Experiments on Helicons in DIII-D: Investigation of the Physics of a Reactor-Relevant Noninductive Current Drive Technology

Full text: Experiments have begun in DIII-D to evaluate noninductive current drive by the Landau absorption of a toroidally-directive spectrum of helicon waves (also known as ^very high harmonic fast waves^{, f}ast waves in the lower hybrid range of frequencies^, or ^whistlers⁾. Modelling has shown that noninductive current drive at midradius (p∼0:5) should be achievable in DIII-D with fast waves at 0.5 GHz, with an efficiency twice as high as with noninductive current drive tools currently available on DIII-D (neutral beams and electron cyclotron current drive) in high-β conditions. An innovative Travelling Wave Antenna (TWA) of the "comb-line" type with 12 radiating modules has been constructed, installed in DIII-D, and is currently being tested at very low power (< 1 kW) to evaluate the antenna coupling in the linear regime, and to prototype technological aspects of such structures in the tokamak environment. Preliminary results indicate strong antenna-plasma coupling, with detailed 3D modelling underway to quantitatively compare the measurements with theoretical expectations. A key input to this model is the edge and far SOL electron density profile, which is being measured with a microwave reflectometer and with fixed and movable Langmuir probes. An important issue for wave coupling in this regime is the degree to which (undesired) quasi-electrostatic slow waves are excited by the structure; evaluation of this is a point of emphasis in the ongoing work. A high-power system is presently being prepared for installation later in 2016 in which a single 1.2 MW klystron at 476 MHz will be used to power a TWA with ∼36 radiating elements in a structure 2 m wide. The goals of the high-power experiments include evaluation of nonlinear effects on excitation of the desired waves (ponderomotive effects, parametric decay) and measurements of the deposition profile and of the current drive efficiency. Ray-tracing predicts an RF-driven current of ∼60 kA per coupled MW of helicon power, which should result in an easily measurable driven current in DIII-D in high-β discharges. (author)