R. W. Motley

Excitation of Electrostatic Plasma Oscillations near the Ion Cyclotron Frequency

Oscillations near the ion cyclotron frequency have been excited in thermal cesium and potassium plasmas by drawing current in a filament along the axis of the plasma column. The oscillations appear to be electrostatic waves propagating radially from the filament. The waves are present if the electron drift velocity exceeds about 10 times the ion thermal velocity, in agreement with the prediction of M. N. Rosenbluth. The measured phase velocity is also in agreement with the phase velocity calculated from the fluid equations.

Electrostatic Oscillations near the Ion Cyclotron Frequency

Measurements of ion wave instabilities in plasma in which the electron drift velocity is a few% of the electron thermal speed and ion and electron temperatures are about equal are reported. Experiments were performed in Cs and K plasmas 3 cm in diameter and about 60 cm long produced by surface ionization of atoms on a hot tungsten plate. If sufficient positive voltage is applied to the collector to raise the electron drift velocity to about 10 times the ion thermal velocity, oscillations appear in the collector current. Results agree with theory. (L.N.N.)

Low-Frequency Oscillations in a Potassium Plasma

Low‐frequency oscillations (10–30 kc/sec) have been observed in thermal potassium plasmas, produced by surface ionization of potassium atoms on a hot tungsten plate. The oscillations are associated with the presence of an ion sheath near the tungsten plate and seem to be reasonably well described on the basis of a simple picture of ion waves propagating across the magnetic lines of force, in the presence of a density gradient.

Current‐driven collisional drift instability

An experimental and theoretical study of the current‐driven collisional drift instability is reported. This very strong [Im(ω) ∼ Re(ω)], low‐frequency (ω ≪ Ωi) instability was observed in a current‐carrying collision dominated, cesium plasma. Positive instability identification is based on a detailed comparison of experimental wave parameters with a linearized two‐fluid theory which includes the effects of electron heat flow, electron temperature fluctuations, parallel resistivity, finite ion Larmor radius and transverse ion viscosity. Theory and experiment are in agreement on: (1) the strong destabilizing effect of axial current; (2) the magnitude of the instability frequency and its parametric dependence on magnetic field, azimuthal mode number, and axial current; (3) the qualitative features of the axial structure and their variation with axial current; (4) the magnitude of the critical current for excitation and its variation with magnetic field and density; (5) the relative stability of the different...

An instrument for measuring the momentum flux from atomic and charged particle jets

We have developed an instrument to measure the momentum flux from an intense plasma stream for which the standard techniques used for low‐pressure gases (<10 Torr) at room temperature are unsuitable. With a response time of < 50 ms, this device, a plasma momentum meter, can measure forces of 10−5–103 N onto surfaces of different materials immersed in dense plasmas (n≳1012cm−3). Such forces are transmitted predominantly by ionic and neutral species, with 10’s of eV’s of kinetic energy, are accompanied by high heat fluxes, and are pulsed. The momentum flux onto a biasable target plate is transferred via a suspended quartz tube onto a sensitive force transducer, a capacitance‐type pressure gauge. This protects the transducer from thermal damage, arcing, and sputtering. An absolute force calibration of the PMM to 1% accuracy has been made and is described. A flat carbon target has been used in measurements of the momentum flux of He, Ne, Ar, and Kr plasmas produced in a magnetized linear plasma device.

Energy Analysis of Cesium Ions in a Q Machine

Energy analysis of cesium ions in the collision‐dominated plasma of a double‐ended Q machine is reported. The ion temperature, found to be between 3000 and 6000°K, is always greater than the temperature of the hot ionizing plates. Also, the measured temperature is almost 50% greater than can be accounted for solely by thermal and electrostatic energy associated with the acceleration of ions in the plasma sheath.

Vortex formation during radio frequency heating of plasma

Experiments on a test plasma show that the linear theory of waveguide coupling to slow plasma waves begins to break down if the rf power flux exceeds approximately 30 W/cm2. Probe measurements reveal that within 30 μsec, an undulation appears in the surface plasma near the mouth of the twin waveguide. This surface readjustment is part of a vortex, or off‐center convective cell, driven by asymmetric rf heating of the plasma column.

Perpendicular bremsstrahlung emission of suprathermal electrons

An interpretation of the x‐ray bremsstrahlung emission by suprathermal electrons perpendicular to a magnetic field is given in terms of the parallel and perpendicular temperature of a three‐temperature distribution function. The slope (i.e., the temperature) of the distribution can be determined relatively well. Factor‐of‐two uncertainties remain for the number of electrons.

Measurement of Drift Wave Potential Oscillations in the Presence of Temperature Oscillations

Floating potential oscillations on a Langmuir probe are found to lead density oscillations in drift wave instabilities of a thermally ionized cesium plasma. This contradiction of the isothermal theory is attributed to oscillations of the electron temperature.

ICRF coil for the IDEAL plasma

We describe an ICRH coil to drive the plasma in the proposed IDEAL device, a linear plasma machine designed to study the physics and engineering problems of the ITER divertor. In initial operation, 2 MW of CW power at ∼40 MHz will be applied to a hydrogen plasma via four 0.75‐m long multiple saddle coils that excite ICRF slow waves. The waves propagate to a 30 % magnetic beach, where they undergo cyclotron absorption. At full heating power the power flow out the ends of IDEAL is designed to equal that in the ITER divertor. Coil loading and the radial distribution of the E+ and E− RF fields have been calculated with the ANTENA Code.