ISSN 10637850, Technical Physics Letters, 2015, Vol. 41, No. 9, pp. 901–904. © Pleiades Publishing, Ltd., 2015. Original Russian Text © M.E. Viktorov, A.V. Vodopyanov, S.V. Golubev, D.A. Mansfeld, A.G. Nikolaev, V.P. Frolova, G.Yu. Yushkov, 2015, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2015, Vol. 41, No. 18, pp. 74–81.
An Experimental Setup for Studying the Interaction of Dense Supersonic Plasma Flows with an Arched Magnetic Field M. E. Viktorova,b*, A. V. Vodopyanova,b, S. V. Golubeva, D. A. Mansfelda, A. G. Nikolaevc, V. P. Frolovac, and G. Yu. Yushkovc a
Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, 603950 Russia b Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, 603950 Russia c Institute of HighCurrent Electronics, Siberian Branch, Russian Academy of Sciences, Tomsk, 634055 Russia *email:
[email protected]nnov.ru Received April 21, 2015
Abstract—We propose a new experimental approach to laboratory investigations of the interaction of super sonic (ionic Mach number up to 2.7) highdensity (up to 1015 cm–3) plasma flows and inhomogeneous mag netic field (up to 3.3 T in magnetic mirrors of arched magnetic trap). This approach offers wide possibilities for modeling processes taking place in both nearEarth and space plasma. DOI: 10.1134/S1063785015090291
The interaction of dense supersonic plasma flows and inhomogeneous magnetic field of arched configu ration is among the key problems in the physics of nearEarth and space plasma. Indeed, this interaction determines the formation of energetic electron com ponent in the Earth’s magnetosphere [1], the motion of plasma flows in planetary magnetospheres [2], energy release in magnetic reconnection [3, 4], the generation of electromagnetic radiation and ejection of energetic particles during solar flares [5]. Labora tory investigations of this interaction are of interest for determining physical mechanisms of processes in space plasma and their detailed investigation under reproducible conditions.
A specially designed plasma generator creates a flow of cathode metal plasma formed in cathode spots of vacuum arc discharge [9]. These cathode spots operate on the edge surface of cathode bounded by a tubular ceramic sleeve. The arc discharge is initiated by spark breakdown over the ceramic sleeve edge under the action of a highvoltage (7 kV) pulse applied
Dense plasma flows for laboratory experiments are usually generated by evaporation and ionization of solid target material by highpower laser pulses [6, 7]. Alternatively, these flows can be formed using Z and θpinches [2, 8]. The present Letter describes a new experimental approach according to which plasma flow is created using vacuum arc discharge, which allows supersonic plasma flows with high degree of ionization and high density to be generated. The obtained plasma flow is directed to an open magnetic trap with an arched magnetic field. Figure 1 gives a schematic diagram of the proposed setup, which is based on a vacuum chamber equipped with three main flanges for mounting plasma genera tor and magnetic coils, and with flanges for diagnostic probes and pumping system. Pumping the discharge chamber with a TV301 Navigator turbomolecular pump ensures residual pressure on a level of 10–7 Torr. 901
6
3
1 5
4
Fig. 1. Schematic diagram of the experimental setup: (1) discharge chamber, (2) plasma generator (mounted on a standard CF DN63 flange), (3) magnetic coils, (4) plasma, (5) Langmuir probe on manipulator (in diag nostics flange), and (6) Langmuir probe at a point that is magnetically conjugate to plasma generator.
2
902
VIKTOROV et al.
the center. The proposed setup allows to study the interaction of plasma flows and magnetic field for both longitudinal and transverse injection of plasma relative to the magnetic field.
3
4
2
1
Fig. 2. Image (inverted) of the optical emission from plasma filling the magnetic field tube (dashed contour indicates boundary of the diagnostic window): (1) plasma generator, (2) Langmuir probe on manipulator (in diag nostics flange), (3) Langmuir probe at a point that is mag netically conjugate to plasma generator, and (4) plasma (see Fig. 1 for more detailed arrangement of parts in the setup).
between the cathode and an auxiliary ring electrode situated on the sleeve outer surface. The main vacuum arc discharge pulse of 20 μs duration between the cath ode and anode is power supplied from a 20 μF high voltage capacitor. The discharge current amplitude is controlled by the capacitor charging voltage. At a maximum voltage of 1.5 kV, the current amplitude reaches 3.5 kA. By changing the discharge current amplitude (with the charging voltage varied from sev eral hundred volts to 1.5 kV), the density of plasma in the cathode region of arc discharge can be controlled within 1013–1015 cm–3. For the arc current pulse dura tion of 20 μs, the length of plasma bunch reaches about 40 cm, which is more than twice as large as the longitudinal size of magnetic trap. The plasma gener ator is mounted on a standard flange, which allows it to be arranged on any of the three available flanges of the vacuum discharge chamber. A supersonic flow of plasma with a density of up to 1015 cm–3 and the ionization degree above 80% (the latter factor is especially important for modeling pro cesses that take place in space plasma) is injected into a magnetic field with variable configuration of field lines, which is generated by current passing in the two coils. Coils are arranged so that their axes are mutually perpendicular and create an arched field configuration of bent open magnetic trap. Each magnetic coil com prises three serially connected sections cooled by air flow via special channels and creates magnetic field in a pulse periodic regime with induction up to 3.3 T at
The first trial experiments were performed with plasma injection via the magnetic mirror of the mag netic trap. The plasma generator was mounted on the flange in which magnetic coil was arranged so that the cathode surface occurred in magnetic mirror of the trap (Fig. 1). These experiments were aimed at study ing how dense plasma fills the magnetic trap with an arched field configuration, which is necessary for sub sequent investigations of the process of “breakage” of magnetic field lines in this configuration by the flow of supersonic dense plasma. The experiments were performed with the alumi num cathode of a plasma generator. In this case, the plasma flow velocity was on a level of V0 = 2 × 106 cm/s, the average ion charge was Z = 2.5, the elec tron temperature was TE = 6 eV, and the kinetic energy of directional ion motion was Ti = 60 eV [10]. For this plasma flow, the ionic (ion sound) Mach number is MS = V0/CS = 2.7, where CS = ZT e /m i is the ion sound velocity and mi is the ion mass. For an ion den sity of ni ~ 1015 cm–3, the ratio of gaskinetic pressure to magnetic field pressure is β = 8πnT/B2 and can vary from 6 to 0.01 for the magnetic induction increased from 0.02 to 0.5 T. This spread of β implies that condi tions for the breakage of magnetic field lines can be realized for accessible parameters of plasma flow. Figure 2 shows an integral photographic image of the optical emission from plasma filling the magnetic trap for an arc discharge current of 2.5 kA and a plasma density of about 1015 cm–3. The current in magnetic coils was 5.6 kA, which corresponded to a magnetic induction of 0.13 T at the center of discharge chamber and 2.83 T in magnetic mirror of the trap. As can be seen, the region filled by plasma has clearly defined boundaries. The spatial arrangement of a plasmafilled magnetic field tube is determined by the radial position the plasma generator cathode. The dynamics of magnetic trap filling by plasma was studied with the aid of Langmuir probes situated in various sites of the discharge chamber. One probe was arranged on the edge of magnetic coil flange free of the plasma generator (behind the second magnetic mirror of the trap). The second probe was situated on a manipulator in the diagnostic flange opposite to the plasma generator. The results of probe measurements revealed variation of the amplitude of response signals and the presence of a delay in plasma arrival to the probes, which was evidence of plasma flow redistribu tion depending on magnetic field changes. Figure 3 shows characteristic waveforms of signals observed with the increase of magnetic field. Both probes oper ated in the regime of ion current saturation at the same
TECHNICAL PHYSICS LETTERS
Vol. 41
No. 9
2015
AN EXPERIMENTAL SETUP FOR STUDYING THE INTERACTION
0
1
−1
−2
−2 (c)
−4
4
2
(d)
−2 −4
−2 −10
0 10 Time, µs
3
2
0 1
0
2
−1
−2
−2
−4 −10
20
Bmax = 2.83 T 4
1 Iarc, kA
Iarc, kA
−2
0 1
0 1
−4
2
−1
0
−2
2
0
2 2
−1
Jprobe, A/cm2
3
1
1
Bmax = 2.3 T
2
4 3
Jprobe, A/cm2
0
2
Iarc, kA
2
Jprobe, A/cm2
3
Bmax = 1.6 T
2
4
1 Iarc, kA
(b)
B=0
0 10 Time, µs
Jprobe, A/cm2
(a) 2
903
20
Fig. 3. Waveforms of signals measured by electric probes for various values of magnetic field in the trap: (1) current density on the upper probe situated at the magnetic trap point that is magnetically conjugate to plasma generator, (2) current density on the probe opposite to plasma generator (10× magnification), and (3) arc discharge current. Values of magnetic induction B refer to magnetic mirror of the trap; zero on the time scale corresponds to maximum of the arc discharge current.
negative voltage (–50 V) applied relative to the dis charge chamber. In the absence of magnetic field (Fig. 3a), the cur rent is detected (as expected) only on the probe situ ated opposite to the plasma generator. At the initiation stage of discharge, plasma with electron energy above 50 eV is generated. These electrons, capable of sur mounting the probe potential, are manifested by the characteristic negative outbursts of current to the probe. Then, a positive signal is observed that corre sponds to the arrival of main plasma bunch at the arc stage of discharge. The time delay of about 10 μs cor responds to the gasdynamic flight of plasma from the plasma generator to the probe. In the presence of a magnetic field (Figs. 3b–3d), the distribution of current to probes dramatically changes. The current density on the upper probe situ ated at the magnetic trap point that is magnetically conjugate to the plasma generator becomes nonzero and exceeds by more than an order of magnitude the current density on the probe opposite to plasma gen erator. In the presence of a magnetic field, the shape of current waveform measured at the point magnetically conjugate to plasma generator is analogous to current to the probe opposite to plasma generator in the absence of the field. The initial stage displays negative current outbursts corresponding to energetic elec trons. As the magnetic field is increased, the signal from the probe situated opposite to the plasma gener ator, measured at the moment of maximum arc cur TECHNICAL PHYSICS LETTERS
Vol. 41
No. 9
rent, remains almost unchanged. The second peak of current to this probe is observed, which is shifted within 15–20 μs rightward relative to the moment of maximum arc discharge current. At the same time, the signal from the probe situated on the upper flange of the discharge chamber increases in proportion to the magnetic field. The observed significant redistribution of current densities on probes situated at various sites of the dis charge chamber implies that more than 90% of the total flow from the plasma generator is effectively cap tured by magnetic field of the trap. The presence of a second maximum of current to the probe situated opposite to the generator observed upon termination of the arc current pulse is probably indicative of the breakage of magnetic field lines of the trap filled by plasma. Thus, an experimental setup has been created for studying the interaction of quasistationary, dense supersonic plasma flows and inhomogeneous arched magnetic fields. The setup design allows investigations to be performed both with plasma filling the open magnetic trap along the magnetic field lines and with plasma flow directed across the magnetic field. This is achieved by changing the arrangement of plasma gen erator on flanges of the vacuum discharge chamber. The proposed setup admits variation of the parameters of plasma flow and magnetic field in a broad range and is intended for laboratory modeling of astrophysical
2015
904
VIKTOROV et al.
processes in which plasma flows interact with mag netic fields of various celestial bodies. Acknowledgments. This work was supported in part by the Ministry of Education and Science of the Rus sian Federation (project no. 14.Z50.31.0007), the Department of Physical Sciences of the Presidium of the Russian Academy of Sciences (program OFN15 “Plasma Processes in Space and Laboratory”), and the Presidential Council for Grants in Science (project no. SP4857.2013.3).
1. H. S. Fu, Yu. V. Khotyaintsev, A. Vaivads, et al., Nature Phys. 9, 426 (2013).
3. M. Yamada, R. Kulsrud, and H. Ji, Rev. Mod. Phys. 82, 603 (2010). 4. N. Katz, J. Egedal, W. Fox, et al., Phys. Rev. Lett. 104, 255004 (2010). 5. S. Masuda et al., Nature 371, 495 (1994). 6. S. K. P. Tripathi and W. Gekelman, Solar Phys. 286, 479 (2013). 7. C. Plechaty, R. Presura, and A. A. Esaulov, Phys. Rev. Lett. 111, 185002 (2013). 8. G. N. Kichigin and N. A. Strokin, Energy Evolution Processes in Space Plasma (Izdat. IrGTU, Irkutsk, 2007) [in Russian]. 9. G. A. Mesyats, Phys. Usp. 38, 567 (1995). 10. A. Anders and G. Yu. Yushkov, J. Appl. Phys. 91, 4824 (2002).
2. I. F. Shaikhislamov, Yu. P. Zakharov, V. G. Posukh, et al., Plasma Phys. Control. Fusion 56, 125007 (2014).
Translated by P. Pozdeev
REFERENCES
TECHNICAL PHYSICS LETTERS
Vol. 41
No. 9
2015
