Xiaoling Zhai

STUDY OF A PLASMA-FILLED X-BAND BACKWARD-WAVE OSCILLATOR

We present experimental studies of a plasma‐filled X‐band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement of a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ≤0.5 to 2 GHz when the BWO was working at the rf power enhancement pressure region. The rf power enhancement appeared over a much wider pressure range in a high beam current case (10–100 mT for 3 kA) as compared to a lower beam case (80–115 mT for 1.6 kA). The plasma‐filled BWO has higher power output compared to the vacuum BWO over a broader region of magnetic guide field strength.

Anomalous decay of Langmuir turbulence

A Stark effect diagnostic yields measurements of the electric field distribution of Langmuir waves, P(E), in beam–plasma turbulence. When the destabilizing beam abruptly cuts off, the form of P(E)∝ exp(−E2) discovered earlier persists, with amplitude decaying exponentially in a microsecond. Strong fields last much longer than other time scales in strong turbulence theory. Exponential decay disagrees with recent power law scalings deduced from cascade theory. A possible explanation envisions Langmuir energy persisting at long wavelengths, slowly coalescing around nucleation density wells left by previous, ‘‘burnt‐out’’ solitons.

Electric field measurement in a plasma-filled X-band backward wave oscillator

Abstract The electric field in a plasma-filled X-band backward wave oscillator (BWO) was measured by the Stark-effect method. Field strengths were as high as 110 kV/cm when the BWO power level was ∼75 MW. Observed electric fields lasted throughout the ∼60 ns high power microwave pulse. After this pulse, fields 30 to 40 kV/cm persisted until beam shutoff, presumably arising from beam-plasma turbulence and low power level electromagnetic radiation.

Statistics of strongly turbulent electric fields

A frequently observed exponential distribution for the probability distribution of strongly turbulent Langmuir fields, P(E,t), may arise from dissipative processes. Experiments show that the distribution is time stationary within the 100 nsec observed resolution, and exhibits exponential time decay after the beam driver ceases. Microwave emission also ceases, compatibly with a beam origin. Return current measures imply that ion waves are much weaker than Langmuir turbulence. Interactions at ion sound wave speeds can mediate the observed μ sec decay, and may also adjust P(E) when the beam driver is on.

Plasma density measurement in a gas-filled X-band backward wave oscillator with a double conversion heterodyne microwave interferometer

Abstract Plasma density was measured with a heterodyne microwave interferometer in both a gas-filled X-band backward wave oscillator (BWO) and in a smooth tube. Plasma is generated by impact ionization of a 650 kV, 2 kA electron beam. For fixed gas pressure we found that the plasma density rise in the operating BWO was much faster than in a smooth tube, indicating that Trivelpiece-Gould modes, or high power microwaves, increase plasma generation. Additional plasma enhanced BWO microwave output power. Measured plasma density at optimum power levels was n cr ≈ 6 × 10 12 cm −3 at onset of emitted microwaves.

Electric microfield distributions in plasma with long-range correlations

Collective and individual particle correlations affect the probability distribution P(E) of the electric microfield in a stationary, turbulent plasma. Extending previous work to include long-range correlations but not dynamics over lengths ⪢λD yields a characteristic distribution P(E) ∝ Ed−1 exp (- E2〈E2〉), with d the dimensionality of the electric field. Here 〈E2〉 is proportional to the square of the correlated particle density. Short-range particle interactions are the usual screening over scales ≲ λD. By ad hoc assumption, long-range correlations arise from strong turbulence which produces a maximally entropic, thermal-like wave distribution of elevated temperature. This leads to 〈E2〉 determined by the amplitude and scale of long-range correlations.

Modeling strong turbulence experiments with long-range correlations

Abstract In strong Langmuir turbulence collective correlations affect the probability distribution P ( E ) of the electric microfield in a stationary, turbulent plasma. Extending previous work to include long-range correlations but not dynamics over lengths ⪢ λ D yields a characteristic distribution P ( E ) ∝ E d −1 exp(− E 2 /〈 E 2 〉), with d the dimensionality of the electric field, and 〈 E 2 〉 proportional to the square of the correlated particle density. This suggests modeling strong turbulence experiments with a maximally entropic, thermal-like wave distribution of elevated temperature, determined by the amplitude and scale of long-range correlations. Implications for recent experiments show agreement between measurements for electric fields, microwave power emitted, and the packing fraction of beam-driven turbulence. Relaxation of the electric fields also yields useful information.

Ion scattering from strong turbulence with long-range correlations☆

Abstract Colliding plasmas can excite strong Langmuir turbulence in which collective correlations affect the probability distribution P ( E ) of the electric microfield. This can be modeled by including long-range correlations but not dynamics over lengths ⪢ λ D . Earlier work yielded a characteristic distribution P ( E ) ∝ E d−1 exp (− E 2 /〈 E 2 〉), with d the dimensionality of the electric field and 〈 E 2 〉 proportional to the square of the correlated particle density. Ions scatter from this localized strong turbulence at rates greatly exceeding classical collisions for solar coronal conditions, while laboratory ion beams have no appreciable scatter beyond the Coulomb rate.

Experimental observations of wall plasma during a 1-microsecond long relativistic electron beam pulse

Experimental observation of wall plasma produced by a relativistic electron beam propagating through a 2.2 cm diameter stainless steel or copper plated stainless steel drift tube has been made at background pressures as low as 3 X 10-6 Torr. An annular electron beam of thickness approximately 1 mm and outer diameter of approximately 1.8 cm was generated using a carbon fiber or graphite cathode and guided down the drift tube by a 15 kG magnetic field. The electron beam energy varied between 500 to 630 kV with total beam currents between 1 to 2 kA and pulse duration of approximately 750 ns. Two viewing ports and lens systems placed approximately 35 cm apart were used to collect light from plasma produced in the drift tube. The light was transmitted to two SPEX 1702/04 monochrometers using quartz optical fibers. Detection was done with photomultiplier tubes sensitive from 300 to 700 nm and lines from hydrogen, oxygen, iron and copper were observed. At the lowest background pressures no light emission was observed until approximately 1.5 microsecond(s) after the beam pulse for the unheated stainless steel drift tube. After baking and pumping the tube for two days light emission was observed approximately 1.8 microsecond(s) after the start of the beam pulse.