Greg Severn

Measurements of Ar+ and Xe+ velocities near the sheath boundary of Ar–Xe plasma using two diode lasers

The Bohm sheath criterion in single- and two-ion species plasmas is studied with laser-induced fluorescence (LIF) using two diode lasers in Xe and Ar–Xe plasmas. The plasmas are generated in a low pressure unmagnetized dc hot filament discharge confined by surface multidipole magnetic fields. Two LIF schemes are employed to measure the argon and xenon ion velocity distribution functions near a negatively biased boundary plate. The results show that the argon and xenon ion velocities approach the ion sound speed of the system near the sheath-presheath boundary and satisfy the generalized Bohm criterion.

Laser-induced fluorescence measurements of argon ion velocities near the sheath boundary of an argon–xenon plasma

The Bohm sheath criterion in single- and two-ion species plasma is studied with laser-induced fluorescence using a diode laser. Xenon is added to a low pressure unmagnetized dc hot filament argon discharge confined by surface multidipole magnetic fields. The Ar II transition at 668.614 nm is adopted for optical pumping to detect the fluorescence from the plasma and to measure the argon ion velocity distribution functions with respect to positions relative to a negatively biased boundary plate. The structures of the plasma sheath and presheath are measured by an emissive probe. The ion concentrations of the two-species in the bulk plasma are calculated from ion acoustic wave experiments. Results are compared with previous experiments of Ar–He plasmas in which the argon ions were the heavier ion species. Unlike the previous results, the argon speed is slower than its own Bohm velocity near the sheath–presheath boundary in the Ar–Xe plasma where argon ions are the lighter ion species. We argue that this result is consistent with the behaviour of the helium ion required by the generalized Bohm criterion in the previous experiments with Ar–He plasmas. Further, our results suggest that the measured argon ion speed approaches the ion sound speed of the system.

Experimental test of instability enhanced collisional friction for determining ion loss in two ion species plasmas a)

Recent experiments have shown that ions in weakly collisional plasmas containing two ion species of comparable densities nearly reach a common velocity at the sheath edge. A new theory suggests that collisional friction between the two ion species enhanced by two stream instability reduces the drift velocity of each ion species relative to each other near the sheath edge and finds that the difference in velocities at the sheath edge depends on the relative concentrations of the species. It is small when the concentrations are comparable and is large, with each species reaching its own Bohm velocity, when the relative concentration differences are large. To test these findings, ion drift velocities were measured with laser-induced fluorescence in argon-xenon plasmas. We show that the predictions are in excellent agreement with the first experimental tests of the new model.

Xenon ion laser-induced fluorescence using a visible tunable diode laser near 680 nm.

Laser-induced fluorescence (LIF) measurements have been performed for the first time in a low temperature (Te approximately 0.6 eV) Xe plasma using a tunable diode laser in the visible range of wavelengths. The transition in Xe II involved the (3P1)5d[3]7/2 metastable state and the excitation wavelength was found to be 680.570+/-0.001 nm (air). LIF measurements of I 2 in a room temperature iodine gas cell were used to monitor the wavelength of the laser during the measurements.

Verifying effects of instability enhanced ion–ion Coulomb collisions on ion velocity distribution functions near the sheath edge in low temperature plasmas

Experiments have shown that ion velocity distribution functions (ivdfs) with non-Maxwellian tails created by ion–neutral collisions and ionizations along pre-sheaths in weakly collisional plasmas can be thermalized into Maxwellian distributions near the sheath edge. A recent theory suggests that ion–ion collisions enhanced by ion-acoustic instabilities can rapidly thermalize ions near the sheath edge into Maxwellian distributions and that increasing either electron temperature or neutral pressure of a plasma suppresses the growth of instabilities and eliminates the thermalization process. Measurements of ivdfs by laser induced fluorescence showed qualitative agreement between experimental data and a marginal stability curve inferred from the new theory.

Mackenzie's Demon with instabilities

MacKenzies Maxwell Demon, consisting of positively biased thin wires, heats plasma electrons without significantly affecting the plasma potential. Experiments were performed on the Maxwell Demon in a multi-dipole confined filament discharge. It is shown that given adequate bias, the Demon reduces a bi-Maxwellian electron distribution function to a single Maxwellian electron distribution function. It is shown that a small planar electrode can perform identical heating as the Demon, provided that the electrode has the area of approximately three times the Demons conductive surface area. The instability that limits the Demons operation is investigated. Time-resolved measurements of changes in global electron temperature, plasma density and plasma potential within a cycle of the instability are considered. It is found that the Demons instability is a repeating pulsed anode spot. Density measurements indicate that the frequency of the instability is dependent on plasma production and loss rates. The neutral pressure dependence of the anode spot instability is measured and modeled for the first time.

Radial control of the electrostatic potential in a tandem mirror with quadrupole end cells

It is demonstrated that the radial electrostatic potential in the Phaedrus tandem mirror [Plasma Physics and Controlled Fusion Research, 1984 (International Atomic Energy Agency, Vienna, 1985), Vol. 2, p. 265] can be altered using plasma potential control rings (PPC rings) situated at each end of the device. With the PPC rings grounded, the radial electric field in the central cell was directed outward and peaked near the plasma edge. By externally controlling the potentials of the PPC rings, the peak value of the radial electric field was varied by over a factor of 2, obtaining plasmas with a maximum value of the radial electric field E(r)≤15 V/cm. A simple calculation to model the change in the central cell potential due to a bias voltage applied to the PPC rings is presented.

Experimental studies of the rotational stability of a tandem mirror with quadrupole end cells

It is demonstrated that the radial electric field in the Phaedrus Tandem Mirror [Plasma Physics and Controlled Nuclear Fusion Research 1984 (IAEA, Vienna, 1985), Vol. 2, p. 265] can be altered using plasma potential control rings (PPC rings) situated at each end of the device, and the azimuthal plasma rotational velocity may thus be varied. Low‐frequency (ω≪ωci), low azimuthal mode number (m=−1 and m=−2) instabilities driven by E×B rotation are observed and shown to be in qualitative agreement with the theory of Freidberg and D’Ippolito [Phys. Fluids 26, 2657 (1983)], and Phillips [Phys. Fluids 27, 1783 (1984)] for the case when Phaedrus is operated as a conventional tandem mirror with minimum‐‖B‖ end cells.

Experimental studies of transverse metastable ion velocity distribution functions in the presheath of a weakly collisional argon plasma

Laser-induced fluorescence measurements of the transverse metastable ion velocity distribution function near a negatively biased plate in a low temperature (Te<1eV), low pressure (p0<1mTorr) dc multi-dipole argon discharge plasma have been made with a diode laser. The metastable argon ions in the 3s23p4(P3)3d4F7∕2 state are found to be characterized by a Maxwellian temperature transverse to the direction normal to the plate. For a neutral pressure of 0.3mTorr, the transverse temperature increases along the presheath from 0.026eV in the bulk plasma to 0.058eV at the presheath sheath boundary.

A note on the plasma sheath and the Bohm criterion

The Bohm criterion is an inequality signifying that the ion flow speed at the plasma boundary must be at least as great as the ion sound speed in order for a sheath to form at the boundary. A physical explanation for this phenomenon is given, and the phenomenon is compared with the flow of falling water.