Applications and Future Work

3.4 Results and Discussion

3.4.5 Applications and Future Work

Our RF plasma source should be widely applicable to other experiments in which the requirements for Paschen breakdown are incompatible with the desired plasma parameters.

Since the source is battery-powered, it can float at arbitrary voltages in a pulsed power system without risk of damage. The RF amplifier can also be used as a stand-alone power source; the combined weight of the amplifier, matching network, and batteries is ∼ 1 kg, making it well suited for a variety of portable applications. With cooling added as described in [61], the amplifier could be operated as a CW 3 kW RF source, or it may be easily modified to operate at much lower power. This has been done for a small dusty plasma experiment at Caltech [86] that operates with 1-3 W of power capacitively coupled to the plasma. Small, high-density RF plasma sources also have applications in space propulsion [94, 100], industrial surface processing [96], and bioengineering [95].

Several outstanding research questions regarding the pre-ionization source warrant fu- ture study. Additional experiments to investigate the conditions necessary for helicon mode operation would likely yield very interesting results. High-frequency magnetic probes could be used to determine if helicon waves are being excited in the plasma, as in other helicon experiments [51, 57, 54]. The discharge tube radius could be increased to test our hypothesis that the small radius was responsible for inhibiting wave heating of the plasma.

If operation with a 1-inch OD discharge tube is to be studied further, it would be beneficial to design a new experimental setup with better diagnostic access. The solenoid could be replaced by two or more discrete coils (e.g., in a Helmholtz coil arrangement),

and the quartz tube could be broken into several segments with probe feedthroughs in between, as in [87]. A probe inserted through a gap in the side of the tube would perturb the plasma much less than one inserted along the axis and would allow for measurements of radial variations in plasma parameters. If possible, the magnetized region should extend for several times the antenna length in order to study downstream plasma properties and limit the influence of end boundary conditions on the discharge.

With the current experimental setup, more thorough experiments could be done with helium, neon, krypton, hydrogen, and nitrogen, and the global discharge model of Sec. 3.3 could be modified with an appropriate energy level structure and set of rate coefficients for each gas. If good agreement were found between the data and model for these gases, it would further validate our modeling framework for unmagnetized high density RF discharges.

Finally, additional study of the high density ICP regime could prove fruitful. It would be interesting to replace the helical antenna with a multi-turn helical coil for which the model of Sec. 3.4.3 is more directly applicable and compare the plasma properties and source efficiency achieved with the quantitative predictions of generally accepted theories of ICP operation (e.g., [5, Chapter 12]).

Chapter 4

Downstream Transport of the RF Plasma

In order for the RF plasma source to be useful in the MHD-driven jet experiment, the pre-ionized plasma needed to flow down the source tube from the antenna region into the main vacuum chamber. Experiments were carried out to determine the downstream density and the timing of the plasma’s arrival into the region in front of the coaxial electrodes. The transport phenomena observed were surprisingly complex and interesting, with unexpected time and spatial dependences. These behaviors are the subject of this chapter.

A simple picture of plasma expansion is as follows: since quasi-neutrality must be obeyed except at the plasma edges, the electron and ion fluids should flow at the same rate. The kinetic energy per unit volume available for expansion is equal to the sum of the mean electron and ion thermal energies in one direction, which from statistical mechanics is

E/V = 1

2nk_BT_e+1

2nk_BT_i≈ 1

2nk_BT_e, (4.1)

whereT_eT_i was assumed. Meanwhile, when the electrons and ions are flowing outwards together at velocityu, their total kinetic energy density is

E/V = 1

2nm_iu²+1

2nm_eu² ≈ 1

2nm_iu². (4.2)

Equating these two expressions, we find u=

rk_BT_e mi

≡c_s, (4.3)

so the plasma should expand at roughly the ion acoustic speed.

While appealing, this energy argument is too oversimplified to describe reality. In the pressure regime of interest for our experiment, ion-neutral collisions slow down the expansion and make it diffusive in nature. Collisionless plasmas at low pressure likewise do not expand uniformly at the ion acoustic speed; instead, the expansion is self-similar with the low-density leading edge of the plasma moving faster thanc_s and a “rarefaction wave”

propagating backwards into the ambient plasma [101, 102, 103]. Understanding the physics of low-pressure expanding plasmas is important for inertial confinement fusion [101] and also for research on the ionosphere and on wakes created by moving bodies such as planets and satellites in space [102]. In inductively-coupled and wave-heated plasma sources used for integrated circuit fabrication, the wafer is typically located some distance from the source region [5, p. 19], and thus transport of plasma downstream from the source is a critical aspect of the operation of plasma processing tools.

4.1 Time Evolution of Fast Camera Images

The Cooke Corporation ICCD camera described in Sec. 3.1.4 was initially used to confirm that some RF plasma was reaching the main chamber and to determine the best time to trigger the main capacitor bank to create a jet. In these experiments, a fast gas valve (see Sec. 1.6.4) was used to supply gas to the RF discharge tube. Unless otherwise noted, the gas valve power supply was set to V_gas,RF = 550 V, and the delay between the triggering of the fast gas valve and the initiation of the RF discharge wastgas,RF =−6.0 ms. The gas pressure in the tube at this time was not precisely known, but a method was discovered for indirectly determining the pressure by comparing photodiode measurements of the time- dependent light emission during these experiments with measurements taken during shots with a known uniform gas pressure in the tube. Details are given in Sec. 5.1. The inferred pressure withV_gas,RF = 550 V andt_gas,RF =−6.0 ms was p_Ar ≈460 mTorr (see Fig. 5.3).

The half-turn helical (HTH) antenna, which was used for all experiments described in this chapter, was mounted with its front end located 5.9 cm behind the plane of the electrodes (i.e., atz=−5.9 cm). Therefore, if the RF plasma flowed out of the source tube along the bias magnetic field at the ion acoustic speed, it would have emerged from the tube roughly 27 µs after the source transitioned to the high-density inductively coupled (ICP)

mode (assuming Te≈2 eV, so cs ≈2.2 km/s). After 120µs, the plasma would have filled the region within∼20 cm of the center electrode, and it would have been a good time to trigger the main capacitor bank. However, the actual behavior observed was very different from this. Examples for three different RF plasma source configurations are shown in Figs.

4.1, 4.2, and 4.3.

In all three image series, the plasma expansion into the chamber appears to be unsteady, with two or more phases in which different behaviors are evident. A region of bright emission appears in front of the RF plasma source tube output within 50µs after RF turn-on—the spatial extent of this region is greater for cases with the bias coil and solenoid turned on¹ (Figs. 4.1 and 4.2) than for the case shown in Fig. 4.3 with no magnetic field. It appears in frames 2–3 of Figs. 4.1 and 4.2 that the emitting region is expanding away from the source tube and into the chamber; however, around t = 150 µs it rapidly dims. In Fig. 4.1, in which the RF power was turned off att= 200µs, a new cloud of plasma emerges from the source tube at around this time, expanding far into the chamber roughly following the bias magnetic field lines and gradually dimming at late times. In Fig. 4.2, on the other hand, in which the RF power was left on until t = 400 µs, no plasma expansion is visible from t= 200–400µs—the expansion appears to be inhibited somehow by continued application of RF power back at the antenna region. Meanwhile, in Fig. 4.3, which also used a 400µs RF pulse but hadB = 0, an expanding cone of plasma is readily visible fromt= 200–300µs.

In both Figs. 4.2 and 4.3, an additional relatively dim puff of plasma appears to emerge from the source tube soon after the RF power is turned off. In all cases in which a cloud of plasma appears be expanding into the chamber, the velocity inferred from the images is on the order of 1 km/s.