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Gas-Puff Z-Pinch Experiment

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Gas-Puff Z-Pinch Experiment INSTITUTE OF PLASMA PHYSICS CZECHOSLOVAK ACADEMY OF SCIENCES GAS-PUFF Z-PINCH EXPERIMENT A. Krejčí RESEARCH REPORT IPPCZ 286 •> November 19M POD VODÁRENSKOU VEZl 4, i80 69 PRAGUE 8 CZECHOSLOVAKIA GAS-PUFF Z-PIHCH EXPERIMENT A. Krejčí IPPCZ-286 November 1988 - 2 1• Introduction Pinch-effect is the tendency of a current-carrying column of a matter to compress itself towards Its axL&« The implosion is caused by the interaction of the current with the self-magnetic field* The idea of a simple arranged experiment in which the high current (on order of MA) thermo- -insulates the plasma by its own magnetic field and simulta­ neously heats it by both Joule heat and adiabatic compression is very attractive! on this principle the first experimental devices for controlled thermonuclear fusion were constructed in early 1950 s [1, 2 J. But the thermonuclear parameters were not reached. MHD Instabilities of compressed plasma fibre, mainly the m = 0 (sausage) and m • 1 (kink) modes, were the main reason of this failure* In addition, the Joule heating proved to be effective only In the beginning of the plasma implosion, because specific resistivity rapidly decre­ ases during the plasma temperature growth (ij r* T ~^' ). Nevertheless, those experiments demonstrated the Z-pinch as an intense ри1зе X-ray source and at the same time they sti­ mulated the progress of high-power pulse technology and plas­ ma diagnostics. Recent concept of linear Z-pinch Is different from that in the 1950 s. It is emphasized the second mechanism of heating now, i.e. adiabatic compression, which Is In some respect the opposite pole to the first mechanism. Pinching plasma is considered as colliding particle beams, performing the targets each other. A similarity of this concept with - 3 - the ideology of inertial confinement is evident: during the "clean*1 implosion only the kinetic energy of accelerated particles increases until they reach a close surroundings of the axis and only there the plasma stagnates and therma- lizates» The experimental research has changed from the classi­ cal Z-pinches imploding from the whole volume of discharge chamber, completely filled Ъу working gas, to other pinches with pulse injection of the gas into the vacuum chamber (gas-puff Z-pinches) [з] • The absence of gas contact with the wall enables more uniform ignition of the discharge and a minima li zat i on of impurities in plasma» Usage of a hollow gas cylinder is then more advantageous to achieve higher implosion velocity, compression and plasma temperature than in the ease of ful cylinder [4, 5>] . Shorter period of dis­ charge circuit (on order of us or hundreds ns) provides better prevention of MHD instabilities» large experimental devices with gas-puff Z-pinches ere nowadays the most effective Impulse source of soft X-rays (the conversion of applied electric energy into X-rays achieves up to 22 % [б]) and on their basis the X-ray lasers are developed. As regards the thermonuclear research, linear Z-pinch is again studied as one of less conventional alterna­ tives of thermonuclear reactor Í7, 8 J • - 4 - 2. Experiment At the Institute oi' Plasma Physics (Czechosl. Acad* Sci., Prague) the device of gas-puff Z-pinch was completed for experimental study of its physical properties and fur­ ther applications as an impulse soft X-ray source (calibra­ tions of X-ray detectors, X-ray lithography). The apparatus consists of capacitor bank (10.8 oP).9 charging and switching circuits, main spark gap switch, flat low-inductance high-voltage conductors, discharge chamber (0 260 mm), pulse electromagnetic valve (with filling volume 1,2 cnrvshot) and vacuum pumping system (10""^ Pa), Working gas (Ar or Ne with initial pressure from 1 x 1CK to 7 x 1(r expands into the chamber and annular nozzle forms the hollow gas shell and directs it to the space between the electrodes (fig, 1), The filling of interelectrocle gap was measured independently by ionization gauge and by means of high voltage breakdown between the electrodes. On the basis of these measurements a proper delay between gas valve openning and main spark gap switching was selected [9J • An open-air main spark gap working in multichannel regime is triggered by this chain: time unit —» hydrogen thyratron —* coaxial trigatron spark gap —» main spark gap. It is desirable that the pinch should occur as close as possible the current peak and maximum energy of magnetic field should transform, into the kinetic energy of particles. Considering that the current period is determined - 5 - Ъу construction of main circuit, it is necessary to comply with this requirement by changes of other discharge parame­ ters. Working regime of our Z-pinch was consequently optimi­ zed Ъу electrodes geometry, initial gas pressure p , capaci­ tor charging voltage U and the delay between valve closure and main spark gap switching S^rp, Following initial confi­ gurations of gas cylinder were studied (length: diameter): A. 2,5 1 (42/17 mm), B. 1 1 (21/21 mm), С 1 2»5 (21/52 mm). The most suitable working regime in described experiment (not only from the energetic point of view, but also from point of view of discharge stability and X-ray radiation) proved to be such regine where U = -30 kV, S^p = 350 ps 5 for We (400 ps for Ar), p0 = (4 - 7) x 10 Pa, configuration B. Particularly, the measurement of soft X-ray emission of pinch was exploited for high-temperature plasma diagnos­ tics. X-ray detection methods with temporal, spatial and spectral resolution were utilized. The X-ray diagnostic tilth temporal resolution was carried by 2 types of detectors. The vacuum X-ray diode (XRD) with an alluminium cathode for the region of ultra-soft X-rays (hv = 10 - 1000 eV) takes advantage of the photo-effect as a dominant process in the X-ray interaction with matter. The XHD is censitive neither to visible light nor to hard X-rays and makes possible the temporal resolution better than 1 ns. For the measurements - 6 - in the region ofhv =1-10 keV, the semiconductor surface- -barrier detector (SBD) was used. In order to improve the response speed (below 10 ns) and signal-to-noise ratio, a voltage* in the reverse direction was applied to the SBD. Some filters of thicknesses <1 p (ZRD) or several Ш (SBD) and 4-channel set-up of both types of detectors were employed for a simple spectrometry. The XRD a SBD were placed in radial distances 35 - 190 cm from the axis of the discharge chamber. Other used methods of X-ray diagnostics (pinhole cameras, crystal spectrographs) are described elsewhere |_10J . 3. Results a) Electric characteristics of the discharge In a short-circuit connection, high-current generator is working with period T = 7*8 jusj the inductance of main circuit is 143 nH. Charging the capacitor bank up to the voltage -40 kV, we obtained the initial current rate about 2 x 10'1 A s"1. Other characteristics of the circuit are shown in table 1; Short-circuit current I_ was measured by Rogowski colls (fig. 1)f the maximum current I was calcula­ ted for lossless LC-^ircuit. The difference between both currents is caused by the energy loss in the circuit, espe­ cially in the spark channels of the main spark gap switch* During the discharge in gas (plasma) load, its resis­ tance and inductance are changing. The imploding cylinder - 7 - inductance rises from some nH to several tens of nH. This rapid growth of inductance at the end of implosion causes typical break (drop) of current signal (see time 9p in fig*2) which coincides with a moment of the imploded plasma shell collapse on discharge axis. On the contrary, relati­ vely big load resistance in the discharge beginning decreases rapidly with rising current, at a time t<žC9p due to avalan­ che ionization and then due to Coulomb collisions. So the resistance does not affect the discharge later. Because our Z-pinch occured before current maximum, increasing current and magnetic field at Sp < t < T/4 makes further plasma implosions possible as seen from fig. 2. « b) Dynamics characteristics of the implosion We assume that the plasma cylinder axis is parallel to the z-axis and that only the components 32 and B^ of the current density and the magnetic induction are not equal zero, (r, 1^ and z are the cylindrical coordinates). The dynamics of Z-pinch implosion is then described by mo­ tion equation <1> V*- 9& + Ш J where 9, p, vr are the density /kg m / , pressure and radial velocity of the gas (plasma)» respectively. Approxi­ mate solution is given by snowplow model [l1J : neglecting gae pressure it is considered a fully ionized thin layer at the cylinder surface which sweeps up the cold gas below this' layer. We simplified snowplow model by the assumption - 8 - that the mass M(t) of the accelerated plasma remains cons­ tant during the implosion of hollow cylinder. The numeri­ cal solution of non-linear differential equation C2) dV Pol2(t)1 ^? s * 4*а(Ш with initial conditions (3) aCt-O) = A, (ff )t=o = 0, gives the results shown in fig* 3. Here a, 1 are the cylin­ der radius and length, respectively, and the current I(t) = Ie sin w t. By fitting the values of the mass M we reached an agreement between calculated and measured implosion times Sp. However, we -have to take into account that this amount of accelerated mass is only some frac'tion of all mass betweet „7 the ^-strodes. This total mass is on the order of 10 ' kg (after a model of adiabatic expansion of the injected gas Into vacuum).
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