by C. P. Moeller General Atomics, San Diego, California , USA RF Launchers That Survive Presented at the Renew theme III workshop at UCLA, March 2-6,2009 This work was performed under contract DE-AC03-97ER with the U.S. Department of Energy.

The Corresponding White Paper: The White Paper on this topic has the title and authors listed below. The title of this presentation was provided by Rick Temkin, but the bias is mine, and the figures are from Fusion Science and Technology, Vol 53, pp ECH for DEMO M. A. Shapiro1, J. R. Sirigiri1, R. J. Temkin1, D. A. Rasmussen2, R. L. Ives3, J. Lohr4 and C. Moeller4 1. MIT Plasma Science and Fusion Center, Cambridge, MA 2. Oak Ridge National Laboratory, Oak Ridge, TN 3. Calabazas Creek Research, Inc., San Mateo, CA 4. General Atomics, San Diego,

ECH vs Other Wave Heating ECH is the only wave heating scheme that requires no coupling structure near the plasma. In principle, only an aperture in the vacuum vessel wall and a path to the outside is required. In reality, the “aperture” is thick and becomes part of a waveguide subject to heating, coating, and neutrons. If the aperture is an oversize corrugated waveguide, however, the losses are low even when the surface conductivity is degraded.

Controlled Deposition ECH can provide heating or current drive at a chosen flux surface by either poloidal steering or frequency tuning. If widely tunable gyrotrons are developed, the launcher can be just a waveguide, unless focusing is required. For a fixed frequency, steering and focusing are presently accomplished ( in ITER, for example) by fixed and movable plasma facing mirrors. The full survival potential of the ECH launcher could be realized if the internal mirrors were eliminated.

Steering Mirror Challenges Removal of heat due to RF dissipation, plasma heating, and neutron absorption. Increase in dissipation due to coating by wall material. Survival of the flexible cooling lines and bearings in the Demo environment. It is possible to avoid these challenges by removing the mirrors from the vacuum vessel.

Remote Focusing-Remote Steering Schematic As shown, the steering angle is ±13˚, but >±20˚ is possible with enhancements.

Why Was Remote Steering Abandoned for ITER? The selected port still required fixed steering mirrors which would be subjected to high power density RF and plasma heating. The spot size in the plasma was too large without a focusing mirror, but focusing reduces the steering range. The steering range was not sufficient.

Improved Focusing Without Mirrors The remote focusing from a single aperture is limited by diffraction. With coherent sources, the effective aperture can be enlarged.

Superposition of 3 Tilted Beams

Evolution of Transverse Profile with Distance from Launcher for 3 Beams

Phase Locked Oscillators A coherent array of launchers requires that several gyrotron oscillators be phase locked to each other. A general theory of phase locking of self excited oscillators has been developed by J.C. Slater, and demonstrated with magnetrons. The required relative locking amplitude is proportional to Q ext Δω/ω. NRL demonstrated phase locking of a gyrotron at 35 GHz using a TWT. In the present case, the gyrotrons would simply lock to each other. The essential hardware component is a directional coupler for the HE11 mode, having a suitable coupling factor (-20 to -30 dB), to mutually couple the oscillators to each other.

Curved Waveguide Launchers The use of fixed mirrors following the RS launcher complicates the design, because the beam from the launcher scans the mirror. The mirrors are necessary both to redirect the beam and as neutron shields. If the launcher were curved poloidally, it would provide neutron shielding, and only one cylindrical mirror would be need for toroidal redirection. A 20˚ poloidal bend would be suitable.

Items Requiring Development for ECH For internal steering mirrors: robust mirrors, flexible cooling lines, and bearings. For ECH without internal mirrors: – Phase locked gyrotrons for beam focusing – Curved waveguide launchers to reduce neutron flux – Frequency tunable gyrotrons – Enhanced remote steering: Develop a tapered waveguide launcher Make the input launch position variable with angle