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

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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

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

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

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 )

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

  • 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

Backup slides

Non-resonant rotation braking produced using n = 2 field

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

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