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1340. Orifice probe for plasma diagnostics: III. Experimental considerations
Classified abstracts 1337-1346 rate from the balance, walls, e t c , being ~ 8 , 10 -a torr. liter, sec- ~. Low oxygen pressures were maintained by a Granwlle-Phlllips automatic pressure controller and either a conventional wire-wound resistance furnace or a radiant heating furnace using quartz-iodine lamps was employed to heat the samples. The performance of the balance is illustrated by some results on the oxidation kinetics of iron and mckel at 500 and 70ff'C. The total noise level during an experiment was typically about 3 t~g. (Canada) R J Hussey et al, J Vae Sci Teehnol, 11 (1), 1974, 397-399. 17 1337. Thermionic emission from the Si/Cs/O (100) surface. (USA) Thermionic emission from the Si/Cs/O (100) surface has been investigated. Identical emission characteristics are seen from both n- and p-type bulk samples, implying that the emzssmn arises from the surface region of the crystal. Adsorbed hydrogen accelerates the decay of thermiomc emission. F r o m the temperature dependence of the thermionic current, the temperature-independent portion of the work function ~bo is found to be 1.06 eV R U Martinelli, J ApplPhy.s, 45 (3), 1974, 1183 1190 17 1338. Conversion of Si to epitaxial SiC by reaction with C2H2. (USA) The growth of fl-S1C films on Si by reaction e r a Si single crystal with C2H2 has been studied for the condltions 10-7 f P c z n 2 5 : 10 -4 torr, 800 < T < - I 1 0 0 ° C , m both high- and ultratugh-vacuum chambers. At C2H2 pressures below approximately 10 -5 torr, linear growth kinetics were observed over the temperature range lnvesugated and the reaction probabihty was determined as 0 024).03. In this pressure range growth occurs by the diffusion of $8 through porous defects incorporated in the growing film. We have studied in detail the structure of defected films formed under various growth conditions by scanning electron microscopy, scanning transm~ssmn electron microscopy, and transmission electron microscopy We conclude that the occurrence of defects is intrinsic to the m e c h a m s m of film growth. The predominant defect type consists of a shallow ( ~ 2 0 0 0 A) pit in the Si substrate, over which the growing SiC assumes a porous polycrystalline morphology. The number and areal densities of these defects are proportional to the C2H2 partial pressure and the SIC film thickness, respectwely The defects act as sources of Si for reaction, and film growth occurs via diffusion of Si from the substrate through the porous overgrowth to the epitaxlal SiC/vacuum interface, where reaction occurs. For C2H2 pressures exceeding approximately 10- 5 torr the porous defects are sealed off at an early stage in the growth and further reaction is virtually arrested due to the extremely small bulk and/or grain boundary diffusivity for Sl in S~C over the experimental temperature range. No significant effect on growth rate due to the type of vacuum system used was found. C J Mogab and H J Leamy, J Appl Phys, 45(3), 1974, 1075-1084.
18. G A S E O U S E L E C T R O N I C S 18 1339. Absolute photon-flux measurements in the vacuum ultraviolet. (USA) Absolute photon-flux measurements in the vacuum ultraviolet have been extended to short wavelengths by use of rare-gas ionization chambers The technique involves the measurement of the ion current as a function of the gas pressure in the ion chamber. The true value of the ion current, and hence the absolute photon flux, is obtained by extrapolating the ion current to zero gas pressure Examples are given at 162 and 266 A. The short-wavelength limit is determined only by the sensitivity of the current-measuring apparatus and by our present knowledge of the photoionization processes that occur in the rare gases. A R Samson and G N Haddad, J Opt Soc~_Am,¢64 (I), 1974, 47-54 18 1340. Orifice probe for plasma diagnostics: lII. Experimental considerations. (UK) Experimental methods are presented for the detection and elimination of instrumental effects arising from field penetration and space charge for an orifice probe m low-pressure ( ~2 "" 10 -2 torr) R F discharge plasmas in argon A channel orifice, compared with a sharp-edged orifice, provides number density values which are too low. The creation of a dual-peak ion dlstnbuUon by the insertion of a grid into the plasma is illustrated (Canada) S M L Prokopenko et al, J Phv~" D Appl Phw, 7 (4), 1974, 563-568,
408
18 1341. Cathode spot structure and dynamics in low-current sacuum arcs. (UK) A study has been made of cathode spot structure and dynamics m copper vacuum arcs for currents between 25 and 100 A using a high-magnification optical techniqu ~. The cathode spot consists of one or more electron emission zones or fragments with areas fluctuating between 10 Jo m 2 and 5 -" 10 - s m 2. The spot generally exists in one of two types In the second type the average current density is about 5 , 10 t° A m -2 and in the first type it is about (05-1) l0 ~° A m -2 The cathode spot undergoes continuous rebuddlng in the form of fluctuations in the number and size of the fragments such that transitions in spot type, from type 1 to type 2 and vice versa, occur on average every 100 ~s A possible explanation of the observations m terms of a cathode spot model based on temperature-field electron emission is d~scussed B E Djakov and R Holmes, J Phys D : Appl Phy~, 7 (4), 1974, 569-580 18 1342. Photoelectric yield from a pure mercury pool cathode. (UK) EsUmates have been made of the absolute photoelectric yield from a pure mercury cathode in a low-pressure (0.9 ~ Po = 5.6 torr) Townsend discharge in mercury vapour over the range of 160 - E,/Po - 422 V c m - ~ torr-2. These estimates lie between 2 77 10-3 and 47 l0 -3 electrons/quantum, when non-resonance photons of energy 5 43 eV are incident on a pure mercury pool cathode of workfunctlon 4 52 eV. (Egypt) A A W Garamoon, J Phy~ D: Appl Phys', 7 (4), 1974, 594-596 18 1343. A h~personic interpretation of the development of the spark channel in gases. (UK) A new mechanism for the development of the spark channel m gases Is proposed, m which It is assumed that the shock wave, known Io exist after the channel is complete (Freeman and Craggs 1969), actually develops while the channel is still In formation The paraboloidal shock is thought to derive its power from the transfer of energy only from the electrons pre-heated in front of the developing channel by field concentration to the neutrals and ions behind ~t (detonation wave mechanism) Neural gas in front of the channel is considered to be cold. Analytical expressions for the velocity and posmon of the detonation wave are derived, and experiments by the authors and by prevmus workers tend to confirm the mechanism for certain low-density gases. The "radiation overshoot" characteristic of shock transmission through some gases has been observed, suggesting that a shock indeed exists prior to completion of the spark channel. (Canada) M M Kekez and P Savic, J Phy~ D ApplPhya, 7 (4), 1974, 620-628. 18 1344. Investigation of the ions emitted from a copper-vapour arc in vacuum. (UK) The average velocity and the average degree of ionization of Jon~ emitted from a copper-vapour arc in vacuum have been determined from ion current, energy, and ~mpulse measurements The average ion velocity found was 1 4 ~ 10~ m s -1, and on the average these ions are ionized 1 7 times. Assuming single ionization, about 50°o of the atoms emitted are iomzed. It was found that the angular distributions of the emitted neutrals a n d l o n s were not equal (Norway) W G J Rondeel, J Phys" D Appl Phy,s, 7 (4), 1974, 629-634 18 1345. Prebreakdown current growth in crossed electric and magnetic fields at low gas pressures: III. Temporal growth of current. (U K) An experimental investigation of the temporal growth, In cyhndrJcal geometry, of ionization currents m crossed electric and magnetic fields has been made in molecular hydrogen at gas pressures where the electron mean free path ~s much greater than the dimensions of the discharge chamber An extension of the spatial growth model advanced previously (Blevm and Stock 1973) is proposed which satisfactorily accounts for most of the phenomena observed (Australia) H M P Stock et al, J Phy,s D Appl Phys, 7 (4), 1974, 635-646 18 1346. Dynamic mass-spectrometer system for pulsed gas-discharge studies. (UK) A brae-of-flight mass-spectrometer system hah been dc~cloped to analyse and time-resolve the events m the afterglow of pulsed ga~
18. G A S E O U S E L E C T R O N I C S 18 1339. Absolute photon-flux measurements in the vacuum ultraviolet. (USA) Absolute photon-flux measurements in the vacuum ultraviolet have been extended to short wavelengths by use of rare-gas ionization chambers The technique involves the measurement of the ion current as a function of the gas pressure in the ion chamber. The true value of the ion current, and hence the absolute photon flux, is obtained by extrapolating the ion current to zero gas pressure Examples are given at 162 and 266 A. The short-wavelength limit is determined only by the sensitivity of the current-measuring apparatus and by our present knowledge of the photoionization processes that occur in the rare gases. A R Samson and G N Haddad, J Opt Soc~_Am,¢64 (I), 1974, 47-54 18 1340. Orifice probe for plasma diagnostics: lII. Experimental considerations. (UK) Experimental methods are presented for the detection and elimination of instrumental effects arising from field penetration and space charge for an orifice probe m low-pressure ( ~2 "" 10 -2 torr) R F discharge plasmas in argon A channel orifice, compared with a sharp-edged orifice, provides number density values which are too low. The creation of a dual-peak ion dlstnbuUon by the insertion of a grid into the plasma is illustrated (Canada) S M L Prokopenko et al, J Phv~" D Appl Phw, 7 (4), 1974, 563-568,
408
18 1341. Cathode spot structure and dynamics in low-current sacuum arcs. (UK) A study has been made of cathode spot structure and dynamics m copper vacuum arcs for currents between 25 and 100 A using a high-magnification optical techniqu ~. The cathode spot consists of one or more electron emission zones or fragments with areas fluctuating between 10 Jo m 2 and 5 -" 10 - s m 2. The spot generally exists in one of two types In the second type the average current density is about 5 , 10 t° A m -2 and in the first type it is about (05-1) l0 ~° A m -2 The cathode spot undergoes continuous rebuddlng in the form of fluctuations in the number and size of the fragments such that transitions in spot type, from type 1 to type 2 and vice versa, occur on average every 100 ~s A possible explanation of the observations m terms of a cathode spot model based on temperature-field electron emission is d~scussed B E Djakov and R Holmes, J Phys D : Appl Phy~, 7 (4), 1974, 569-580 18 1342. Photoelectric yield from a pure mercury pool cathode. (UK) EsUmates have been made of the absolute photoelectric yield from a pure mercury cathode in a low-pressure (0.9 ~ Po = 5.6 torr) Townsend discharge in mercury vapour over the range of 160 - E,/Po - 422 V c m - ~ torr-2. These estimates lie between 2 77 10-3 and 47 l0 -3 electrons/quantum, when non-resonance photons of energy 5 43 eV are incident on a pure mercury pool cathode of workfunctlon 4 52 eV. (Egypt) A A W Garamoon, J Phy~ D: Appl Phys', 7 (4), 1974, 594-596 18 1343. A h~personic interpretation of the development of the spark channel in gases. (UK) A new mechanism for the development of the spark channel m gases Is proposed, m which It is assumed that the shock wave, known Io exist after the channel is complete (Freeman and Craggs 1969), actually develops while the channel is still In formation The paraboloidal shock is thought to derive its power from the transfer of energy only from the electrons pre-heated in front of the developing channel by field concentration to the neutrals and ions behind ~t (detonation wave mechanism) Neural gas in front of the channel is considered to be cold. Analytical expressions for the velocity and posmon of the detonation wave are derived, and experiments by the authors and by prevmus workers tend to confirm the mechanism for certain low-density gases. The "radiation overshoot" characteristic of shock transmission through some gases has been observed, suggesting that a shock indeed exists prior to completion of the spark channel. (Canada) M M Kekez and P Savic, J Phy~ D ApplPhya, 7 (4), 1974, 620-628. 18 1344. Investigation of the ions emitted from a copper-vapour arc in vacuum. (UK) The average velocity and the average degree of ionization of Jon~ emitted from a copper-vapour arc in vacuum have been determined from ion current, energy, and ~mpulse measurements The average ion velocity found was 1 4 ~ 10~ m s -1, and on the average these ions are ionized 1 7 times. Assuming single ionization, about 50°o of the atoms emitted are iomzed. It was found that the angular distributions of the emitted neutrals a n d l o n s were not equal (Norway) W G J Rondeel, J Phys" D Appl Phy,s, 7 (4), 1974, 629-634 18 1345. Prebreakdown current growth in crossed electric and magnetic fields at low gas pressures: III. Temporal growth of current. (U K) An experimental investigation of the temporal growth, In cyhndrJcal geometry, of ionization currents m crossed electric and magnetic fields has been made in molecular hydrogen at gas pressures where the electron mean free path ~s much greater than the dimensions of the discharge chamber An extension of the spatial growth model advanced previously (Blevm and Stock 1973) is proposed which satisfactorily accounts for most of the phenomena observed (Australia) H M P Stock et al, J Phy,s D Appl Phys, 7 (4), 1974, 635-646 18 1346. Dynamic mass-spectrometer system for pulsed gas-discharge studies. (UK) A brae-of-flight mass-spectrometer system hah been dc~cloped to analyse and time-resolve the events m the afterglow of pulsed ga~