As discussed in Sec. 6.2, the pre-ionized jet experiment was unable to create jets with masses more than a factor of 2–4 lower than those studied in the original Caltech MHD-Driven Jet
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Figure 6.22: Langmuir probe ion saturation current as a function of radius measured at z = 3.8 cm at three different times during the RF plasma expansion (here the time of RF amplifier turn-on is defined ast= 0µs). Gas was supplied through the rear end of the RF plasma source tube with the fast gas valve bank set toVgas,RF = 550 V.
Experiment, because a jet would not form unless a substantial quantity of neutral gas was injected through the 8 gas inlets on the outer electrode. A portion of this gas was ionized and dragged along with the jet, limiting the maximum jet velocity that could be achieved (see Sec. 6.4.2). One probable reason for needing high Vgas,outer was that pre-ionized RF plasma was injected only at the center of the inner electrode. If this plasma diffused out of the RF discharge tube along the magnetic field lines shown in Fig. 2.2, as suggested by the fast camera images in Figs. 4.1 and 4.2, then it could never reach the region near the outer electrode15. Therefore, the RF plasma alone could not enable the path for poloidal current flow needed to drive a jet, and breaking down additional neutral gas in the vicinity of the outer electrode was necessary.
In order to confirm that no RF plasma reached the region near the outer electrode, the straight Langmuir probe described in Sec. 3.1.1 was used to measure the ion saturation current as a function of radius at z = 3.8 cm. These measurements, shown in Fig. 6.22, verified that the RF plasma density was negligible beyond r = 10 cm (the inner radius of the outer electrode). The radius of the expanding plasma cone was only slightly greater with no confining magnetic field than with the bias and solenoid fields on, in agreement with the impression given by the camera images in Figs. 4.2 and 4.3.
The Isat. measurements in Fig. 6.22 were taken at t > 200µs after RF amplifier turn-
15Furthermore, Fig. 4.11 showed that an appreciable downstream RF plasma density could only be achieved at locations where there was a pre-existing neutral gas cloud present, so even if the magnetic field geometry had been modified, no RF plasma could have reached the area near the outer electrode if gas was not injected through the outer electrode inlets.
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z = 3.0 cm z = 5.0 cm z = 7.0 cm z = 11.0 cm t = 11.3 µs
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Figure 6.23: Left and center: Imacon camera image and Rogowski coil current measurement (smoothed over 500 ns using a moving average filter) for a pre-ionized argon shot with Vmain= 4 kV,Vgas,RF = 550 V,Vgas,inner= 460 V, and Vgas,outer = 650 V. Right: Poloidal current distribution measured with the magnetic probe during a set of shots with these same parameters.
on, when a high density RF plasma had already expanded into the chamber. However, the beneficial effects of pre-ionization on the main plasma breakdown were realized even if the main bank was triggered only 10–150µs after RF turn-on, when only a low density plasma created by fast electrons streaming out of the RF source was present in front of the electrodes (see Figs. 4.4, 4.5, and 4.6, and the discussion in Sec. 4.2). Most of the pre-ionized jets discussed in this chapter were created with the RF amplifier triggered 150µs before the main bank. Increasing this time delay to 250–300µs so that a plasma with n&1018m−3 was present in the chamber when the electrodes were energized did not enable any additional decrease in the amount of neutral gas input necessary for jet formation. Although our initial vision for the experiment was to form a jet primarily out of pre-ionized plasma, this did not turn out to be possible.
Although a well-defined pre-ionized jet would only form with Vgas,outer >700 V, when lower values of Vgas,outer were used breakdown could still be achieved, and the main bank was able to discharge through the plasma. Fig. 6.23 shows an example of the experiment behavior with Vgas,outer = 650 V, slightly below the threshold for good jet formation.
The gun current ramped up slowly at first, indicating that the plasma impedance was high initially, and the plasma that was visible at later times in camera images did not propagate into the chamber as a jet. The magnetic data shows that the current channel radius was ∼ 6 cm, roughly two times wider than was measured for pre-ionized jets with Vgas,outer = 750 V (see Fig. 6.19). Interestingly, I(r) shown in Fig. 6.23 dipped suddenly
at r = 12–14 cm, indicating that most of the return current16 flowed back to the outer electrode at this radial location. For successful jets, on the other hand, the return current flowed primarily at r > 16 cm (see Figs. 6.5 and 6.19), probably because the hoop force caused the outer segments of the spider legs to expand beyond their initial location at r≈17.8 cm.
It is not clear whether the location of the return current could be important for deter- mining the basic jet propagation properties: the simple model of jet acceleration presented in Sec. 6.4.1 considered only the current flowing through the central jet column and ne- glected the return current, and recent numerical simulations of the Caltech experiment [28]
were able to accurately replicate many aspects of the jet behavior despite predicting a return current radius that did not match the data. The magnitude of the poloidal current flowing through the plasma in the Vgas,outer = 650 V case was comparable to the total discharge current measured by the Rogowski coil, so the lack of jet formation could not be blamed on missing current that flowed to ground through an arc in the small gap between the elec- trodes or in the vicinity of the re-entrant port. It is possible that the slow initial ramp-up of the gun current (see the central panel of Fig. 6.23) was problematic—perhaps theJ×B force during this period was sufficient to push the plasma away from the electrodes, so that when the current began to increase rapidly att≈6µs, the necessary initial conditions for jet formation no longer existed.
To overcome the problems discussed in this section and further explore the limits on low mass operation of the Caltech arc discharge experiments, a new setup has been devised to create a single pre-ionized arched flux rope using two RF plasma sources. By pre-ionizing plasma behind both of the experiment’s electrodes instead of only behind the cathode as in the MHD-driven jet experiment, the need for a large amount of additional neutral gas injection will be obviated. The design of this experiment will be described in Appendix I.
16The direction of positive current flow was away from the outer electrode (anode) and toward the inner electrode (cathode), so the “return current” was actually a current in the +zdirection, while the current in the central jet column was in the−zdirection.