The influence of electric nonsymmetry in plasma electron source with hollow cathode as well as gas mixture with various mass numbers of gases on performances of the source was investigated. The operating pressure ranges for various working gases were determined. The intensive low-energy electron beams were obtained in fore-vacuum working pressure range at relatively low extraction potentials

Borisko, V.N.

Petrushenya, A.A.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

INFLUENCE OF GAS MIXTURE WITH VARIOUS MASS NUMBERS OF GASES ON OPERATION PRESSURE RANGE OF PLASMA ELECTRON SOURCE WITH HOLLOW CATHODEV.N. Borisko, А.А. PetrushenyaKharkov National University, Department of Physics and Technology,31 Kurchatov Ave., 61108, Kharkov, UkraineE-mail:Borisko@pht.univer.kharkov.uaThe influence of electric nonsymmetry in plasma electron source with hollow cathode as well as gas mixture with various mass numbers of gases on performances of the source was investigated. The operating pressure ranges for various working gases were determined. The intensive low-energy electron beams were obtained in fore-vacuum working pressure range at relatively low extraction potentials.PACS: 52.50.Dg INTRODUCTIONAt  present  effective  electron  sources  operating  at fore-vacuum pressures [1, 2] are required for a row of technological processes, such as annealing and melting of materials, treatment of surfaces etc [2, 3]. For these purposes it are widely used the plasma electron sources on  basis  of  reflective  discharge  with  hollow cathode. However, such sources effectively operates only using of high extraction potentials owing to existence of the potential barrier for electrons between emitting plasma and extraction electrodes. Moreover, increased working gas pressure in accelerating gap results in reduction of the breakdown strength of accelerating gap [1, 2]  that complicates the application of such sources at fore-va-cuum pressures. Therefore, the perspectives of their use are connected with extension of the operating pressure range and providing of the needed current and power ef-ficiencies at relatively low extraction potentials.In  the  present  paper,  we  suggested  to  use  the mixture with light and heavy gases, such as argon and hydrogen, as well as the electric nonsymmetry between reflective cathodes to extend the operating characteristic of the plasma electron sources on basis of reflective dis-charge with hollow cathode.SOURCE CONSTRUCTIONThe construction of the plasma electron source on basis of penning cell with hollow uncooled cathode is schematically  shown  in  Fig.1.  The  electrode  system consisted of hollow cathode 1, reflective cathode 2, cyl-indrical anode 3, extraction electrode 4 and collector 5. The anode was 18 mm in inner diameters and 18 mm in length. The hollow cathode cavity diameter and length were 4,7 mm and 35 mm, respectively. The hollow cath-ode 1 and the reflective cathode 2 were made of graph-ite, but the anode 3 was stainless steel. The interelec-trode  clearances  between  cathodes  and  anode  were 9 mm each. The electrode system was placed in the uni-form axial magnetic field.The electron beam was extracted from discharge in axial direction from emissive orifice of the cathode 2, which was 3,5 mm in diameter. The extraction voltage Uext =  (0 ÷ 600) V was applied between the cathode 2 and the extraction electrode 4, but the collector potential was about extraction voltage.  To increase the electron extraction efficiency of the source the electric  nonsymmetry was created between cathodes [4]. In run of experiments it were used both the symmetric and nonsymmetric electric circuits connect-ing the cathodes.  In the first  case both cathodes were grounded. In the other case, the cathode 2 was groun-ded, but the potential of the hollow cathode 1 was var-ied in range of  UC1 = (0 ÷ −600) V. The electric  non-symmetry  was  created  by  the  potential  difference between cathodes dU = UC1 – UC2, where UC1 is the po-tential of the cathode 1, but UC2 is the potential of the cathode 2. The anode was always under positive poten-tial.The base pressure of vacuum chamber was about 4⋅10−6 Torr. Fig. 1. Plasma electron source diagram.1 - hollow cathode,  2– reflective  cathode,  3  –  anode,  4 - extraction electrode, 5 – collector, 6, 7, 8, 9 - power  supplies.EXPERIMENTAL METHODS AND OPER-ATING CONDITIONSThe  investigation  were  carried  out  in  a  stationary discharge  burning  regime  at  operating  pressures Р = (0,01 ÷ 4)⋅10−2 Torr,  at  magnetic  induction B = (0,01 ÷ 0,1) T ,  at  anode  voltages  Ua = (0 ÷ 4) kV and anode currents Ia = (0 ÷ 0,9) А. The plasma density no ∼ (0,1 ÷ 2)⋅1011 cm−3 and  the  electron  temperature Problems of Atomic Science and Technology. 2002. № 4. Series: Plasma Physics (7). P. 179-181                  179                                            R                                            Z                                                                                          С                                                                                                                      dU    Uа                        mА                                                                mА                  mА                                             mА +   _1                 3 2           4          5Leak-in + _ + _ B6                        789 Uext0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,60100200300400500600700800argonhydrogen argon - hydrogen mixtureI a, mAUа, kV10-4 2 3 4 5 6 7 8 9 10-3 11 12 13 14 15 16 17 18 10-2050100150200250300350400argonargon - hydrogen mixturehydrogen  2    3    4    5    6    7    8    9   I a, mAР, Torr.Те ∼ (1 ÷ 4) eV in emitting region were determined by probe techniques. Collector 5 measured the total extrac-ted electron current. The retardation curves of extracted electrons were obtained by using a multigrid electrostat-ic analyzer. Argon, clear hydrogen and argon-hydrogen mixture were used as working gases. The working gases, such as argon  and  hydrogen,  flowed into  the  hollow cathode. When the source was operated with the gas mixture, ar-gon and hydrogen flowed into the hollow cathode via gas mixer. The working gas flow rate was varied in rang of vflow = (0,1 ÷ 1,1) Atm⋅cm3⋅c-1. EXPERIMENTAL  RESULTSThe reflective discharge initiated a discharge burn-ing regime with hollow cathode. The high voltage dis-charge burning regime with growing volt-ampere char-acteristic was realized at low discharge currents (Fig.2). The initiation of the hollow cathode burning regime was accompanied by increase of the discharge current and reduce of the anode voltage. These changes of discharge burning regimes are marked in Fig.2 by dotted lines. For hydrogen  the  initiation  currents  Ia  ∼ (250 ÷ 550) mA were  significantly  higher,  but  the  discharge  voltages Ua = (0,3 ÷ 0,4) kV were lower, than their values for ar-gon (Ia  ∼ (50 ÷ 180) мA, Ua = (0,55 ÷ 0,8) kV).  For ar-gon-hydrogen  mixture  the  initiation  currents (Ia ∼ (40 ÷ 200) мA) corresponded to those for argon, as to the discharge voltages (Ua = (0,3 ÷ 0,4) kV) for same mixture they corresponded to those for hydrogen. When hydrogen was used the initiation of the hollow cathode discharge burning regime was observed under a higher operating pressure (Рo > 3⋅10−3 Torr), then for ar-gon  and  argon-hydrogen  mixture  (Рo > 10−4  Torr),  as seen from Fig.3. Fig.2. Volt-ampere characteristics of the plasma elec-tron source. H = 600 Oe, dU = 0 V, Uext ∼ 400 V.Working gas - hydrogen.P = 10−2 Torr, v flow = 1,1 Atm⋅cm3/c.Working gas - argon.P = 10−3 Torr, v flow = 0,5 Atm⋅сm3/c.Working gas – argon - hydrogen mixture.Р = 10−3 Torr,  hydrogen - v flow = (0,7 ÷ 0,8) Atm⋅сm3/c, argon - v flow = (0,3 ÷ 0,4) Atm⋅сm3/c.Such behaviour of a reflective discharge is responsible for peculiarities of existence of high-current discharge burning  regime  at  various  working  pressures,  anode voltages and working gas sorts [3, 5].Upper bound of the operating pressure range was de-termined by appearing the electrical breakdown of the accelerating  gap.  The  electrical  breakdown  was  oc-curred at the operating pressures:  Рmax > 4⋅10−3 Torr for argon, Рmax > 3⋅10−2 Torr for hydrogen and Рmax > 1,5⋅10−2 Torr for argon-hydrogen mixture. The operating pres-sure  ranges  for  various  working  gases  are  shown  in Fig.3 by arrows. Fig.3. Dependencies of initiating current values Ia on a working gas pressure P. H = 600 Oe, dU = 0 V, Uext ∼ 400 V.Working gas - hydrogen.v flow = 1,1 Atm⋅cm3/c.Working gas - argon.v flow = 0,5 Atm⋅сm3/c.Working gas – argon - hydrogen mixture.Hydrogen – v flow = (0,7 ÷ 0,8) Atm⋅сm3/c,argon – v flow = (0,3 ÷ 0,4) Atm⋅сm3/c.The main disadvantages of application of clear gases were the restriction of the operating pressure for argon because of electrical breakdown of accelerating gap and the  impossibility  to  operate  at  low working  pressures and discharge currents for hydrogen. The application of argon-hydrogen mixture allowed to extend the operating characteristics of the source at the expense of reduce of discharge currents and voltages, as well as extension of operating pressure range. The electric nonsymmetry in the discharge resulted in significant increase of the extracted electron currents and the power efficiency of the source for all working gas  sorts.  The  dependencies  of  the  extracted  electron current Ibeam on a potential difference dU between cath-odes is shown in Fig.4. At an operation with hydrogen, the  power  efficiency  of  the  source  and  the  extracted electron  current  reached  of  Н = 0,77 mА/W  and Ibeam = 250 mA, respectively, at  dU = -100 V. At  dU < -100 V it was observed disruption of a hollow cathode regime.      In this case, the anode voltage increased, but the anode current decreased abruptly.180Fig.4. Dependencies of extracted electron currents Ibeam on a potential difference between cathodes dU.Uext ∼ 400 V.Working gas - hydrogen.P = 10−2 Torr, Ia = 800 mA, H = 800 Oe, v flow = 1,1 Atm⋅cm3/c.Working gas - argon.P = 10−3 Torr, Ia = 300 mA, H = 600 Oe, v flow = 0,5 Atm⋅сm3/c.Working gas – argon-hydrogen mixture.Р = 10−3 Torr, Ia = 300 mA, H = 600 Oe,hydrogen – v flow = (0,7 ÷ 0,8) Atm⋅сm3/c,argon – v flow = (0,3 ÷ 0,4) Atm⋅сm3/c.For  argon  the  power  efficiency  and  the  extracted electron current were Н = 0,43 mА/W and I = 145 mА, respectively at dU = -600 V.For  argon-hydrogen  mixture  their  values (Н = 1 mА/W, Ibeam = 460 mA) at  dU = −500 V,  were a higher, than for clear gases. However, further increase of the extracted electron current was impossible without forced cooling of constructive elements of the source.When the  electric  nonsymmetry  was  used,  the amount of change of the extracted electron current cor-responded to change of the hollow cathode current for all  working gas sorts.  Besides,  the extraction of  elec-trons was effective at relatively low extraction poten-tials Uext.  As can been seen from Fig.5 (curve 1), with increasing  Uext,  the  electron  emission  rises  and  at Uext > 100 V the extracted electron current is saturated. In this case,  the beam energy  was about 15 eV in all range of operating pressures (Fig.5, curves 2, 3). CONCLUSIONSAs a result of performed experiments it was shown, that the electric nonsymmetry in plasma source of given type resulted in increase of the electron stream incoming into discharge from hollow cathode cavity.Fig.5. Dependence of extracted electron current on an extraction potential (1) and retardation curves of ex-tracted electrons (2, 3).Working gas – argon-hydrogen mixture.Р = 10−3 Torr, H = 600 Oe,hydrogen – v flow = (0,7 ÷ 0,8) Atm⋅сm3/c,argon – v flow = (0,3 ÷ 0,4) Atm⋅сm3/c.1 – Р = 10−3 Torr, Ia = 300 mA, dU = -400 V, 2 – Р = 10−3 Torr, Ibeam = 100 mA, dU = -440 V, Uext = 300 V.3 – Р = 10−2 Torr, Ibeam = 100 mA, dU = -270 V, Uext = 300 V.This provides the high electron extraction efficiency at relatively low extraction potentials. The application of argon-hydrogen mixture permits to extend the operation pressures in fore-vacuum range at conservation of elec-tron beam performances.The obtained experimental  results  can be used for create  of  new  technological  plasma  electron  sources with high performances.REFERENCES1. Yu.А. Burachevskiy, V.А. Burdovitsin, A.V. Mit-nikov,  Ye.M. Oks.  Journal  of  technical  physics, 2001, Vol.71, No.2, p.48-50.2. V.А. Burdovitsin, M.N. Kuzmechenko, Ye.M. Oks. Journal  of  technical  physics,  2002,  Vol.72,  No.7, p.134-136.3. Yu.Ye. Krendel. Plasma electron sources. M.: Ener-goatomizdat, 1977.4. V.N.  Borisko,  A.A.  Petrushenya.  The  Journal  of Kharkov  National  University,  No.559,  physical series  «Nuclear,  Particles,  Fields»,  Issue  2/18/ 2002, p.67-71.5. E.M.  Reiyhrudel,  G.V.  Smirnitskay,  G.A.  Yegiz-aryan.  Journal  of  technical  physics,  1973, Vol.XLIII, No.1, p.130-135.Problems of Atomic Science and Technology. 2002. № 4. Series: Plasma Physics (7). P. 179-181                  181-600 -500 -400 -300 -200 -100 00100200300400500argonhydrogen argon - hydrogen mixtureI beam , mAdU, V 0 100 200 300 400 500 6000306090120150180210240270300321I (1- mA; 2,3 - mkA)U , V

Influence of gas mixture with various mass numbers of gases on operation pressure range of plasma electron source with hollow cathode

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Influence of gas mixture with various mass numbers of gases on operation pressure range of plasma electron source with hollow cathode

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