Noah Hershkowitz

Temporal evolution of collisionless sheaths

We have performed experimental measurements to determine the temporal evolution of Langmuir sheaths near an electrode to which a negative step bias is applied in a collisionless argon plasma. The plasma was produced by a hot‐filament discharge in a multidipole device. Plasma potential data were obtained using emissive probes with two different techniques: time resolved sampling and time averaged techniques. The sheath is found to initially form close to the electrode, to extend to a maximum separation, and to contract to a steady‐state value. The time scale required to reach a steady state is close to the time scale of the presheath relaxation. Characteristics of sheaths in rf plasmas are also measured using a parallel‐plate plasma capacitor. It is observed that the plasma potential profile has significant variation with frequency, even for frequencies as low as 1 kHz which are far below the ion plasma frequency (∼1 MHz).

Langmuir probe measurements of electron temperature and density scaling in multidipole radio frequency plasmas

We have constructed a multidipole rf plasma system in a bell jar with a single planar rf powered electrode inserted at one end of a ‘‘magnetic bucket.’’ Capacitive probe measurements revealed that the rf fluctuations are particularly large in these capacitively coupled plasmas. A Langmuir probe has been developed, in which a large shunting capacitor and several self‐resonant inductors are used to allow the probe tip to follow the rf fluctuations. With this new probe, we are able to adequately suppress the effect of these large rf fluctuations and measure time averaged plasma densities and electron temperatures or electron energy distribution functions (EEDFs). We report here a series of experiments in argon with the Langmuir probe fixed at the center of the plasma. The EEDFs were found to be Maxwellian with temperatures from 2 to 5 eV. The value of Te depends primarily on pressure, which was varied from 0.5 to 2.5 mTorr. The plasma density exhibited an approximately linear relationship with AelVrf/Te, whe...

Electrodeless Plasma Cathode for Neutralization of Ion Thrusters

Significant progress has been made in the characterization of the necessary parameters required to create an effective Radio Frequency (RF) plasma based electron source. Such a device has the promise of comparable current densities to and longer operational lifetimes than hollow cathodes. The operational lifetime of hollow cathodes is ultimately limited by cathode deterioration. RF sources provide an alternative approach that does not consume electrode material while providing electrons. Preliminary experiments suggest that an electron current of 3.75 Amps was extracted through an electron sheath near a grounded ring within an a rgon plasma using 340W of RF power. Permanent magnets provided an axial magnet field of 150 Gauss at the antenna. Experimental evidence from the RF plasma cathode suggests that the amount of neutralizing electron current that can be extracted is equal to the random electron flux and is limited by the plasma density, the neutral argon flow rate, and the ion loss area provided in the source.