N. Rynn

A new plasma source based on contact ionization

A new type of plasma source is presented: A collisionless plasma is formed by producing ions on one end and electrons on the other of a cylindrical vacuum chamber in a solenoidal magnetic field. The ions are produced by contact ionization of potassium on tungsten. The source of electrons is a LaB6 plate. In the usual single‐ended Q machine the elements rhenium, iridium, and platinum are tested as ionizing metals for potassium and barium.

Onset, growth, and saturation of the current‐driven ion cyclotron instability

The temporal evolution of the current‐driven electrostatic ion cyclotron instability was investigated experimentally. The critical destabilizing electron drift velocity for different values of mode phase velocity was measured. The phase velocity was changed by varying the effective plasma column length and hence the parallel wavelength. Ion cyclotron damping was observed to dominate over electron Landau damping at low phase velocities. The temporal growth rate was experimentally determined for several values of electron drift and found to agree very well with linear theory. The observed saturated mode amplitudes compare favorably with the saturation levels predicted by a theory of nonlinear stabilization due to wave induced collisions.

Temporal evolution of ion temperatures in the presence of ion cyclotron instabilities

Spectroscopic techniques to determine the temporal evolution of the ion temperature in a collisionless barium plasma are presented. The theory of temperature relaxation in plasmas with external magnetic field is modified to include wave‐induced collisions. Experiment and theory are compared. The current‐driven electrostatic ion cyclotron instability is shown to be an effective method for wave heating of ions.

Ion‐beam excitation of electrostatic ion‐cyclotron waves

Electrostatic ion‐cyclotron waves are excited by a cesium ion beam injected into a cesium plasma in a Q machine. Strong interactions are observed. Spatial growth rates are measured as a function of beam and plasma parameters.

Negative‐ion plasma sources

Three designs for negative‐ion plasma sources are described. Two sources utilize metal hexafluorides such as SF6 and WF6 to scavenge electrons from electron‐ion plasmas and the third relies upon surface ionization of alkali halide salts on heated alumina and zirconia. SF6 introduced into electron‐ion plasmas yielded negative‐ion plasma densities of 1010 cm−3 with low residual electron densities, (ne/ni∼0.01–0.05). On alumina, plasma densities of 5×109 cm−3 were obtained for CsCl, CsI, and KI and 109 cm−3 for KCl. On zirconia 1010 cm−3 densities were obtained for CsCl. For alkali halide sources, electron densities of ne/ni≲10−4 have been achieved.

Ion beam excitation of ion-cyclotron waves and ion heating in plasmas with drifting electrons

Ion-cyclotron waves are excited by a cesium ion beam in a cesium Q-machine plasma with drifting plasma electrons. These interactions differ significantly from those in the case of drifting ions in that the drifting electrons play an active role in the instability mechanism. The observed mode frequencies are slightly below those of the electron current driven modes. These waves can be convectively or absolutely unstable, depending on the ion beam velocity. For low beam velocities the instabifities are convective in character with large spatial growth rates k i /k r ∼ 0.2. For larger beam velocities the instabilities are absolute in character with temporal growth rates 0.04. The absolute instabilities are similar to two-stream instabilities. Plasma ion heating is observed and is consistent with a model in which mode amplitudes are saturated by diffusion effects.

Turbulent dispersion of ion cyclotron waves: theory and experiment

Particles in or near resonance with a plasma wave undergo perturbations of their equilibrium orbits due to stochastic fields. This contributes added diffusion damping to any wave in the plasma. In a steady‐state experiment this mechanism can cause the saturation of a wave. A theory of the current‐driven ion cyclotron instability modified by turbulent damping based on addition of a damping term to the linear dispersion relation is presented. The turbulent damping is interpreted as an upper bound on the linewidth of the wave. As current varies, the frequency and line width of the mode alter. Once the frequency of the ion cyclotron wave is lowered to nearly the ion cyclotron frequency, mode coupling to an ion‐acoustic wave seems to occur, damping the oscillations. Until this point, detailed predictions of theory agree well with experiment.

Ion fluctuations and velocity distribution in the presence of ion cyclotron waves

Ion density fluctuations and the ion velocity distribution function in the presence of the current‐driven electrostatic ion cyclotron instability are determined using resonance fluorescence of the ions. The optical line intensity modulation shows that the ion density modulation can be as large as 90%. From the Doppler broadening of the lines it is found that the distribution function of ions heated by the unstable ion cyclotron waves is Maxwellian with an uncertainty of 5%.

Electrostatic ion-cyclotron instability driven by a slow electron drift

The electrostatic ion-cyclotron instability can be excited in a single-ended Q-machine by a positively biased collector. According to infinite plasma theory the instability is triggered by the drifting electrons. Recently this explanation has been questioned since experiments by another group have shown that the excitation takes place solely near the collector. The excitation has been ascribed to the collector sheath which has strong axial and radial gradients. The authors try to prove that drifting electrons are able to excite the ion-cyclotron instability. They produce an electron drift without the formation of a localized two-dimensional sheath. To that end the plasma electrons are delivered by a source which is spatially separated from the ion source. If the central section of these electrons is slightly accelerated by a grid they observe the instability.

Barium ion beam probe for magnetic field, diamagnetism, and space potential measurement in plasmas

A diagnostic has been developed to simultaneously determine the magnetic field and space potential in fusion‐like plasmas. Initial tests have demonstrated the ability to measure an absolute change of 17 G in the magnetic field strength; under the same test conditions a 6‐V change in space potential could have been detected. The spatial resolution in these tests is a volume of 0.25 cm3. The magnetic field is determined by a measurement of the Zeeman resonance absorption pattern with a frequency scanning narrow‐band laser of an energetic (<50 keV), low current (∼10 μA) barium ion beam. The laser is injected tangentially to the ion beam to take advantage of the narrow Doppler spread in the direction of the beam. Space potentials are determined by a measurement of the Doppler shift due to a change in beam energy E=E0−eφ. The diagnostic will be used initially on the TRW STM device to measure diamagnetic depression of the ELMO rings in STM.