5.2 Plasma Density Peak in the Afterglow
5.2.3 Bi-Maxwellian Electron Energy Distribution
νmr−1 ≈ 10 µs. However, the plasma density in the afterglow didn’t peak until ∼ 100 µs after RF turn-off, so some metastable atoms must have survived this long, if they were indeed responsible for the density increase (we have not identified any likely source of new metastables in the afterglow other than three-body recombination, which can be neglected in this discussion since recombination followed by pooling ionization did not lead to a net increase in the ion density). It is notable that all other instances of afterglow density rises due to pooling ionization that we know of [118, 119, 120, 121] occurred in plasmas with ne≤5×1016m−3, so the metastable lifetime in the afterglow was 100µs or longer.
Based on the results of experiments with argon-hydrogen mixtures shown in Fig. 5.12, it seems clear that atomic processes producing new ionization were the fundamental cause of the afterglow density rise in our RF discharge. However, the detailed sequence of processes involved, which may have been more complex than pooling ionization alone, remains to be conclusively determined. Preliminary numerical calculations were carried out in which pool- ing ionization terms were added to the 1D discharge model equations in Sec. 4.4.2: only very small afterglow density increases were observed in simulations of unmagnetized discharges with uniform gas pressure. This result is consistent with the data shown in Figs. 5.9 and 5.10, but calculations with nonuniform pressure and radial confinement will be required if the dramatic afterglow density peak observed in discharges with the bias field and fast gas valve used is to be reproduced. Direct measurements of the time-dependent metastable den- sity (for example, using laser absorption) would be very valuable for confirming or refuting the proposed pooling ionization mechanism.
1.45 eV, 9%
0.21 eV, 91%
B off z = 0.3 cm
0.35 eV, 100%
B on z = 0.0 cm
Figure 5.13: Electron temperature fits to ln|Ie| measured by the Langmuir probe near the output of the RF discharge tube. Te is labeled for each Maxwellian component of the distribution function, along with the percentage of the electrons belonging to each population. Left: Probe measurement averaged from t= 170–180µs for an unmagnetized discharge in which the RF power was turned off att= 100µs. Right: Probe measurement averaged fromt= 670–680µs for a discharge with the bias field coil and solenoid turned on, and the RF power turned off att= 500µs. The fast gas valve was used withVgas,RF = 550 V in both cases.
visible in the measured distribution function if the process is occurring at a high rate [121].
However, the temperature of the “hot” component in our experiment was only 1–2 eV, so it was not definitively associated with energy pooling. Nevertheless, it appears that some process was reheating a portion of the electrons. In the absence of reheating, evaporating cooling was expected to reduce Te to below 0.5 eV within the first 50 µs after RF power turn-off (see Figs. 4.22 and 4.26). Furthermore, the electron-electron collision frequency withne ∼1018m−3 and Te ∼1.5 eV was νee∼2×107s−1 [4, p. 596], so the two electron populations would have quickly equilibrated unless new hot electrons were continuously being created.
Interestingly, bi-Maxwellian electron distributions were only observed in experiments with no magnetic field—in the magnetized discharges that exhibited a large afterglow den- sity rise, the Langmuir probe I-V curves were always fit well by a singleTe distribution, as shown in the right panel of Fig. 5.13. In the B = 0 case, the fraction of hot electrons de- creased in time and was lower further from the RF antenna—these trends are illustrated in Fig. 5.14. Further investigation into the conditions necessary for a bi-Maxwellian distribu- tion to develop may yield insights into the dominant atomic processes in the afterglow and
−10
−5 0
5 10 100
200
300
400 10−2
10−1 100
z (cm) Time (µs)
Hot Electron Fraction
Figure 5.14: “Hot” (> 1 eV) electron fraction during the afterglow for unmagnetized dis- charges with the fast gas valve used. The RF amplifier was turned off at t= 100µs.
could possibly provide useful information for understanding the afterglow density peak.
Chapter 6
Pre-Ionized MHD-Driven Jets
This chapter will describe the MHD-driven jets created with the aid of the RF pre-ionization system. Secs. 6.1, 6.2, and 6.3 cover the diagnostics used to study the jets, the effects of pre-ionization on the initial plasma breakdown, and the impact of the jet structure on the discharge circuit behavior, respectively. The reader who is mostly interested in results relevant to astrophysics or basic plasma physics may choose to skip to Sec. 6.4;
note, however, that detailed knowledge of the diagnostic techniques described in Sec. 6.1 is invaluable for understanding the data.
Pre-ionized jet experiments were carried out primarily with argon gas, which produced the most interesting results. In principle, the gross jet behavior should have been similar for all gas species, as in the original Caltech MHD-Driven Jet Experiment (see [130], for example); however, the relative ease of breaking down argon in either DC or RF electric fields (see Fig. 1.10) allowed for a broader region of parameter space to be accessed than was possible with other gases.
As discussed in Sec. 1.7, the primary motivation for pre-ionization was to create hotter, faster MHD-driven jets and spheromaks with increased relevance to astrophysics and fusion plasmas. These goals were achieved, to some extent—Secs. 6.4.2 and 6.4.3 will show that the pre-ionized jets were indeed faster and hotter than those created previously. However, the density, temperature, and velocity differed by less than an order of magnitude from the corresponding properties of jets created without pre-ionization. The main impediment to further improvements will be discussed in Sec. 6.6.
Fast camera images of a typical argon jet formed without pre-ionization are shown in
1.3 µs 4.3 µs 7.3 µs 10.3 µs
Outline of inner electrode
Figure 6.1: Imacon camera images of an argon jet formed without pre-ionization. The experiment parameters for this shot were Vmain = 5 kV, Vbias = 80 V, Vgas,inner = 700 V, Vgas,outer = 750 V, andVgas,RF = 0 V.
Fig. 6.1 (see also Fig. 1.5 for images of a typical hydrogen jet), while the evolution of a pre-ionized argon jet is shown in Fig. 6.2. The morphology of the pre-ionized jet is notably different, with a bright, narrow central column near the electrodes and a more diffuse flared structure at the front of the jet. Detailed investigations (see Secs. 6.4.3 and 6.5.2) revealed that the different location of neutral gas injection (through the RF discharge tube output at the center of the inner electrode rather than through the 8 holes located atr= 4.8 cm), rather than the lower initial gas pressure, was primarily responsible for the difference in jet structure between the pre-ionized and non-pre-ionized cases. This result has possible implications for astrophysical jets, suggesting that the intial mass distribution above the accretion disk may be critical for determining the ultimate jet properties.
Although the installation of the pre-ionization system altered the jet morphology, the fundamental driving mechanism was still the axial gradient in the toroidal magnetic field pressure (or equivalently, the hoop force exerted by the poloidal current), and many aspects of the jet evolution were similar to that described in Fig. 1.5. For example, the pre-ionized jets underwent a current-driven kink instability (see Sec. 1.2), as shown in the left panel of Fig. 6.3. The appearance of the kinking laboratory jet is strikingly similar to that of the AGN jet 3C31 shown in the right panel of the figure, and indeed, recent numerical simulations [131] have suggested that the wiggling structures observed in 3C31 and other AGN jets are manifestations of the kink instability. Systematic study of the kinking behavior in our pre-ionized laboratory jets and quantitative comparisons with astrophysical jets have not yet been carried out, however.
2.3 µs 3.3 µs 4.3 µs
5.3 µs 6.3 µs 7.3 µs
Outline of inner electrode
Figure 6.2: Imacon camera images of a pre-ionized argon jet formed with Vmain = 4 kV, Vbias= 80 V, Vgas,RF = 550 V, Vgas,inner = 460 V, and Vgas,outer = 709 V.