M. Hashmi

Laser‐induced desorption of gas from metallic surfaces

The processes associated with the interaction of focused laser radiation and stainless‐steel targets suspended in a UHV chamber have been investigated. Plasma formation in the vicinity of the target surface was observed with increasing laser energy. Simultaneously, the laser‐induced desorption of gas increases by three orders of magnitude. The experimental results show that most of the gas is not released from the small laser‐illuminated area of the target but from the considerably larger surrounding surfaces of the vaccum chamber by impact of the rapidly expanding plasma cloud.

Study of a laser-produced plasma by Langmuir probes

The structure, the parameters and the expansion of the plasma produced by focusing a 7 J, 20 ns Nd-glass laser on stainless-steel and glass targets suspended in a high-vacuum chamber were investigated by Langmuir probes. It was observed that the probe signals consisted of a photoelectric-emission peak and the main plasma from the target. The flow velocity, density and electron temperature of the plasma were determined. The expansion of the plasma was found to be adiabatic, yielding gamma =5/3. The spatial distribution of the plasma was observed to be strongly anisotropic.

LANGMUIR PROBES COMPARED WITH OTHER DIAGNOSTIC TECHNIQUES IN A Q-DEVICE.

A comparison of density measurements between cylindrical Langmuir probes, resonance fluorescence scattering and microwave methods is performed in a singly ionized barium plasma of a Q-device by varying the following parameters: 1) Density n: 3 × 108 cm−3 ≤ n ≤ 3 × 1011 cm−3; 2) Plate temperature Tp: 2100°K ≤ Tp ≤ 2500°K; 3) Plate material: Re and W; 4) Magnetic field B: 2 kG ≤ B ≤ kG; 5) Mode of operation: Double and single ended operations; 6) Ion temperature Ti down to about room temperature, keeping the electron temperature Te = Tp; 7) Degree of ionization γ: 0.1% ≤ γ ≤ 100%; 8) Probe dimensions.If the density n is evaluated by equating the extrapolated ion saturation current Is at the plasma potential Vs to the random ion current, it is shown that this yields a reliable relative value of the plasma density, which is by a factor of about 2.5 higher than the most probable value.

Basic loss processes in thermally ionized magnetoplasmas: Part I, Theory

A theoretical explanation for the experimentally observed enhanced surface recombination of the ions in a thermally ionized magnetoplasma in a Q-device is proposed. It is based on a collisionless treatment which yields a non-Maxwellian velocity distribution for the plasma particles (section 2). In section 3 the influence of rate collisions on the velocity distribution , particularly in double-ended operation, (D.E., bothendplates hot) is discussed and it is shown that even in the case of a partially thermalized plasma with a thermalization rat e of about 50% the surface recombination causes still a J0 ~ n dependence instead of a J0 ~ n2 dependence as found in the case of a fully thermalized plasma.

Production and Study of an Alkali Plasma with Negative Ions

The method of contact ionization is used to produce a three‐component plasma (Cs+, Cl−, e−) in the Q‐device BARBARA using a beam of Cesium chloride. The end‐plate temperature controls the concentration of Cl−, which can be varied in a wide range. The density of negative ions is obtained from the difference between the positive ion density given by a Langmuir probe and the electron density given by a microwave resonator. A high concentration of negative ions is achieved.

On the oscillator strength of the resonance transitions of Sr

Abstract Density measurements by the resonance flourescence scattering method and other diagnostic techniques in an Sr plasma, produced by contact ionization, yield for the ratio of the oscillator strengths of both resonance transitions of Sr + a value of about unity.