electric field. The development of the array is explained as a result of diffraction of the beam around the filaments, leading to the sequential generation of high intensity spots, at which new filaments are created, about a quarter wavelength upstream from each existing filament. Electromagnetic wave simulations corroborate this explanation and show very good correlation to the observed pattern of filaments.open424

Plasma structures observed in gas breakdown using a 1.5 MW, 110 GHz pulsed gyrotron

Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments using a pulsed 1.5 MW, 110 GHz gyrotron. The gyrotron Gaussian output beam is focused to an intensity of up to 4 MW/ cm2. The plasma filaments develop in an array with a spacing of about one quarter wavelength, elongated in the electric field direction. The array was imaged using photodiodes, a slow camera, which captures the entire breakdown event, and a fast camera with a 6 ns window. These diagnostics demonstrate the sequential development of the array propagating back toward the source. Gases studied included air, nitrogen, SF6, and helium at various pressures. A discrete plasma array structure is observed at high pressure, while a diffuse plasma is observed at lower pressure. The propagation speed of the ionization front for air and nitrogen at atmospheric pressure for 3 MW/ cm2 was found to be of the order of 10 km/s.open322

Experiments and simulations of a large area ECR source as an electric propulsion neutralizer.

The windowed electron cyclotron resonance (ECR) source, invented by Getty, was modified into a windowless ECR source to investigate the potential use of this device as an electron source for the neutralizer of an ion thruster system. This plasma source utilizes linear arrays of permanent magnets placed at the end of a large S-band microwave horn. These magnets are held inside a grill with alternating rows of open spaces and aluminum cross bars. The metal bars are cross-polarized so that microwave radiation transmits through the grill with low reflection. The peak electron density and electron temperature measured 1 cm from the grill surface were 5 x 1010 cm-3 and 10 eV, respectively, for 200 W input microwave power and 1 mTorr argon gas pressure. At the axial distance of 25 cm from the grill surface, these values were 1 x 1010 cm-3 and 4 eV, respectively. The extracted electron current with pulsed bias increased with collector areas and with input microwave powers, as expected, though the increase was not linear. Unexpectedly, however, the current increased as the distance of the collector from the resonance zone was increased. With microwave circuit optimization, the pulsed electron current achieved 0.77 A with a 30-V bias voltage applied to a graphite collector (7.3 cm x 10.7 cm, located 28.3 cm from the grill surface), 200 W input microwave power, and 1 mTorr argon gas pressure. For DC extraction with the same settings, an electron current of 0.51 A was extracted. The 2D simulations using the MAGIC computer code demonstrated electrons gain energy only near the ECR zone (875 gauss contour). The effectiveness of the steel pole pieces designed for the windowless Getty source was also confirmed. The curvature in either electric or magnetic field profile was verified as a necessary condition for ECR to take place. Electron trajectory plots using the TriComp computer code showed electron trapping by the magnetic mirrors. Both experiments and simulations indicated that electron cross-field diffusion near the grill surface was weak due to the strong magnetic field. However, the experiments suggested that the diffusion far from the grill is significant due to the weak magnetic field there, where more electron current can be extracted. Hence, a large portion of electrons in regions far from the grill might be supplied by electrons streaming along the edge field lines, instead of electrons diffusing across the center field lines.Ph.D.Aerospace engineeringApplied SciencesElectrical engineeringNuclear engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125662/2/3208466.pd

Imaging of atmospheric air breakdown caused by a high-power 110-GHz pulsed Gaussian beam

We present the images of regular filamentary plasma arrays produced upon the breakdown of air at atmospheric pressure at the focal region of a high-power 110-GHz pulsed Gaussian beam. The source of the millimeter wave beam is a gyrotron that can generate up to 1.5-MW output power with 3-mu s pulselength. This unique plasma structure exists only at high pressures. With decreasing pressure, the structure changes into layers of curved plasma sheets and into more familiar diffuse plasma. A main cause of the formation of the regular array structure appears to be the reflection from filaments. The successive generation of conductive filaments modifies the incident field pattern and creates local hot spots upstream of the existing filaments with regular spacing of roughly a quarter wavelength.close81