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6217. Construction, operation, and enhancement of JET
Classified
abstracts
6213-6223
40 6213. Surface flashover of solid insulators in atmospheric air and in vacuum
The surface flashover of Teflon, Plexiglas, quartz, Pyrex glass, Macor glass-ceramic, and sapphire solid insulators has been measured in vacuum (5 10-s torr, N 10m6 Pa) and in atmospheric air using dc, ac (60 Hz), and 1.2/50+s lightning impulse voltages. The dependence of the flashover voltage on the following parameters is investigated: (1) spacer material, (2) diameter of the spacer, (3) spacer length, (4) number of spacers stacked in series, (5) air pressure in the range 10-6-105 Pa, (6) electrode material, (7) spark conditioning, and (8) the external resistance in series with the gap. At a fixed insulator length the flashover voltage decreases with increasing spacer diameter. The withstand voltage of spacers stacked in series increases with increasing the number of spacers. The dc flashover voltage of different insulating materials is theoretically calculated in vacuum as a function of the length of the insulator and compared with the experimentally obtained results. Good agreement is obtained. A Sivathanu et al, J appl Phys, 58, 1985, 146-153. 40 6214. Energy and density measurements of sloshing ions in tandem-mirrorexperiment upgrade using solid-state probe techniques The energy and flux of charge-exchange neutrals from the sloshing ions in the endplug of tandem-mirror-experiment upgrade were measured using solid-state probes. An average energy of the sloshing ions of 6 keV was inferred from the depth profile of deuterium implanted in a silicon sample exposed to the charge-exchange neutrals. A bounce-averaged FokkerPlanck code was used to calculate the sloshing-ion energy distribution. The calculated depth profile of deuterium in the silicon sample resulting from this energy distribution is in good agreement with the measured profile. Carbon resistance probes were used to measure the chargeexchange flux from which the central chord sloshing-ion line density was inferred. The line density of particles with energies r keV was deduced to account for 60% of the total plasma line density in the endplug. By folding the sloshing-ion line density into the diamagnetic loop data, it was shown that the l/e radial extent of the sloshing ions remained relatively constant from shot to shot at 14 cm. W L Hsu et al, J appl Phys, 58, 1985, 179-183. 40 6215. Low-pressure flashlamps: influence of the wall ablation phenomena on their output characteristic When operating with linear low-pressure flashlamps (0.1-20 torr) two different linear zones of the E,,,/E,, characteristics are described: high-energy density zone corresponds with a wall-ablation regime having a strong influence from the material extracted from the walls; low-energy density zone corresponds with an isolated normal discharge regime. Spectroscopic measurements also demonstrate this behavior. The input energy density needed for the regime transition is constant when varying tube geometry. Pyrex and quartz tubes with 10 ps pulses; the value of E ,r_i,ion was _ 38 J crnm3 at 5 torr air pressure. J SLnchez Sanz and J M Guerra PCrez, Appl Optics, 24,1985, 194@1946. 40 6216. Current characteristics of a high voltage 60 Hz crossed-field discharge This paper deals with the continuation of research on vacuum pumping with 60 Hz ac voltages and permanent magnets arranged to form a crossed-field configuration. It is well known that dc voltages in a crossedfield discharge and in the presence of a titanium cathode will cause a decrease in residual gas pressure. In our previous research it was demonstrated that this result could also be achieved with 60 Hz ac voltages. Before applying this knowledge to commercial vacuum pumps it is important to gain an understanding of the underlying physical mechanisms. Of particular interest are the initiation and maintenance of the discharge under ac conditions. Results reported in this paper deal with a radial magnetic field in the butt-gap between parallel disk electrodes. In this particular crossed-field arrangement, it was found that the electron space charge is unstable, but ion production is more or less continuous with both dc and ac excitation. The pumping yield per ion was found to be low, but could probably be increased by replacing the glass tube with a metallic vessel so that fewer of the sputtered titanium atoms combine in situ with adsorbed gas molecules on the glass wall. E E Ames et al, J Vat Sci Technol, A3, 1985, 18-21. 40 6217. Construction, operation, and enhancement of JET The paper highlights features of the construction of JET with an emphasis on the vacuum system. It reports on the operation of the vacuum system. The experimental results of JET operation up to August 1984 are 738
summarized. Three areas of enhancement of the device are dealt with--limiters and walls, heating systems, and tritium handling. G Duesing and K J Dietz, J Vat Sci Technol, A3, 1985, 1151-1156. 40 6218. Electrical characterization of rf glow discharges using an operating impedance bridge The impedance and its inverse, admittance, of nitrogen rf glow discharge are measured with an operating impedance bridge in two geometrically similar, radial-flow, parallel-plate plasma reactors. Separation of these electrical parameters into their constituents, resistance and capacitive reactance for impedance and conductance and susceptance for admittance, permits an evaluation of the electrical behavior of these discharges. At power levels between 0.06 and 0.4 W cm-* and electrode spacings between 2.5 and 8.2 cm, the discharge conductance generally increases with rf power and electrode spacing. However, the exact trends depend upon the specific reactor. Based upon these results, an electrical similarity criterion is proposed that may assist transfer of plasma etching and deposition processes from one reactor to another. W E Mlynko and D W Hess, J Vat Sci Technol, A3, 1985, 499-503.
41. OPTICAL
ELECTRONICS
AND LASERS
41 6219. Automated pressure scanning of tunable dye lasers Tunable laser spectroscopy has recently been used in diagnosing the chemistry and physics of plasma, combustion, and chemical vapour deposition reactors. However, the impact of these spectroscopic techniques on real-time process monitoring has been limited to date. One of the reasons for this is the difficulty in tuning the laser frequency to a specific transition and having it remain there for more than a few minutes. I describe a simple method by which these difficulties can be circumvented and frequency tuning can be controlled remotely. A microprocessor is used to regulate the gas flow of a pressure tuned oscillator so that constant pressure (and, indirectly, frequency) is maintained (for excitation of a particular transition) or swept (to record an excitation or absorption spectrum). As a natural extension the pressure can be slaved to a wavemeter so that direct frequency control can be achieved. R A Got&ho, Rev Sci Instrum, 56, 1985, 629-631. 41 6220. Temperature rise induced by a cw laser beam revisited The problem of spatial distribution of the temperature rise due to a cw laser beam focused on the surface of an absorbing material is reexamined. The effect of temperature dependence of the absorption and reflection coefficients is incorporated in the calculations in a self-consistent way. A Green’s function has been developed for the heat diffusion equation in the axially symmetric case and a general steady-state solution is obtained for an arbitrary source function. Compared with previous results, our calculations predict melting at lower laser power densities and changes in the spatial temperature distribution. In the limit of large absorption constants these differences are small. However, they become increasingly significant as the absorption constant is decreased. E Liarokapis and Y S Raptis, J appl Phys, 57, 1985, 5123-5126. 41 6221. Cavity length optimisation in acousto-optically mode-locked argonion lasers The optimisation of the cavity length of acousto-optically mode-locked argon-ion lasers is described. Measurement of the discharge current as the cavity is adjusted indicates that the correct length can be identified. The generation of the second harmonic of the argon-ion laser for the same purpose is also discussed. I S Ruddock and R Illingworth, J Phys Ec Sci Instrum, 18, 1985, 121-123. 41 6222. Optogalvanic fluctuations in a waveguide CO, laser The optogalvanic effect has been investigated in a CO, laser over a wider pressure range than studied previously. The use of square wave rather than sinusoidal intensity modulation led to clear evidence for a temperature relaxation model. The usefulness of the optogalvanic effect for wavelength stabilisation of waveguide lasers at high modulation frequencies is examined. C J Walsh, J Phys D: App[ Phys, 18, 1985, 789%794. 41 6223. Statistics of mode competition in a miniature TEA CO, laser Measurements are presented of the statistics of the energy distribution between the longitudinal modes of a miniature TEA CO, laser. Correlations exist between the fluctuations on different modes, on account
abstracts
6213-6223
40 6213. Surface flashover of solid insulators in atmospheric air and in vacuum
The surface flashover of Teflon, Plexiglas, quartz, Pyrex glass, Macor glass-ceramic, and sapphire solid insulators has been measured in vacuum (5 10-s torr, N 10m6 Pa) and in atmospheric air using dc, ac (60 Hz), and 1.2/50+s lightning impulse voltages. The dependence of the flashover voltage on the following parameters is investigated: (1) spacer material, (2) diameter of the spacer, (3) spacer length, (4) number of spacers stacked in series, (5) air pressure in the range 10-6-105 Pa, (6) electrode material, (7) spark conditioning, and (8) the external resistance in series with the gap. At a fixed insulator length the flashover voltage decreases with increasing spacer diameter. The withstand voltage of spacers stacked in series increases with increasing the number of spacers. The dc flashover voltage of different insulating materials is theoretically calculated in vacuum as a function of the length of the insulator and compared with the experimentally obtained results. Good agreement is obtained. A Sivathanu et al, J appl Phys, 58, 1985, 146-153. 40 6214. Energy and density measurements of sloshing ions in tandem-mirrorexperiment upgrade using solid-state probe techniques The energy and flux of charge-exchange neutrals from the sloshing ions in the endplug of tandem-mirror-experiment upgrade were measured using solid-state probes. An average energy of the sloshing ions of 6 keV was inferred from the depth profile of deuterium implanted in a silicon sample exposed to the charge-exchange neutrals. A bounce-averaged FokkerPlanck code was used to calculate the sloshing-ion energy distribution. The calculated depth profile of deuterium in the silicon sample resulting from this energy distribution is in good agreement with the measured profile. Carbon resistance probes were used to measure the chargeexchange flux from which the central chord sloshing-ion line density was inferred. The line density of particles with energies r keV was deduced to account for 60% of the total plasma line density in the endplug. By folding the sloshing-ion line density into the diamagnetic loop data, it was shown that the l/e radial extent of the sloshing ions remained relatively constant from shot to shot at 14 cm. W L Hsu et al, J appl Phys, 58, 1985, 179-183. 40 6215. Low-pressure flashlamps: influence of the wall ablation phenomena on their output characteristic When operating with linear low-pressure flashlamps (0.1-20 torr) two different linear zones of the E,,,/E,, characteristics are described: high-energy density zone corresponds with a wall-ablation regime having a strong influence from the material extracted from the walls; low-energy density zone corresponds with an isolated normal discharge regime. Spectroscopic measurements also demonstrate this behavior. The input energy density needed for the regime transition is constant when varying tube geometry. Pyrex and quartz tubes with 10 ps pulses; the value of E ,r_i,ion was _ 38 J crnm3 at 5 torr air pressure. J SLnchez Sanz and J M Guerra PCrez, Appl Optics, 24,1985, 194@1946. 40 6216. Current characteristics of a high voltage 60 Hz crossed-field discharge This paper deals with the continuation of research on vacuum pumping with 60 Hz ac voltages and permanent magnets arranged to form a crossed-field configuration. It is well known that dc voltages in a crossedfield discharge and in the presence of a titanium cathode will cause a decrease in residual gas pressure. In our previous research it was demonstrated that this result could also be achieved with 60 Hz ac voltages. Before applying this knowledge to commercial vacuum pumps it is important to gain an understanding of the underlying physical mechanisms. Of particular interest are the initiation and maintenance of the discharge under ac conditions. Results reported in this paper deal with a radial magnetic field in the butt-gap between parallel disk electrodes. In this particular crossed-field arrangement, it was found that the electron space charge is unstable, but ion production is more or less continuous with both dc and ac excitation. The pumping yield per ion was found to be low, but could probably be increased by replacing the glass tube with a metallic vessel so that fewer of the sputtered titanium atoms combine in situ with adsorbed gas molecules on the glass wall. E E Ames et al, J Vat Sci Technol, A3, 1985, 18-21. 40 6217. Construction, operation, and enhancement of JET The paper highlights features of the construction of JET with an emphasis on the vacuum system. It reports on the operation of the vacuum system. The experimental results of JET operation up to August 1984 are 738
summarized. Three areas of enhancement of the device are dealt with--limiters and walls, heating systems, and tritium handling. G Duesing and K J Dietz, J Vat Sci Technol, A3, 1985, 1151-1156. 40 6218. Electrical characterization of rf glow discharges using an operating impedance bridge The impedance and its inverse, admittance, of nitrogen rf glow discharge are measured with an operating impedance bridge in two geometrically similar, radial-flow, parallel-plate plasma reactors. Separation of these electrical parameters into their constituents, resistance and capacitive reactance for impedance and conductance and susceptance for admittance, permits an evaluation of the electrical behavior of these discharges. At power levels between 0.06 and 0.4 W cm-* and electrode spacings between 2.5 and 8.2 cm, the discharge conductance generally increases with rf power and electrode spacing. However, the exact trends depend upon the specific reactor. Based upon these results, an electrical similarity criterion is proposed that may assist transfer of plasma etching and deposition processes from one reactor to another. W E Mlynko and D W Hess, J Vat Sci Technol, A3, 1985, 499-503.
41. OPTICAL
ELECTRONICS
AND LASERS
41 6219. Automated pressure scanning of tunable dye lasers Tunable laser spectroscopy has recently been used in diagnosing the chemistry and physics of plasma, combustion, and chemical vapour deposition reactors. However, the impact of these spectroscopic techniques on real-time process monitoring has been limited to date. One of the reasons for this is the difficulty in tuning the laser frequency to a specific transition and having it remain there for more than a few minutes. I describe a simple method by which these difficulties can be circumvented and frequency tuning can be controlled remotely. A microprocessor is used to regulate the gas flow of a pressure tuned oscillator so that constant pressure (and, indirectly, frequency) is maintained (for excitation of a particular transition) or swept (to record an excitation or absorption spectrum). As a natural extension the pressure can be slaved to a wavemeter so that direct frequency control can be achieved. R A Got&ho, Rev Sci Instrum, 56, 1985, 629-631. 41 6220. Temperature rise induced by a cw laser beam revisited The problem of spatial distribution of the temperature rise due to a cw laser beam focused on the surface of an absorbing material is reexamined. The effect of temperature dependence of the absorption and reflection coefficients is incorporated in the calculations in a self-consistent way. A Green’s function has been developed for the heat diffusion equation in the axially symmetric case and a general steady-state solution is obtained for an arbitrary source function. Compared with previous results, our calculations predict melting at lower laser power densities and changes in the spatial temperature distribution. In the limit of large absorption constants these differences are small. However, they become increasingly significant as the absorption constant is decreased. E Liarokapis and Y S Raptis, J appl Phys, 57, 1985, 5123-5126. 41 6221. Cavity length optimisation in acousto-optically mode-locked argonion lasers The optimisation of the cavity length of acousto-optically mode-locked argon-ion lasers is described. Measurement of the discharge current as the cavity is adjusted indicates that the correct length can be identified. The generation of the second harmonic of the argon-ion laser for the same purpose is also discussed. I S Ruddock and R Illingworth, J Phys Ec Sci Instrum, 18, 1985, 121-123. 41 6222. Optogalvanic fluctuations in a waveguide CO, laser The optogalvanic effect has been investigated in a CO, laser over a wider pressure range than studied previously. The use of square wave rather than sinusoidal intensity modulation led to clear evidence for a temperature relaxation model. The usefulness of the optogalvanic effect for wavelength stabilisation of waveguide lasers at high modulation frequencies is examined. C J Walsh, J Phys D: App[ Phys, 18, 1985, 789%794. 41 6223. Statistics of mode competition in a miniature TEA CO, laser Measurements are presented of the statistics of the energy distribution between the longitudinal modes of a miniature TEA CO, laser. Correlations exist between the fluctuations on different modes, on account