RF current drive antenna. Final report, August 15, 1993--August 14, 1995 Page: 4 of 58
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The major part of the results obtained in this project are reported in the paper "Design and
Initial Tests of the Stacked Stripline Antenna in the Phaedrus-T Tokamak" which has
been accepted for publication in Nuclear Fusion. A copy of that paper is attached to this
report. We present in the next few sections some additional results, and especially
descriptions of technology developed along the way to achieving our main results. These
technology developments include: 1)Development of a high current multiconductor
feedthrough, 2)Development of high current matching capacitors, 3)Development of a
load-insensitive external decoupling circuit. We describe these briefly below.
High Current Feedthrough
In Fig. 1, we show a rough sketch of the RF antenna geometry in Phaedrus-T. The
Stacked Stripline (SSL) antenna in Phaedrus-T was designed for a very low frequency, 7
MHz, due to the nature of the Alfv6n wave current drive experiments. This, plus the
desire to maintain- maximum flexibility, dictated that the resonating capacitors be placed
outside the machine's vacuum chamber. This posed a technical challenge, because the
circulating current of the antenna's 10 straplets adds up to over 10 kA for normal
operation at the few hundred kilowatt level. In order to keep the losses within bounds,
the multiple foil feedthrough shown in Fig. 2 as developed. It uses vacuum autoclaved
epoxy-fiberglass composite as the sealing material. Despite concerns that this would pose
a risk to the vacuuiiquality, we routinely obtained base pressures of 5x10~ Torr, as good
as we obtained before the feedthrough was installed. We note that this feedthrough, using
its multiple foil construction, presented an additional inductance in the circuit that was
negligible compared to the antenna inductance.
High Current Matching Capacitors
The multiple foil construction of the feedthrough was continued to the matching
capacitors, providing essentially an independent connection to each straplet. The
inductance of each straplet was about 9 nHy, which implies at 7 MHz a resonating
capacitance of 57000 pF per straplet. This is an extremely large capacitance as RF
capacitors go, and finding capacitors this large and with inductances small compared to 9
nHy was a challenge. Though the operating voltage was less than 500 volts, the currents
were large, upwards of 1000 amperes each. Our first attempt at meeting these
requirements was to use commercially available "reconstituted mica" capacitors available
from the Cornell Dubilier as kVx brand capacitors. We paralleled 10 such capacitors for
each straplet, operating them at 100 amperes each. Unfortunately, their internal
construction consisted of a plasma sprayed bond from the foil edges to the leads, and this
joint formed a series arc at high currents and destroyed the units after several shots. It
became clear that a custom design was necessary.
We built our own capacitors by winding .005 inch thick copper foils sandwiching .003
inch thick polypropylene dielectric, and the leads were formed by directly leading the
foils tangentially out from the sides of the rolled units, so that there were no joints to fail.
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Probert, P.H. RF current drive antenna. Final report, August 15, 1993--August 14, 1995, report, September 1, 1995; United States. (digital.library.unt.edu/ark:/67531/metadc668932/m1/4/: accessed December 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.