The current paper presents the results of experimental researches to determine the mode features of plasma-beam discharge (PBD) generation by an electron beam injected by a low-vacuum gasdischarge electron gun (LGEG) with the cold cathode and hollow anode on the basis of the high-voltage glow discharge and in the range of helium pressure of P ? 10 ÷ 130 Pa. The PBD boundaries and their dependences on parameters of an electron beam are found. The influence of PBD on parameters of low-vacuum gas-discharge electron gun is revealed. It causes an avalanche increase of electron beam current and burning of plasma-beam discharge in the whole space of the vacuum chamber volume and generation of electromagnetic radiation is revealed. Achieved results will be used for implementation of various vacuum technologies in the medium of reaction gas and generated electromagnetic radiation

Gasik , Michail I.

Maslenikov , Dmitriy V.

Ovcharuk , Anatoliy N.

Tutyk , Valery A.

Digital Library of the Czech Technical University in Prague

Acta Polytechnica 53(2):249–253, 2013 © Czech Technical University in Prague, 2013available online at http://ctn.cvut.cz/ap/IGNITION FEATURES OF PLASMA-BEAM DISCHARGEIN GAS-DISCHARGE ELECTRON GUN OPERATIONValery A. Tutyka,∗, Anatoliy N. Ovcharukb, Michail I. Gasikc,Dmitriy V. Maslenikovda Department of physics, National Metallurgical Academy of Ukraine, Dnepropetrovsk, Ukraineb Department of electrometallurgy, National Metallurgical Academy of Ukraine, Dnepropetrovsk, Ukrainec Department of electrometallurgy, National Metallurgical Academy of Ukraine, Dnepropetrovsk, Ukrained Department of physics, National Metallurgical Academy of Ukraine, Dnepropetrovsk, Ukraine∗ corresponding author: tutykva@ua.fmAbstract. The current paper presents the results of experimental researches to determine the modefeatures of plasma-beam discharge (PBD) generation by an electron beam injected by a low-vacuum gas-discharge electron gun (LGEG) with the cold cathode and hollow anode on the basis of the high-voltageglow discharge and in the range of helium pressure of P ≈ 10÷ 130Pa. The PBD boundaries and theirdependences on parameters of an electron beam are found. The influence of PBD on parametersof low-vacuum gas-discharge electron gun is revealed. It causes an avalanche increase of electron beamcurrent and burning of plasma-beam discharge in the whole space of the vacuum chamber volumeand generation of electromagnetic radiation is revealed. Achieved results will be used for implementationof various vacuum technologies in the medium of reaction gas and generated electromagnetic radiation.Keywords: plasma-beam discharge, vacuum, electron beam, electron gun, electromagnetic radiation.1. IntroductionPerspectives of application of plasma-beam discharge(PBD) for creation of new electron beam and plasma-chemical technologies is determined by high effi-ciency of beam electrons kinetic energy transformationinto the HF (high-frequency) fields and plasma par-ticles energy [12, 5, 1]. The phenomenon of beaminstability is in the effective initiation of fluctuationsand waves in plasma by means of electron beam, whichcame into being after publication of the fundamentalworks by Akhiyezer and Faynberg, Boma and Gross.This is used for the scientific issues and applicativepurposes of the researches to investigate the operatedthermonuclear synthesis, to find new methods of ac-celeration plasma, to work with plasma electronics,to carry out the experiments in the space etc. [2, 9, 4].The use of PBD in plasma-chemical reactors withthe purpose to receive chemically pure substancesand implement the CVD and PCVD technologieson their basis for coatings and crystal growth in the re-action gas medium is of great practical interest [2].This implementation is rational in the media of inter-mediate and low vacuum. PBD ignition at the pres-sure of neutral gas reaching the values of approxi-mately 6650Pa is theoretically proved [2]. Havingbeen investigated out of the magnetic field and withinthe neutral gas pressure range of ∼ 0.05÷10Pa by wellknown experimental works [7, 3, 6], PBD attractspractical, scientific and technical interest in the fieldof low pressure. For work in this range, low-vacuumgas-discharge electron guns (LGEG) with the hollowanode on the basis of the high-voltage glow discharge(HVGD) applied to implement various electron beamtechnologies [10, 8, 11] is created. The present workstudies the features of PBD formed by the electronbeam which is injected with the low-vacuum gas dis-charge electron gun in the medium of low pressureneutral gas (P ≈ 10÷ 133Pa).2. AimsThe aim of the work is experimental researches of fea-tures of plasma-beam discharge (PBD) generationmode in the gas discharge electron gun operationwith cold cathode and the hollow anode on the basisof the high-voltage glow discharge in a range of heliumpressure P ≈ 10÷ 130Pa.3. Materials and methodsExperimental studies of PBD created by LGEG, werecarried out on the plant, which scheme is shownin Fig. 1.LGEG (1) is fed from the high-voltage power sup-ply (2) of direct current of 5 kW. The parameters arethe following: with accelerating voltage of 0÷ 20 kVand the high-voltage pulse generator forming ten-sion impulses with amplitude of 1 ÷ 40 kV, timeof ∼ 10µs and repetition frequency of 50 ÷ 100Hzwere used. High voltage of negative polarity is in-troduced to LGEG cold aluminum cathode. The an-ode made from stainless steel is earthed on the caseof the vacuum chamber (6).Measurement of anode current IA is carriedout by means of the R3 resistor. The current of an elec-249Valery A. Tutyk et al. Acta PolytechnicaFigure 1. Schematic diagram (a) and general view (b) of the experimental plant; 1 – low-vacuum gas dischargeelectron gun, 2 – power supply, 3 – system of blowing, 4 – double probe, 5 – photo-electronic multiplier of the FEU-19type, 6 – case of the vacuum chamber, 8 – collector, 9 – vacuum pump.tron beam IE coming to the collector (8) is measuredfrom R4 resistor. High voltage on the cathode deter-mined by the voltage divider R1, R2. LGEG is locatedon a dielectric flange. Inside of the vacuum cham-ber (6) the dynamic vacuum is supported. Evacuationis formed by the vacuum pump (9), while the workinggas of helium is continuously blown from the blow-ing system (3) into the area of HVGD gun. Stud-ies are made when helium pressure is in a rangeof 10÷ 133Pa. Measurement of EB current distribu-tion on the cross section is performed with the useof “the hole chamber” (Faradey’s screened cylinder).Parameters of plasma are measured by a double elec-tric probe, which could move transversely of the vac-uum chamber. During the operation in the pulse mode,the following EB parameters are measured by os-cillographic method: current, accelerating voltage,pulse type, etc. The radiation of light from the areawhere EB interacted with neutral gas and plasmais recorded by means of the photo-electronic multi-plier (5) of the FEU-19 type.In the experimental research, low-voltage gas-dis-charge guns with hollow anode and cold cathodeof EGP-9 type (Fig. 2), [11] are used, includingthe main components shown below.LGEG operation can be described by the following.After giving of high accelerating voltage UA to the elec-trodes of LGEG (Fig. 3) HVGD is ignited along the ax-ial line of the anode opening. The formed ions i+start to bombard the cold cathode and as a resultof γ-processes beam fast electrons e–b appear. The lat-ter during their movement create the new ions i+and slow electrons e–m due to the collisions with neu-tral particles A = i+ + e–m.The new-formed ions are partly grasped by elec-tric field and move to the cathode. On their waysome of them undergo recharging to cause fast neutralparticles (A) formation, which continue their move-ment to the cathode. Due to the cathode bombingFigure 2. General view of LGEG of EGP-9 type;1 – cathode water-cooled joint, 2 – high-voltage ce-ramic insulator, 3 – hollow anode block, 4 – elementsof gas blowing system, 5 – replaceable anode.by ions i+ and fast particles, new fast electrons e–bcome into being and the process repeats.Leaving HVGD plasma, the electron beam (EB) cre-ates. The other part of ions free from towards – cath-ode traction is attracted by EB to perform its ionicfocusing. Slow electrons e–m are drawn to the an-ode and compensate the charge of the fast electronse–b from the beam, which have left LGEG, allowingthe gun to work without the electron collector.4. Results and discussionExperimental researches have shown that LGEG per-formance is characterized by the two modes of work:(1.) ion focusing mode (Fig. 4a–4c); and(2.) plasma-beam discharge mode (Fig. 4b–4d).In small currents, the mode of ionic focusing(Fig. 4a) is observed and the presence of the ions250vol. 53 no. 2/2013 Ignition Features of Plasma-Beam DischargeFigure 3. Schematic of LGEG perfomance.Figure 4. LGEG modes.focused by EB ions and created by impact ionization,is its main feature. The luminescence of the longitudi-nal crossover of EB in helium is blue, that can be re-lated to high energy of electron beam (UA ∼ 1÷10 kV),which makes possible actuation the highest levelsof helium atoms and ions. The form of current pulse(Fig. 4c) indicates the absence of fluctuations in sys-tem. The increase in EB current higher than the lim-ited one (at P = const) causes ignition of PBD(Fig. 4b). Thus plasma with a pink luminescence fillsall the internal space of the vacuum chamber. Changein luminescence color towards the longer waves meansconsiderable decrease in energy of electrons partic-ipating in actuation and ionization of neutral gasatoms. Ignition of PBD in the pulse mode leads(Fig. 4d) to formation of current pulse of HF fluctua-tion at the top.As researches showed, the PBD peculiarity is thatthe discharge ignition takes place in the wholeinternal space of vacuum chamber when LGEGwork while ignition under the same conditionsbut without the magnetic field [7, 6] is character-ized by plasma formation around the electron beam.This essential difference can be explained by as bel-Figure 5. Condition of PBD ignition.low. The electron beam, injected by LGEG interactsboth with own plasma, created by itself with impactionization, and with HVGD plasma, which developsthis electron beam (Fig. 5). Analyzing the resultsof PBD formation mechanism in LGEG operation,it is possible to report on the following. Havingcertain parameters of EB and neutral gas pressure,the HVGD plasma concentration ne and concentra-tion of electrons in beam nb reach the required valueto develop plasma beam instability and PBD ignition.However, PBD ignition provokes increase in plasmadensity in HVGD because of the neutral gas atomsadditional ionization by electric fields of flame fluctu-ation. This is accompanied with the increase of ionswhich are bombarding the cathode. The avalancheincrease of EB current occurs. Then the processes arebeing intensified and PBD fills the whole internal spaceof vacuum chamber where EB is injected. The mostpossible explanation is by the relation to the positivefeedback which appears between EB and PBD. It in-tensifies the processes of EB energy transformationinto the energy of PBD. Thus, due to this feedback,in PBD mode, LGEG creates the plasma formationwhich is considerably bigger in size. The similar forma-tion is created by thermionic guns as it fills the workingvolume of the vacuum chamber.The experimental researches determine the bound-aries of PBD ignition at various pressure P dependingon current I = f(P ) (Fig. 6) and EB capacityN = Θ(P ) (Fig. 7) with the varying diameters of an-ode hole dA. The graphs curves 1 are measuredat dA1 = 10mm, and for curves 2 at dA2 = 8mm.Increase of pressure cause increase of the currentand EB capacity, these conditions are requiredfor PBD ignition. Its regular character can be ex-plained in such a way. It is well known [2], that the nec-essary conditions for beam instability development arethe following:(1.) size of plasma d is not to exceed the length of freerun of beam electrons λb in the gas,d ≤ λb , (1)(2.) the energy of electron beam is to be efficientlydelivered to the plasma [2] when increment of beaminstability,δ = ωpe(nbne)1/3> 5 · νen , (2)251Valery A. Tutyk et al. Acta PolytechnicaFigure 6. Dependence of EB limit current for PBDignition on helium pressure.Figure 7. Dependence of EB limit power requiredfor PBD ignition on helium pressure.where νen is collision frequency of plasma electronswith neutrals, ωpe = 5.64 × 104n1/2e is Langmuireelectron frequency of plasma. This phenomenonis related to the fact that PBD in a certain sense isanalog of a classic HF discharge, where HF waves(ω  νen) are actuated by electron beam [4].The increase in pressure leads to increase in con-centration of neutral particles nn = P/kT , (wherek is Boltzmann constant and T is gas temperature).This increases in collision frequency of plasma elec-trons with neutrals νen = 3.4 · 107 · a2√T · nn s−1 [3],where a = 0.95× 10−8 cm is an efficient radius of he-lium atom, T is temperature of plasma electrons or en-ergy of electrons (eV) and condition Eq. 2 of PBDignition fails because νen increased. It is necessaryto increase current and EB power in order to restore it.Main influence on PBD ignition renders neitherto the current value nor to EB capacity, but to elec-tron concentration nb. In the Fig. 8 the dependencesof critical EB concentration in PBD ignition from he-lium pressure for various diameters dA1 = 10mmand dA2 = 8mm is shown. Decrease of the beamdiameter in constant flow to the decrease of anodehole diameter results in increasing in electron concen-Figure 8. Dependence of EB critical concentrationin PBD ignition on helium pressure.tration in the beam and, as Figs. 6, 7 and 8 illustrate,decrease in both EB flow and power, is the necessarycondition for PBD ignition.Let’s consider the correspondence of the experimen-tally obtained conditions of PBD ignition to the the-oretically developed ones. Optical measurements,done by photographing the area of PBD burningnear the anode hole, evidence the fulfillment of the con-dition Eq. 1 in the whole range of LGEG operatingpressure. So, for example, when EB with UA = 5000Vat P1 = 13Pa, the measured values d1 ∼ 8 cm,λb1 ≥ 36 cm; when P2 = 100Pa — d2 ∼ 3 cm,λb2 = 13 cm and correspondingly in both cases d < λb.Then we estimate condition of PBD ignition describedby Eq. (2). The data provided by double probe mea-surement are as follows: for P1 = 13Pa, Ib1 = 0.1A,UA1 = 5000V value of νen1 ∼ 107 s−1. Concentrationof plasma electrons ne1 ∼ 1010 cm−3, when electronLangmuire frequency of plasma isωe1 = 5.64× 104√ne1 ≈ 6× 109 s−1.Value of nb1 ∼ 2 × 108 cm−3 (for anode holedA = 10mm). Value of increment of beam instabilityis expressed byδ1 = ωe1 (nb1/ne1)1/3 ≈ 2× 109 s−1.It become obvious, that condition Eq. 2 is trueas indicates the possibility of PBD ignition with theseparameters. Let’s consider a case with higher pres-sures, when P2 = 133Pa, Ib2 = 0.5A, UA2 = 5000V,when plasma parameters are as follows: value ofνen2 ∼ 108 s−1; concentration of electron of a beamis nb2 ∼ 7 × 108 cm−3; concentration of electronsof plasma made ne2 ∼ 3× 1011 cm−3; electron Lang-muire frequency of plasma ωe2 ≈ 3× 1010 s−1. Takinginto account the presented parameters, the value canbe expressed as here:δ2 = ωe2 (nb2/ne2)1/3 ≈ 1011 s−1.Condition Eq. 2 for this case is also true as welland PBD is ignited, that corresponds to experimentalmeasurements.252vol. 53 no. 2/2013 Ignition Features of Plasma-Beam DischargeResults achieved by the present work prove that it ispossible to have PBD with the use of LGEG un-der the pressure of P ≈ 10÷133Pa. The phenomenonis accompanied by the plasma formation where elec-tron concentration is ne ≈ 1010÷1011 cm−3, and occu-pies the whole working volume of the vacuum chamber.The existence of EB, HF-fields, plasma of large volumeand low vacuum make it possible to implement newCVD and PCVD technologies under such conditionsin the area of reactionary gas to receive the coatings,grow crystals and for the needs of various technologiesof plasma chemistry.5. ConclusionsThe present work proves experimentally that the twoworking modes of LGEG operation with pressuresof P ≈ 10÷ 133Pa are possible. They are the modeof ion focusing and the mode of plasma-beam dis-charge. Transition from one mode to another is deter-mined by the parameters of an electron beam and gaspressure.Areas of PBD ignition with elevated helium pressure(P ≈ 10÷133Pa) are identified within dependenceon current, capacity and concentration of EB elec-trons. It is shown that the increase in pressure leadsto the necessity to increase both current and EB capac-ity in order to ignite PBD. However, the determiningfactor is the increase of electron concentration in EBwhich change in the range of nb ≈ 107 ÷ 108 cm−3.The current researches of the presented work findout the influence of PBD on LGEG operation. This re-sults in avalanche increase of EB current and PBD igni-tion which take the space of the whole working volumeof the vacuum chamber where EB is injected. The mea-surements of concentration of PBD plasma electronswere carried out under conditions of increasing pres-sure and they showed that electrons concentrationwas within the range of ne ≈ 1010 ÷ 1011 cm−3.Generation of large volume of plasma in LGEGoperation when it works in a PBD mode is a per-spective direction for implementation of variousCVD and PCVD technologies in the medium of re-action gas. Transition from one mode to anotheris defined by parameters of electron beam and gaspressure.References[1] A. I. Akhyiezer, L. A. Akhyiezer, R. V. Polovin, et al.Electrodynamics of plasma. Nauka, Moscow, 1974. (inRussian).[2] A. A. Ivanov, V. G. Leiman. O zagiganiiplazmenno-puchkovogo razriada moschnym electronnympuchkom v gase bolshoj plotnosti. Physics of plasma3(4):780–785, 1977. (in Russian).[3] E. A. Kornilov, et al. Losses of energy of beam invapor-gas channel. Physics and chemistry of treatmentof materials 2:20–23, 1975. (in Russian).[4] V. E. Mishin, Yu. A. Ruzhin, V. 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Ignition Features of Plasma-Beam Discharge in Gas-Discharge Electron Gun Operation

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