27.12 MHz Oscillator
Antenna Capacitive matching
network Transformer Logic
modules
DRF1301 MOSFET Push-Pull
Module
Gating Pulse 300 V in
RF Amplifier
13.56 MHz output UV Lamp
Fast Gas Valve Quartz Tube
Solenoid Alternating 13.56 MHz trigger pulses
Figure 2.1: Block diagram for the RF plasma source, showing the main sections of the RF amplifier and the other key components described in the text.
surface processing of materials used in industry [5]. Usually a plasma source is designed to take advantage of a particular energy transfer mechanism, but in some cases all three coupling regimes have been observed in a single device as the RF input power was varied [54].
A particular class of wave heated discharges that operates with helicon waves has been found to be exceptionally efficient at producing a high plasma density with low input power [51]. The impressive properties of helicon sources [55, 56] made them an attractive candidate to use for pre-ionization in the Caltech experiments, so we designed our RF plasma source with helicon mode operation in mind. However, detailed density scaling measurements (described in Chapter 3) ultimately showed that wave heating was probably not important in our source; instead, the discharge was found to be primarily inductively coupled.
Figure 2.2: Schematic showing the pre-ionization plasma source installed on the MHD- driven jet experiment. Some minor elements of the source such as the coaxial cable leading to the RF antenna and the support structure holding up the gas feed flange have been omitted for clarity. The jet experiment’s electrodes appear as thin rectangles at the right- hand side of the figure in this side-on, cross-sectional view. The anode was attached to the grounded vacuum chamber, while the cathode and all attached components, including the pre-ionization source, charged up to (−3) – (−6) kV when the main capacitor bank was triggered. The RF plasma expanded into the chamber along the background magnetic field through a hole in the center of the cathode, as illustrated in the figure. The magnetic field lines shown were calculated using an IDL program written by Bao N. Ha.
with the aid of ongoing consultations with Professor Bellan. A block drawing of the main components of the RF plasma source is shown in Fig. 2.1, and a 2D CAD drawing of the final jet experiment setup is shown in Fig. 2.2. The coaxial planar electrodes shown in Fig.
1.3 were modified with a 1” diameter hole in the center of the cathode. A custom battery- powered RF amplifier was used to create plasma in a quartz tube behind the electrodes, and it flowed into the chamber through this hole, with radial confinement provided by an axial magnetic field.
The dimensions of the pre-ionization source were chosen so that the entire system could fit behind the cathode with minimal modification to the original plasma gun design [17].
The outer electrode supports, bias coil, and overall size and geometry of the “re-entrant
port” (consisting of a 10” CF flange welded to a 7.50” outer diameter [OD], 7.375” inner diameter [ID] stainless steel tube with a 3/8” thick circular stainless steel plate welded to the opposite end) were all unchanged. The ability to inject gas through 8 inlets on each of the inner and outer electrodes was retained, with the aim of allowing the experiment to access a full range of jet densities between the ∼1022 m−3 achieved using DC Paschen breakdown alone and the 1018–1019 m−3 expected when only the pre-ionized plasma was used.
Three fast gas valves (see Sec. 1.6.4), one each for the inner electrode gas inlets, outer electrode gas inlets, and RF plasma source tube, were used for the experiment. Two of these were mounted on the chamber above the jet experiment gun, with 1/4” OD polyethylene tubing carrying the gas to the appropriate gas feeds, while the gas valve supplying the inner electrode inlets was suspended from the ceiling behind the gun1. Custom stainless steel pieces with Swagelok fittings were used to split a single gas line into eight for each of the inner and outer electrodes. These gas splitters, which were designed so that the flowing gas would never encounter a right angle turn where it would tend to reflect, were built by DV Manufacturing, as was the re-entrant port itself. Gas was delivered to the RF plasma source tube through a 2 3/4” CF double-sided flange (similar to MDC part number 140013) modified with a 1/4” Swagelok tube fitting welded onto one side. This flange was attached to a 2 3/4” flange with a 1” “quick-disconnect” welded on (MDC part number 412010) that slid onto the rear end of the tube as shown in Fig. 2.2.
The 33.0 cm long, 2.54 cm OD, ∼2.2 cm ID quartz discharge tube was blown by Rick Gerhart in the Caltech glass shop. The tube attached to the rear side of the jet experiment’s cathode via a weldable 1-inch quick-disconnect (MDC part number 410010). It had a small raised ring about 1” from its end (known as a “maria” in glassblowing terminology) to prevent it from being pushed through the quick-disconnect into the chamber by the air- vacuum pressure difference.
The solenoid used to produce an axial magnetic field in the discharge tube was wound with Litz wire on a bobbin made of laser-cut 1/4” thick ABS plastic (custom-ordered from Pololu Robotics and Electronics). The solenoid had 78 turns spread over a 20.4 cm length, or n= 382 turns/meter. From Ampere’s law and symmetry, the magnetic field strength inside
1This arrangement was adopted because it was found that argon gas would not break down in the standard jet experiment configuration with no RF pre-ionization if the gas travel distance between the fast gas valve and the inner electrode was too long.
Figure 2.3: Antenna designs tested for the RF plasma source. Left: Half-turn helical
“Shoji” (HTH) antenna [52] wound by hand directly onto the quartz discharge tube using thin copper strips soldered together. Center: Straight “Nagoya type III” (NIII) antenna [57]
constructed from 1/4” OD copper tubing attached to small printed circuit boards. Right:
Single loopm= 0 antenna wound by hand from a copper strip.
when a currentI was passed through the windings was thusB =µ0nI = 4.8 Gauss/Ampere.
The solenoid was powered by a floatable capacitor bank similar to those used to power the bias field coils and fast gas valves (see Sec. 1.5). The bank’s 1.8 mF capacitance was provided by a single aluminum electrolytic capacitor rated to 450 V, and switching was accomplished with an SCR. With the leads between the bank and the solenoid twisted, calibration with a current monitor showed that the relationship between the charging voltage displayed on the bank’s LED meter and the peak current through the solenoid wasIpeak≈ (2.44 A/V)VLED, orBsol. ≈(11.7 Gauss/V)VLED. The current peaked ∼650µs after the bank was triggered.
RF power was transferred to the plasma using an antenna that fit snugly around the quartz discharge tube. We tested three different types of antennas known to excite helicon waves: a straight “Nagoya type III” (NIII) antenna [57], a [right-handed] half-turn helical
“Shoji” (HTH) antenna [52], and a single loop antenna [58, 59]. Photos of the antennas are shown in Fig. 2.3. RF plasmas were successfully produced with all three antennas, but the
Gas feed for RF plasma source UV Flashlamp
Light-Pac Trigger Module for Flashlamp Gas feeds for inner electrode gas inlets
Re-entrant port
Figure 2.4: View from behind the pre-ionized MHD-driven jet experiment gun. The UV flashlamp was mounted on a custom 2 3/4” CF flange assembly (built by Mike Gerfen at Caltech Central Engineering) attached to the end of the quartz tube behind the gas feed—it emitted a burst of light directed down the tube at the beginning of each discharge.
highest level of ionization was obtained with the HTH antenna (see Sec. 3.4), so this design was ultimately used when the pre-ionization source was installed on the MHD-driven jet experiment.
In initial tests of the RF plasma source with argon gas2, plasma breakdown was difficult to achieve, occurring only over a small range of gas pressures near 20 mTorr, and even within this range only after the RF amplifier was pulsed repeatedly for a minute or more with a 1 Hz rep rate. To remedy this problem, an ultraviolet (UV) flashlamp (Excelitas Technologies model FX-1165 Metal Can Xenon Flashlamp with Reflector, powered by Lite- Pac Trigger Module FYD-1150 and Power Supply PS-1120) was attached to the end of the quartz tube behind the gas feed connection (see Fig. 2.4). Firing the UV flashlamp at the time of RF turn-on produced enough seed ionization in the discharge tube to enable full plasma breakdown to occur consistently on every shot, with minimal temporal jitter.
A critical aspect of the RF pre-ionization system design was that all components could float electrically. This was important for safety because the source was installed inside the re-entrant port (see Fig. 2.2), which charged up to−3 to−6 kV along with the jet experi- ment’s cathode when the electrodes were energized3. Floatable operation was accomplished
2The Nagoya type III antenna was used for these tests.
3The re-entrant port was isolated from the grounded vacuum chamber by a ceramic break (MDC part number 9632006), and the electrical connections to the main capacitor bank were made on either side of the break, as shown in Fig. 1.8. Referring to Fig. 2.2, it is notable that the re-entrant port fit tightly through the port on the end dome of the vacuum chamber, with only a small vacuum gap separating the conductors.
by powering the capacitor banks for the bias coil, solenoid, and fast gas valves through a 20 kV isolation power supply, as discussed in Sec. 1.5, and by designing the RF power amplifier and UV flashlamp power supply to run off of AA batteries. The batteries charged up capacitors to store up to∼2 J of energy per pulse for the RF amplifier and 0.16 J per pulse for the UV flashlamp. RF pulse durations of ≤1 ms were quite sufficient given the jet experiment’s∼10µs timescale, so 2 J of stored energy was enough to supply relatively high RF power levels (2–3 kW).
The “ground” references of the floating RF amplifier, capacitive matching network (which was isolated from the RF amplifier by the amplifier’s output transformer, see Sec.
2.3), and floating capacitor banks for the bias coil, solenoid, and fast gas valves were all connected to a single node (to avoid ground loops), and this point was attached to the re-entrant port through a 15 Ω high pulse energy non-inductive resistor (Carborundum Co.
887AS series). The UV flashlamp and its circuit box were left floating, as were the fast gas valves themselves. The 15 Ω resistor value was chosen so that the RC time for the floating circuits to follow changes in the cathode voltage was1µs (the total parallel capacitance to ground of the floating capacitor banks was measured to be∼1 nF), but it was desirable that R not be too small so that the peak current flowing between the cathode and the floating circuits was limited.