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Electron extraction from a hollow cathode plasma discharge through a primary anode, biased with a dc accelerating potential, is modulated to produce pulsed or continuous electron beams without the interruption of the plasma cathode discharge. This is achieved with the addition of a secondary anode, within the hollow cathode. When this secondary anode is electrically connected to the primary anode, beam production ceases; when the secondary anode is electrically isolated, beam production resumes. Previous works demonstrated the utility and operation of this device; however, the precise physical principles allowing beam modulation were not well characterized. In this paper, we show that beam modulation can be understood in terms of changes in plasma potential within the hollow cathode that either promote or prevent the formation of an electron sheath at the exit of the hollow cathode, which is mediated by the electrical state of the secondary anode. Maintaining a plasma within the hollow cathode during a state change of the secondary anode depends on the secondary anode meeting the criteria for global nonambipolar flow. These effects are explored with both the Langmuir probe measurements within the hollow cathode during the steady-state operation, and the high-speed current and voltage monitoring during beam pulses.

Impedance probe measurements are made at ionospheric (F-region) plasma conditions ( $n_{e}\approx 10^{4}$ – $10^{6}$ cm $^{-3}$ and $\lambda _{D}\approx 1$ –10 cm) created in the Space Physics Simulation Chamber at the Naval Research Laboratory. Measurements of probe-plasma impedance are used to provide comparative data and identify possible problems, with the goal of developing flight-capable diagnostics for sounding rocket experiments. The ability of the diagnostic technique to infer electron density and plasma potential in an ionospheric plasma is demonstrated with laboratory measurements. In addition, preliminary data from prototype instruments built in our laboratory are presented.

Electron extraction from a hollow cathode plasma discharge through a primary anode, biased with a DC accelerating potential, is controlled to produce either a pulsed or a continuous electron beam without interruption of the discharge. The beam is on while a secondary anode, interior to the hollow cathode, is electrically isolated. While the primary and secondary anodes are electrically connected, the beam is off. Pulses as short as 10 μs have been produced, rise times are

Standing whistler wave patterns have been investigated in the Naval Research Laboratory's Space Physics Simulation Chamber. In the original experimental configuration, partial reflection of the antenna-launched whistler waves from the chamber end boundaries occurs, setting up a combination of standing and traveling waves. By controlling the axial magnetic field strength profile, cyclotron absorption of the whistler waves can be induced before reflection occurs, leaving only the forward propagating waves. By comparing standing-wave amplitudes to that when the wave is prevented from reflecting, cavity Q's in excess of 30 have been observed. Under uniform axial magnetic field conditions, the addition of planar conducting grids across the vacuum chamber cross section at the ends of the plasma column provides improved reflecting surfaces and corresponding increases in the value of Q.

Cold cathode plasma electron beam sources are robust alternatives to hot filaments and field emission sources. In applications requiring pressures above 1 mtorr or reactive gas backgrounds, cold cathode plasma sources can produce high-power electron beams while also exhibiting long lifetimes. The images presented here are produced by electron beams extracted from cold cathode plasmas.