Research advances to understanding mode stabilization physics and reliably maintaining the high beta plasmas

Research advances to understanding mode stabilization physics and reliably maintaining the high beta plasmas

• Motivation

• Maintenance of high N with sufficient physics understanding allows confident extrapolation to ITER and CTF

• Outline

• Active control of beta amplified n = 1 fields / global instabilities
• Mode dynamics and evolution during active control
• Control performance compared to theory, connection to ITER
• Kinetic effects on resistive wall mode (RWM) stabilization
• Non-axisymmetric field influence on plasma rotation profile

NSTX equipped for passive and active RWM control

• Stabilizer plates for kink mode stabilization

• External midplane control coils closely coupled to vacuum vessel

• Varied sensor combinations used for feedback

• 24 upper/lower Bp: (Bpu, Bpl)
• 24 upper/lower Br: (Bru, Brl)

Active RWM control and error field correction maintain high N plasma

Probability of long pulse and <N>pulse increases significantly with active RWM control and error field correction

• Standard H-mode operation shown

• Ip flat-top duration > 0.2s (> 60 RWM growth times)

During n=1 feedback control, unstable RWM evolves into rotating global kink

• RWM grows and begins to rotate

• With control off, plasma disrupts at this point
• With control on, mode converts to global kink, RWM amplitude dies away
• Resonant field amplification (RFA) reduced

• Conversion from RWM to rotating kink occurs on w timescale

• Kink either damps away, or saturates

• Tearing mode can appear during saturated kink

Soft X-ray emission shows transition from RWM to global kink

Experimental RWM control performance consistent with theory

• VALEN code with realistic sensor geometry, plasmas with reduced V

Significant N increase expected by internal coil proposed for ITER

• 50% increase in N for RWM stability

Stronger non-resonant braking at increased Ti

• Examine Ti dependence of neoclassical toroidal viscosity (NTV)

• Li wall conditioning produces higher Ti in region of high rotation damping

• Expect stronger NTV torque at higher Ti (-d/dt ~ Ti5/2 )

• At braking onset, Ti ratio5/2 = (0.45/0.34)5/2 ~ 2
• Consistent with measured d/dt in region of strongest damping

Advances in global mode feedback control, kinetic stabilization physics and magnetic braking research

• Active n = 1 control, DC n = 3 error field correction maintain high N plasma over ideal Nno-wall limit for long pulse

• Growing RWM converts to kink that stabilizes; can yield tearing mode

• Control performance compares well to theory

• Significant N increase expected for ITER with proposed internal coil

• Kinetic modifications to ideal stability can reproduce behavior of observed RWM marginal stability vs. V

• Simple critical rotation threshold models for RWM stability inadequate

• Non-resonant V braking observed due to n = 2 applied field

• Braking magnitude increases with increased Ti

Backup slides

Non-resonant rotation braking produced using n = 2 field

• n = 2 has broader braking profile than n = 3 field (from field spectrum)