Off-axis Current Drive and Current Profile Control in JT-60U T. Suzuki, S. Ide, T. Fujita, T. Oikawa, M. Ishikawa, G. Matsunaga, M. Takechi, M. Seki, O. Naito, K. Hamamatsu, M. Sueoka, H. Hosoyama, M. Nakazato, and the JT-60 Team Japan Atomic Energy Agency 21th IAEA Fusion Energy Conference (19 Oct. 2006, Chengdu) EX/6-4 (17+3min.) This work was partially supported by Grand-in-Aid for Young Scientists (B) No and Grand-in-Aid for Scientific Research No and from MEXT.

Introduction Optimization or control of q/j profile is crucial.  For higher confinement and stability such as in ITER SS & hybrid scenarios  In a self-organized plasma, real-time feedback control is essential.  Minimum but efficient control is preferable. Minimum q (q min ) is one of the key parameters for stability at high  and resulting confinement.  Raising q min can avoid low m/n MHD, such as NTM and sawtooth. For the q min control, off-axis current drive is required.  LHCD & NBCD

Contents Current drivers and MSE diagnostic for current profile measurement Real-time control of q min using off-axis LHCD in high  plasmas Measurement of beam driven current profile by off-axis NBCD Summary

Off-axis NBCD  2MW/2MW for co/ctr  Investigated here Off-axis Current Drivers and high resolution MSE diagnostic in JT-60U LHCD (2GHz, <1.5MW for co)  Continuous control of power and N // possible at every 10ms  Used as current driver for real-time control of q min q(r) measurement using 30ch MSE  dR~ m (a~1m)  at every 10ms for real-time calc.

Real-time control of q min using off-axis LHCD in high  plasmas

In 2006, q min is controlled in real-time using LHCD in high  plasmas In 2004, q profile control at low  demonstrated  Real-time evaluation of q(r) using MSE  Real-time control of CD location through N // In 2006, q min control at high  for better core performance  Control of key parameter at high  : q min  Real-time control of LH power: P LH  I LH  N // is fixed to keep CD location at higher T e region for better CD efficiency.  LH power control: dP LH /dt=-  (q min - q min,ref )   : positive constant  LH power increases when q min < q min,ref.

Real-time q min control demonstrated at  N = , f BS =0.46, f CD =0.87 High-  p ELMy H-mode  I p =0.8MA, B t =2.5T, q 95 =5.8, n e =1.8x10 19 m -3,  p = q min followed the given reference q min,ref.   =2MW/s Without control, q min goes down to  1.7 interlocks by arcing N // =1.7 q min raised q min =1.7

Complex behavior in current profile evolution during q min control in high  LHCD spatially localized at  =  decrease j by reduction of j OH inside  =0.3 Complex interaction between j, p, and T e (or  NC ) at high  during q min control q(r) change through q min control  T e increases  j OH  NC and j BS increase  q min decreases  Control system increases P LH contour plot j increase j LH j BS +j OH q min control

m/n=2/1 NTM in high-  p mode plasma at  N ~1.7,  p =1.5 I p =0.8MA, B t =2.4T, q 95 =5.4  22% decrease in W dia q min rises above 2 overshoot of control during q min control to 1.7  m/n=2/1 NTM disappears  W dia recovers  Central n e, T e, T i increase.  No change in peripheral MHD stabilization and confinement recovery by q min control in high  m/n=2/1 NTM confinement recovery

Measurement of beam driven current profile by off-axis NBCD

Off-axis NBCD investigation for real-time current profile control Off-axis NBCD has been known to drive current, raise central q and decrease V loop in JT-60U.  working as one of standard off-axis current drivers in JT-60U But, off-axis NBCD “profile” has not been fully recognized, yet.  difficult to measure, veiled by large Ohmic current. Important to know its character in order to use for optimization and real-time control of j(r).  NBCD profile measurement exploiting  progress in diagnostics (high resolution MSE, neutron emission profile)  flexible NBI system (perp. NB w/o NBCD)  improvement in analysis technique typical configuration for off-axis NBCD

Spatially localized off-axis NBCD profile is measured using MSE diagnostic. I p =1.2MA, B t =3.8T, q 95 =5.4, fixed n e =3.3x10 19 m -3, ELMy H-mode, No MHD except ELMs Temporal evolution of increment of current  I from reference current profile, measured by MSE  spatially localized NBCD at  = Off-axis beam driven current profile j BD dS agrees with increase in neutron emission S n (r) representing beam population profile.  th-th neutron < 10%, bulk density constant tangential minor radius  min of diag. sight line current increase (reference)

Measured I BD agrees with calc., but CD location in calc. shifts inward than meas. I p =0.8MAI p =1.2MA Measured off-axis NBCD is spatially localized both at I p =0.8&1.2MA.  consistent with neutron emission profile Peak CD location in calc. shifts inward than exp. by d  ~0.2 in Ip=0.8&1.2MA.  But, total NBCD current in exp. agrees with calc. in Ip=0.8&1.2MA. Difference between CD locations in exp. and calc.  further study required 127  32kA 103kA 106  41kA 90kA

Summary Real-time control of q min was demonstrated in high  plasma having  N = , f BS =0.46, f CD =0.87.  q min was evaluated by MSE, and current profile was modified by off- axis LHCD at  =  q min followed the given reference rising in time.  Complex interaction between j, p and T e was observed. NTM (m/n=2/1) was stabilized through q min increase above 2 during q min control, and confinement was recovered.  Development of current profile control system is important for higher confinement and stability in advanced scenarios in ITER. Spatially localized off-axis NBCD is possible.  Measured CD location agrees with neutron emission profile.  Measured total driven current agrees with the ACCOME calc.  CD location in the ACCOME code shifts inward than measurement by d  ~0.2.  Some mechanism is required for the explanation.  further study