Volume 91, Issue 8, 15 April 2002
Index of content:
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
Measurement of Ar metastables near a dielectric surface in barrier and plasma display panel discharges91(2002); http://dx.doi.org/10.1063/1.1456961View Description Hide Description
The density of Ar metastable excited atoms in the vicinity of a surface in barrier and display panel discharges was measured by the laser-induced evanescent-mode fluorescence technique. The temporal and spatial distributions of excited atoms were also measured by conventional spontaneous emission and laser absorption methods. From these measurements at various pressures, the behavior of the metastable atoms is clarified and the flux of the metastable atoms on the barrier surface is estimated.
Determination of the ionization and acceleration zones in a stationary plasma thruster by optical spectroscopy study: Experiments and model91(2002); http://dx.doi.org/10.1063/1.1458053View Description Hide Description
A stationary plasma thruster is experimentally studied using different optical spectroscopies of xenon ions. Doppler shift in laser induced fluorescence is used for velocity determination while the ion density is determined by emission spectroscopy. These experiments show unambiguously that the ionization and the acceleration zones are spatially distinct inside the thruster channel. Moreover, it is shown that these results can be easily taken into account with a very simple quasineutral stationary one-dimensional model.
Ion extraction from a laser-ionized plasma produced between parallel plate cathodes and an anode above them91(2002); http://dx.doi.org/10.1063/1.1454191View Description Hide Description
We studied the behavior of a laser-photoionized plasma in atomic-vapor laser isotope separation under an external electric field with a two-dimensional one-fluid model, in which electrons are assumed to be in thermal equilibrium. Sheath-formation and ion-extraction processes are investigated in both a conventional parallel-electrode system and an M-type electrode system consisting of two parallel cathodes and one anode above them. The process of ion extraction in the M-type electrode was made clear and it is shown that ions are collected twice as fast as in the parallel-electrode system.
91(2002); http://dx.doi.org/10.1063/1.1459619View Description Hide Description
Because plasma production at vacuum cathode spots is approximately proportional to the arc current, arc current modulation can be used to generate ion current modulation that can be detected far from the spot using a negatively biased ion collector. The drift time to the ion detector can used to determine kinetic ion energies. A very wide range of cathode materials have been used. It has been found that the kinetic ion energy is higher at the beginning of each discharge and approximately constant after 150 μs. The kinetic energy is correlated with the arc voltage and the cohesive energy of the cathode material. The ion erosion rate is in inverse relation to the cohesive energy, enhancing the effect that the power input per plasma particle correlates with the cohesive energy of the cathode material. The influence of three magnetic field configurations on the kinetic energy has been investigated. Generally, a magnetic field increases the plasma impedance, arc burning voltage, and kinetic ion energy. However, if the plasma is produced in a region of low field strength and streaming into a region of higher field strength, the velocity may decrease due to the magnetic mirroreffect. A magnetic field can increase the plasma temperature but may reduce the density gradients by preventing free expansion into the vacuum. Therefore, depending on the configuration, a magnetic field may increase or decrease the kinetic energy of ions.
91(2002); http://dx.doi.org/10.1063/1.1433175View Description Hide Description
The feasibility of using hot plasmas as strippers for low-energy ion beams has been investigated by measuring charge state distributions of 350 keV/u ions after their passage through a laser-produced plasma target. The plasma target was produced by irradiating a small pellet of lithium hydride with a Nd-glass laser. The profiles of electron densities of the plasma target were estimated from the intensity profiles of an Ar laser refracted by the plasma. The intensities of ions with different charge states were simultaneously measured using a time-resolved magnetic spectrograph. It was found that this plasma can yield higher charge states than conventional gaseous or solid strippers. Results of a numerical analysis are compared with the experimental data to explain the observed stripping capability of the plasma.