Active Stimulation of the Quasi Coherent Mode by External Antenna - - PowerPoint PPT Presentation

Active Stimulation of the Quasi Coherent Mode by External Antenna

• T. Golfinopoulos, B. LaBombard, R.R. Parker, W. Burke, R. Granetz, M.

Greenwald, R. Leccacorvi, R. Vieira, S. Wolfe, S. Wukitch et al.

◮ Quasi Coherent Mode (QCM) believed responsible for

favorable outward transport during EDA H-modes

◮ QCM has ˜

B ≈ 5 G around midplane ⇒ perhaps opportunity to excite mode via inductively-coupled antenna

◮ Visualize QCM with current filaments following field lines with

alternating direction - design antenna to mimic this geometry. Goals

◮ Couple power from external antenna to QCM ◮ Attempt to strongly drive marginally stable mode ◮ Modify transport at edge, extend EDA H-Mode conditions ◮ QCM physics ◮ Mode-locking - antenna+mode ⇔ rotor+stator, AC motor,

chance to impart torque?

Antenna Design

(a) (b) Figure: (a) Antenna design Jan. ’11, Src.: R.

Leccacorvi; Concept: B. LaBombard; (b) Wiring schem. for QCM sys. M⇒matching network, f -dependent

◮ La-doped Mo wire strung

back and forth.

◮ Rapid radial decay

∝ e−kθz ⇒close to plasma

◮ Excited by variable-freq.

source, 1 kW amp. or greater.

◮ Sweep frequency across

range of QCM; detect resonances, f response.

◮ Dwell at fixed f , mod. signal

with slow square wave.

◮ Bandwidth 50-300 kHz so

WCM may also be targeted.

Run Time

◮ Piggyback 0.1 s or less at end of flattops, especially for EDA

H-mode runs - allows us to assess antenna loading with plasma and calibrate matching network.

◮ Dedicated runs - start with Ohmic EDA H-mode plasmas

described in Snipes2001 (QCM is well-characterized for these plasmas):

• 1. Low-field (3.2 T) to reduce H-mode threshold
• 2. Ramp up field (4.2 T) to achieve desired q95 > 3.5 for QCM
• 3. Slowly reduce L-mode target density below

¯ ne = 1.3 × 1020 m−3 to make QCM marginally stable

• 4. Drive antenna, swept f and constant f with square-wave mod.

◮ Diagnostics: Fast Magnetics, PCI, GPI, Reflectometer,

scanning probes (Langmuir+magnetic), HIREX Sr, Lyman-α