Speaker
Robert I. Pinsker
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1065.pdf
Design Considerations and Research and Development of a Comb-line
Traveling Wave Antenna for Helicon Current Drive in DIII-D*
R.I. Pinsker1, C.P. Moeller1, J.S. deGrassie1, M.W. Brookman1, A. Nagy2,
H. Torreblanca1, R.C. O'Neill1 and M. Porkolab3
1
General Atomics, P.O. Box 85608, San Diego, CA 92186-5608, USA
2
Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08543-0451, USA
3
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
A project to demonstrate high-efficiency off-axis current drive intends to couple 1 MW of
power at 476 MHz to DIII-D plasmas in the fast wave polarization, also known as the whistler
or helicon wave, to enable a proof-of-principle experiment on helicon current drive [1]. A
traveling wave antenna of the comb-line type is in the final stages of design for installation in
the DIII-D tokamak in late 2018. The antenna consists of a toroidal array of 30 modules, each
5 cm wide, so that the array is 1.5 m wide. Power is fed from one end of the array to generate
a wave traveling in one toroidal direction at a value of n|| = 3 to drive current non-inductively.
A 12-element prototype comb-line was operated at low power (< 0.5 kW) in DIII-D in 2016,
where it was demonstrated that the plasma-antenna coupling was adequate to transfer at least
75% of the power to propagating helicon waves in the plasma, rather than being resistively
dissipated in the structure or coupled out of the antenna at the 'downstream' end [2]. The
scaling of the measured coupling efficiency for the low-power prototype to the high-power
antenna depends crucially on two factors: the resistive losses in each element and the strength
of the mutual reactance between adjacent elements. In this contribution we quantify the
importance of these parameters in determining the optimum number of modules for a wave-
launching structure of this kind. To assess the significance of multipactor discharge in the
antenna and feed structures and to qualify the module design for operation at high electric
fields, a test stand has been constructed with up to 0.1 T dc magnetic field available and with
more than 10 kW of rf power in the operating band of the antenna (~0.5 GHz). Initial testing
of one quarter of one module at high 'Q' has already demonstrated conditioning out of
multipactor in the module, and investigation of the effect of the magnetic field on multipactor
and on high-voltage standoff has begun. In the near future, we will use the test stand to
qualify the vacuum transmission line that will convey 1 MW of power from the feedthroughs
to the input end of the comb-line antenna and to investigate the effect of anti-multipactor
coating techniques.
[1] R. Prater, C.P. Moeller, R.I. Pinsker, et al., Nucl. Fusion 54, 083024 (2014)
[2] R.I. Pinsker, et al., IAEA Conference EX/P3-22 (2016)
__________________________________
*Work supported in part by the US Department of Energy under DE-FC02-04ER54698 and DE-AC02-
09CH11466.
