Ion emission from plasma-beam discharge

Classified abstracts 31-38 18 31. Photoemission and band structure. ( G e r m a n y ) T h e i m p o r t a n t aspects o f the use o f p h o t o e m i s s i o n for the determination o f the electromc structure o f s o h d s are discussed. T h e present state o f the art o f quantitative m e t h o d s for the analysis o f t h e optical data a n d p h o t o e m , s s i o n ~s also reported. T h e energy dtstrtbutton o f photoelectrons for direct a n d md,rect electron trans,tions tn t h e b a n d m o d e l is cons,dered. It is s h o w n that the shift o f m a m m u m in energy d , s t n b u t t o n of photoelectrons for opttcal direct transitions takes place m o r e slowly t h a n w o u l d c o r r e s p o n d to t h e increase o f p h o t o n energy, ff the p h o t o n energy is increased. T h e relaUonsh,p between density o f states a n d energy d t s t n b u t t o n o f photoelectrons for mdirect transition is presented. T h e role o f melastic s c a t t e r m g in the d e t e r m i n a t i o n of the opt,cal trans,tlon p r o b a b d i t y f r o m the m e a s u r e d energy d i s t n b u t t o n ts described. It is s h o w n that m the analysis o f the energy d,stnbut~on o n e h a s to take into c o n s t d e r a t m n that m the case o f inelastic scattering a stronger falsificatmn o f the energy distribution occurs with growing p h o t o n energy. T h e influence o f surface contact field, adsorbed layers a n d surface b a n d bending o n the density state determination is d~scussed. Q u a n t u m y,eld, optical conductwity a n d m u l t i p h o t o n p h o t o e m i s s i o n are also constdered. P Gorlich and K Sumi, Phys Status Sohdt (a), 2 (3), July 1970, 4 2 7 4 4 0 . 18 32. Schlieren method and its application for plasma diagnostics. (Czechoslovakm) T h e Schlieren m e t h o d h a s f o u n d a w,de applicatton m the study o f s t r e a m i n g a n d other p r o b l e m s related to mvestigatton o f the change m the density o f a reed,urn, a n d recently tt h a s been applied to stud,es of plasma. By th,s m e t h o d ,t is poss,ble to find the f o r m o f the density change o f a neutral a t o m or molecule, ,on a n d electron concentrat,ons. It is s h o w n that the Schlieren m e t h o d allows the measurem e n t o f the c o n c e n t r a t i o n c h a n g e o f 7 ~ 10 t~ c m -3 of electrons a n d 6 -: 10 ~ c m -3 of neutral particles in a Schlieren region o f 2.5 cm. T h e use of a laser as a light source can b r m g a b o u t stgnificant improvem e n t in sensitivity. It ts expected that the Schheren m e t h o d wdl find wider a p p h c a t l o n m part,cle density d e t e r m m a t m n m v a c u u m techtuque. V Hermoch, Czech J Phys, B20 (8), 1970, 939-949. 18 33. Ion emission from plasma-beam discharge. (USSR) P l a s m a - b e a m dtscharge allows p l a s m a to be obtained with full gas ionization. T h e application o f p l a s m a - b e a m discharge as a n ton source a n d ,on emitter ts proposed. T h e h i g h t e m p e r a t u r e o f electrons m this discharge m a k e it possible to increase current density in m n b e a m s due to ion space charge c o m p e n s a t , o n by fast electrons from the discharge T h e results o f experimental mvestigat,on of the ,on em,sslon o f h y d r o g e n along magnetic field are gwen. T h e current a n d energy o f emitted ,ons as a function o f gas pressure m the discharge c h a m b e r a n d the current o f mjected electrons are m e a s u r e d . At the reJect,on o f an electron current o f 0.6 A w,th an energy o f 1 keV, the ~on current of 3 A at the h y d r o g e n pressure o f 10 -3 torr ,s reached. T h e average energy o f emitted ions ts 50-70 per cent o f injected electron energy with a relatwe spread o f 20-50 per cent in energy. R A Demirkhanov et al, Zh tekh Fiz, 40 (7), July 1970, 1351-1354 (m

Russtan). 18 34. The energy distribution of photoelectrons from the K2CsSb photo cathode. ( G e r m a n y ) T h e p o t a s s m m - a n t i m o n y - c a e s m m p h o t o c a t h o d e is notable for its very low dark current o f the order o f 10 ~7 A / c m 2 at r o o m temperature. Photoelectron energy distribut,ons have been determined u s m g a retarding potential m e t h o d in a glass ultrah,gh v a c u u m system with spher,cal p h o t o t u b e s Between 2.1 a n d 3 1 eV of incident p h o t o n energy the d~stnbution consists o f a single peak, t h e energy o f which increases m o n o t o m c a l l y with increasing p h o t o n energy. F o r incident p h o t o n energies less t h a n a b o u t 2 5 eV, p h o t o e m t s s l o n is mainly d u e to excitat,on f r o m impurity levels. F o r p h o t o n energies above 4.0 eV, the dlstr,butions show fast-electron structure which m a y be due to excitation f r o m m a x i m a m the valence b a n d density o f states at depths o f 2.3 a n d 2.8 eV below v a c u u m a n d the conservation of crystal m o m e n t u m m a y n o t be a n i m p o r t a n t rule m these transtt~ons T h e fast-electron structure m a y be correlated with structure a p p e a r m g at 1 9 a n d 2.4 eV in the optical a b s o r p t i o n of the p h o t o c a t h o d e . A slow peak at an electron energy o f 0.5 eV ascribed to pair production, first a p p e a r s for an incident p h o t o n energy o f 4.3 eV a n d this peak be-

c o m e s m c r e a s m g l y p r o m i n e n t as the p h o t o n energy is increased. (England) R Nathan and C H B Mee, Phys Status Sohdi (a), 2 (1), May 1970, 67-72. 18 35. The mechanism of emission from the silver-oxygen-caesium photoeathode. ( G e r m a n y ) T h e s d v e r - o x y g e n - c a e s m m p h o t o c a t h o d e h a s been of cons,derable , m p o r t a n c e because of tts response in the near infra-red region o f spectrum. T h e c a t h o d e ,s prepared by oxtdlztng a silver substrate m a glow discharge a n d t h e n allowing it to react wtth caesium at a n approprmte temperature. Photoelectron energy distribution measurem e n t s were m a d e by a retarding potential m e t h o d in a glass ultrahigh v a c u u m a p p a r a t u s . F o r tnc,dent p h o t o n energy below 2 1 eV, the d t s t n b u t t o n s consist o f a single peak, a n d as the p h o t o n energy ,s increased, structure due to fast a n d slow electrons m a y be resolved. T h e fast-electron structure is tdentified as being due to em,ssion f r o m a m a x i m u m m the density o f states for sdver o f a b o u t 0.3 eV below the Fermi level. A slow-electron peak appears to be due to pair production in c a e s m m m o n o x i d e , the threshold occurring at a p h o t o n energy o f a b o u t 3.4 eV. T h e dtstributions confirm the S o m m e r Borzlak m o d e l o f the cathode, in which sdver parttcles are s u p p o s e d to be dispersed m a s e m , c o n d u c t i n g caesium m o n o x i d e matrix. F o r p h o t o n energy up to a b o u t 4 eV, em,ssion is due to exc,tation of electrons f r o m the silver parttcles a n d the tunnelling o f these electrons into the c o n d u c t t o n b a n d o f the caesium m o n o x i d e . F o r p h o t o n energ,es greater t h a n a b o u t 4 eV, e m , s s , o n f r o m the valence b a n d of caesium m o n o x i d e m a y become t m p o r t a n t (England) K S Neil and C H B Mee, Phys Status Solidi(a), 2 (1), May 1970, 4 3 53. 18 36. Electron gun with pulsed cathode heating. (USSR) T h e r m , o n l c em,sslon properties o f t u n g s t e n c a t h o d e w,th pulsed heating are mvestlgated. T u n g s t e n h a s a h,gh m e i u n g point, htgh mechanical stablhty tinder ion b o m b a r d m e n t m high electric fields, a n d a low v a p o u r pressure. In order to obtain high pulsed em,ssion currents without excessively high h e a t , n g p~wer a n d short hfe due to copious evaporatton, pulsed heating ts used. A three-electrode system was used as a n electron gun. T h e c a t h o d e t u n g s t e n filament has a diameter o f 0.5 m m a n d a working length of 1.5 cm. T h e c a t h o d e is heated to a t e m p e r a t u r e o f 2000 to 2500°C by dc current. F u r c a t h o d e heating to high t e m p e r a t u r e o f a b o u t 3000°C, a delay h n e p r o d u c i n g a square pulse with a n a m p l i t u d e of 300 V a n d duratton o f 1 msec is employed. T h e c a t h o d e t e m p e r a t u r e ,s m e a s u r e d w,th the a,d o f a photoelectron mult,pher. T h e thermal t,me c o n s t a n t o f the c a t h o d e after pulse cessat,on was 400 msec. In a n experimental dewce with a res,dual gas pressure o f 2 × 10 -6 torr a b e a m current of 25 A was reached at a pulse d u r a t t o n o f 150/~sec a n d a n accelerating voltage of 20 kV. T h e current denstty o n the c a t h o d e surface is 40 A/bin "°. T h e useful hfe of the c a t h o d e is 500 h r at a pulse repet,tlon rate of 0.1 pulse/see. V A Bashko et al, Zh tekh Fiz, 40 (6), June 1970, 1297-1299 (in

Russian). 18 37. The mechanism of exoelectron emission after excitation with electrons. ( G e r m a n y ) U s i n g a retarding potenttal m e t h o d , it ,s s h o w n that the energy o f exoem,tted electrons after excitatton with electrons can achieve values up to 80 eV. A new model o f exoelectron emlsston f r o m insulating materials ~s proposed. J Drenekhan et al, Phys Status Sohdt (a), 2 (1), May 1970, K51-K54. 18 38. On parameter of energy spectrum of exoelectrons from metallic surfaces. ( G e r m a n y ) A n exoelectron spectrometer is described using a mult~channel arnplltude analyzer. T h e m e a s u r e m e n t of exoelectron energy ,s performed in v a c u u m at a residual gas pressure o f 2 "~ 10 5 torr by m e a n s of scanning w,th retarding potential. T h e energy s p e c t r u m p a r a m e t e r s o f exoelectrons have been obtained from the AI, TI a n d W surfaces after polishing. T h e presence of a certain a m o u n t o f electrons whose energy exceeds the photoelectron energy is revealed. All results obtained prove the fact that exoemission m a y not be explained merely by a d e f o r m a t m n reduced photoeffect. ( U S S R ) V S Kortov et al, Phys Status Sohdz (a), 2 (1), May 1970, 55-59 (m

German.) 63