The first section of the collective ions acceleration based on simultaneous temporal and spatial modulation of relativistic electron beam (REB) was studied experimentally. The virtual cathode was originated in the electrodynamic structure consisting of two tubes with different diameters (jump of electrodynamics) by REB, produced in magnetically insulated diode. At plasma assistance the low-frequency oscillations of REB current and the low-frequency microwave radiation were obtained due to the virtual cathode periodical relaxation in the processes of charge compensation by ionized residual gas

Chupikov, P.T.

Faehl, R.J.

Medvedev, D.V.

Onishchenko, I.N.

Panasenko, B.D.

Prokopenko, Yu.V.

Pushkarev, S.S.

Yegorov, A.M.

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

PLASMA ELECTRONICS132  Problems of Atomic Science and Technology. 2002. ? 4. Series: Plasma Physics (7). P. 132-134EXPERIMENTAL INVESTIGATIONS OF INTERACTION OFSUPERCRITICAL ELECTRON BEAMS WITH PLASMAP.T. Chupikov, D.V. Medvedev, I.N. Onishchenko, B.D. Panasenko,Yu.V. Prokopenko, S.S. Pushkarev, R.J. Faehl*, A.M. YegorovNational Science Center " Kharkov Institute of Physics and Technology"Academic St. 1, Kharkov, 61108, Ukraine, e-mail: onish@kipt.kharkov.ua*Los Alamos National Laboratory, USAThe first section of the collective ions acceleration based on simultaneous temporal and spatial modulation ofrelativistic electron beam (REB) was studied experimentally. The virtual cathode was originated in the electrodynamicstructure consisting of two tubes with different diameters (jump of electrodynamics) by REB, produced in magneticallyinsulated diode. At plasma assistance the low-frequency oscillations of REB current and the low-frequency microwaveradiation were obtained due to the virtual cathode periodical relaxation in the processes of charge compensation byionized residual gas.PACS: 52.40.Mj1. INTRODUCTIONThe structures with virtual cathode can be successfulapplied in collective accelerators of charged particles [1,2]. They are an integral part of two-section collective ionaccelerator based on simultaneous space and timemodulation of relativistic electron beam [3,4]. In suchaccelerator the ions are accelerated by a field of spacecharge slow wave generated by a temporally modulatedintensive electron flow, which is propagating in spatiallyperiodic structure. The mechanisms of the low frequencyexcitation by ions moving in electron beams of the currentabove the space charge limiting one were studied in [5,6].This work is a part of efforts aimed researches of acapability for creation of two-sectioned ion accelerator[1]. The essence of approach for the solution of thisproblem concludes to using of space charge fields in REBfor the acceleration of intense ion beams. In the base ofconception the collective ion acceleration by asynchronous slow space charge wave, that originates fromsimultaneous temporal and spatial modulation of REB.The temporary modulation arises during relaxation ofvirtual cathode at the plasma presence and spatial one iscaused by the spatially - periodic magnetic field.The basis of this article is experimental researches ofthe first section of collective ion accelerator. Theconditions of formation of a virtual cathode in cylindricalelectrodynamic structures are studied. The capability oflow frequency modulation of REB by chargecompensation of virtual cathode space charge by ionizedresidual gas in system is realized.2. EXPERIMENTAL SETUPIn the vacuum structures with supercritical electroncurrents (above the space charge limiting current) avirtual cathode (VC) should arise. The problem of ourresearch is to form VC in the anode tube (drift tube) of themagnetically insulated diode. It was achieved by makinga jump of tube diameter. The arrangement for thisexperiment was performed in the first section of two-section collective ion accelerator [1]. The experimentalsetup of it is represented in Fig. 1.In Fig. 1 the structure of the high-current REBaccelerator is also shown. It is capable to generate theelectron beam with energy up to 300 keV and current upto 4 kA at impulse duration of microsecond range. TheMarx generator (MG) total energy is equal 0.9 kJ wheneach stage is charged by voltage of 30 kV.The MG high-voltage pulse is applied to themagnetically - insulated diode (1). The cylindricalcathode was made from stainless steel. Its diameter was31 mm and width of the emission edge was 0.1 mm.The value of electron beam current is determined bydiameter of input cylindrical anode, the diameter of whichwas equal 41, 46, and 50 mm and are provided withfollow-up used constructers in the injection region of thebeam. As it will be shown below the jump ofelectrodynamic structure is necessary for the virtualcathode formation. The inner diameter of the transportelectron cylindrical liner (drift tube) is equal 50 mm.The electron beam is conveyed by a longitudinalexternal magnetic field of the solenoid (2) with inductionup to 1.33 T. In the cathode region the value of magneticfield was ~60 % from the value inside the solenoid. Suchconfiguration of magnetic field forms the electron beamwith diameter of 32 mm and wall thickness of 3 mm.7II-sectionI-sectionVCPG2155 345MG6ASCS EMFFFig. 1. Collective accelerator of ionsMG – Marx generator; ASCS – accelerator starting andcontrol system; EMFF – external magnetic fieldformation; VC – virtual cathode; PG – plasma gun.133In Fig. 2 the cross-sectional portrait of electron beamis shown in the collector region after REB transporting bythe homogeneous longitudinal magnetic field.High-voltage dividers (resistive (3) and capacitive(4)), magnetic sensors of currents (5) used on backcurrent-conductor (6) of MG and together with Faradaycup which is as a current collector were used for thecontrol and the measurements of parameters of the high-current electron accelerator. The periodic magnetic fieldis formed by means of alternating aluminum and ironrings (7), which was placed on the exterior of transportelectron beam liner. The radial thickness of aluminumrings is equal 1 cm, and iron is 0.5 cm. The spatial periodformed by rings on the second section of ion accelerationis equal 4 cm and the overall length of the structure hasthe value of 4¸5 periods. The external periodic magneticfield is necessary for ion acceleration in the secondsection by slow space charge wave of modulated REB.In Fig. 3 the oscillograms of the main parametersof the high-current REB accelerator are shown. The peakvalues of the oscillograms correspond to followingvalues: the input current of the magnetically - insulateddiode is 4.4 kA, and the collector REB current is 3.4 kA,and diode voltage is 280 kV.3.VIRTUAL CATHODE FORMATIONThe conditions of virtual cathode formation in thediode with magnetic insulation were studied using thecylindrical electrodynamic structures. Structures of twotypes: a circular tube of diameter 41 mm and waveguidewith jumping namely “diode anodic unit of diameter41 mm - cylindrical drift liner of greater diameter 50 mm”- were investigated experimentally.In Fig. 4 the oscillograms of the diode input currentand the REB current on the collector (Faraday cup) areshown. As it is seen from Fig. 4, a in the first case theREB current injected from the cylindrical cathode istransported without losses to the collector placed at theend of the drift liner and in the homogeneous magneticfield of induction 0.88 T. In the electrodynamic structurewith jumping the REB current on the collector is muchless than the diode input current and corresponds to valueof a space charge limiting vacuum current in the drift liner(Fig. 4, b). It proves the possibility of the virtual cathodeformation in the structure with jumping.4. PLASMA VIRCATORIn case, when injected electron beam current exceedsthe value of space charge limiting vacuum current in thegiven geometry, REB is locked in the drift liner underoperating of a sagging space charge potential. Theaccumulated electrons form such potential distribution,that it can be looks like a virtual cathode. In Fig. 5 theoscillogram of input diode current (1) and the timeFig. 3. Input (1), and collector (2) currents and diodevoltage (3)Fig. 2. Portrait of electron beam-500,0n 0,0 500,0n 1,0µ 1,5µ 2,0µ-6-5-4-3-2-1021Current, kATime, sa)-500,0n 0,0 500,0n 1,0µ 1,5µ 2,0µ-6-5-4-3-2-1021Current, kATime, sb)Fig. 4. Current oscillograms1 – input diode current; 2 – collector current of REBa) -tube diameter 41 mm; b) -jump of electrodynamicstructure from 41 mm to 50 mm134-500,0n 0,0 500,0n 1,0µ 1,5µ 2,0µ-0,8-0,6-0,4-0,20,00,2Voltage, VTime, sFig. 6. Microwave radiation pulsecharacteristic curve of the limiting vacuum current (2)during accelerator operation pulse are shown. From theFig. 5 follows that in the transportation liner of electronbeam the VC should be formed at beginning of the pulse.In the structure with supercritical electron beams theoscillations of electrons between the cathode and virtualcathode forms a power microwave radiation (so-called,vircator-generator). In our case the frequency of theseoscillations has a value of several GHz. At plasmaassistance the low frequency modulation should takeplace in the system with virtual cathode. As a result ofvolume charge compensation the potential well disappearsin the system, and accelerated ions in the well escape theregion of virtual cathode. Thus, the system is returned inthe initial state and the process is repeated periodically.As a result the high frequency oscillations of virtualcathode become modulated. Besides the relaxing potentialwell of virtual cathode modulates the REB current andaccelerates plasma ions up to energies determined by thewell depth.In Fig. 6 the oscillogram of the detected microwaveradiation of vircator is shown. In our experiments the lowfrequency modulation of plasma vircator microwaveradiation can be explained by charge compensation ofvirtual cathode space charge by ionized residual gas atpressure 2´10-4 Torr. The modulating frequency ofmicrowave radiation has value ~40 MHz. It is necessaryto notice, that the collector current of REB has the samevalue of modulating frequency (Fig. 3).5. CONCLUSIONIn this paper, the first section of the collective ionsaccelerator (Fig. 1) based on temporary and spatialmodulation of the dense relativistic electron beam wasexperimentally investigated. The input current of themagnetically - insulated diode was obtained 4.4 kA fromMarx generator with energy of 0.9 kJ. The transportationof electron beam with pick current of about 4.4 kA wasrealized along all drift liner with diameter of 41 mm. Thevirtual cathode was formed in the electrodynamicstructure, which had the jumping of diameter from 41 mmto 50 mm. High microwave radiation generated of thevirtual cathode was obtained.In the condition of plasma assistance the lowfrequency oscillation of electron beam and low frequencymicrowave radiation were obtained by chargecompensation of virtual cathode space charge by ionizedresidual gas. In the plasma vircator the plasma assistanceleads to virtual cathode disappearance due tocompensation its space charge by ions and to itssubsequent relaxation restoration at plasmadisappearance. Hence, there is an opportunity of low-frequency modulation, determined by the ion time offlight. This modulation is necessary to create the slowwave space charge in the second section. The furtheracceleration of ions will be occurred in a slow wave of atemporary-modulated electron beam at its subsequentspatial modulation by an external magnetic field.The obtained research results confirms the possibilityto realize the idea of collective acceleration of ions by asynchronous slow space charge wave generated bysimultaneous temporal and spatial modulation of REB.This work was supported by STCU Project No.1569.REFERENCES[1] I.N. Onishchenko, S.S. Pushkarev Ion acceleratorbased on plasma vircator // VANT. Issue: Nuclear-PhysicsInvestigations. 2001, V. 38, No.3, p. 41-43.[2] W. Peter, R. J. Faehl, C. Snell et al Collective ionacceleration by means of virtual cathodes // IEEETransactions on Nuclear Science. 1985, V. NS-32, No.5,p. 3506-3508.[3] V. A. Balakirev, A. M. Gorban, I. I. Magda et alCollective acceleration of ions by modulated high-currentREB // Plasma Physics Reports. 1997, V. 23, No.4,p. 323-327.[4] V.V.Belikov, A.G.Lymar, and N.A.Khizhnyak. Ionsacceleration by modulated electron beam. // Pis’ma Zh.Tekh. Fiz. 1975, Vol.1, No.13, p.615.[5] V.A. Balakirev, I.N. Onishchenko, N.I. OnishchenkoLF oscillations excitation by the ion stream in highcurrent relativistic electron beams // Proc. of the 12th Int.Conf. "Microwave and Telecommunication Technology".(Sevastopol, Ukraine) September, 9-13, 2002, p. 373-374.[6] P.I.Markov, I.N.Onishchenko, G.V.SotnikovNumerical simulation of processes in a supercriticalelectron beam at the presence of plasma. //InternationalConference and School on Plasma Physics andControlled Fusion. Alushta (Crimea), Ukraine, September16-21 2002 : Book of Abstracts. P.145.0,0 500,0n 1,0µ 1,5µ-6-4-2021Current, kATime, sFig. 5. Input diode current (1) and limited current (2) ofelectron beam

Experimental investigations of interaction of supercritical electron beams with plasma

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Experimental investigations of interaction of supercritical electron beams with plasma

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