Off-axis Current Drive and Current Profile Control in JT-60U

1. 21th IAEA Fusion Energy Conference (19 Oct. 2006, Chengdu) EX/6-4 (17+3min.) Off-axis Current Drive andCurrent 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 This work was partially supported by Grand-in-Aid for Young Scientists (B) No. 18760650 and Grand-in-Aid for Scientific Research No. 18560798 and 16082209 from MEXT.

2. Introduction • For the qmin control, off-axis current drive is required. • LHCD & NBCD • 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 (qmin) is one of the key parameters for stability at high b and resulting confinement. • Raising qmin can avoid low m/n MHD, such as NTM and sawtooth.

3. Contents • Current drivers and MSE diagnostic for current profile measurement • Real-time control of qmin using off-axis LHCD in high b plasmas • Measurement of beam driven current profile by off-axis NBCD • Summary

4. Off-axis Current Drivers and high resolution MSE diagnostic in JT-60U • Off-axis NBCD • 2MW/2MW for co/ctr • Investigated here • q(r) measurement using 30ch MSE • dR~0.06-0.1m (a~1m) • at every 10ms for real-time calc. • 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 qmin

5. Real-time control of qmin using off-axis LHCD in high b plasmas

6. In 2006, qmin is controlled in real-time using LHCD in high b plasmas • In 2006, qmin control at high b for better core performance • Control of key parameter at high b: qmin • Real-time control of LH power: PLH  ILH • N// is fixed to keep CD location at higher Te region for better CD efficiency. • LH power control: dPLH/dt=-a(qmin - qmin,ref) • a: positive constant • LH power increases when qmin < qmin,ref. • In 2004, q profile control at low b demonstrated • Real-time evaluation of q(r) using MSE • Real-time control of CD location through N//

7. Real-time qmin control demonstrated at bN=1.3-1.6, fBS=0.46, fCD=0.87 1.31.7 qmin raised • High-bp ELMy H-mode • Ip=0.8MA, Bt=2.5T, q95=5.8, ne=1.8x1019m-3, bp=1.2-1.5. • qmin followed the given reference qmin,ref. • a=2MW/s • Without control, qmin goes down to 1.3. N//=1.7 interlocks by arcing qmin=1.7

8. Complex behavior in current profile evolution during qmin control in high b jLH jBS+jOH • LHCD spatially localized at r=0.3-0.5 • decrease j by reduction of jOH inside r=0.3 • Complex interaction between j, p, and Te(or sNC) at high b during qmin control q(r) change through qmin control  Te increases  jOHsNC and jBS increase  qmin decreases  Control system increases PLH j increase contour plot qmin control

9. MHD stabilization and confinement recovery by qmin control in high b confinement recovery m/n=2/1 NTM • m/n=2/1 NTM in high-bp mode plasma at bN~1.7, bp=1.5 Ip=0.8MA, Bt=2.4T, q95=5.4 22% decrease in Wdia • qmin rises above 2 overshoot of control during qmin control to 1.7 m/n=2/1 NTM disappearsWdia recovers • Central ne, Te, Ti increase. • No change in peripheral

10. Measurement of beam driven current profile by off-axis NBCD

11. Off-axis NBCD investigation for real-time current profile control • Off-axis NBCD has been known to drive current, raise central q and decrease Vloop 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

12. Spatially localized off-axis NBCD profile is measured using MSE diagnostic. current increase • Ip=1.2MA, Bt=3.8T, q95=5.4, fixed ne=3.3x1019m-3, ELMy H-mode, No MHD except ELMs • Temporal evolution of increment of current dI from reference current profile, measured by MSE spatially localized NBCD at r=0.65-0.8 • Off-axis beam driven current profile jBDdS agrees with increase in neutron emission Sn(r) representing beam population profile. • th-th neutron < 10%, bulk density constant (reference) tangential minor radius rmin of diag. sight line

13. Measured IBD agrees with calc., but CD location in calc. shifts inward than meas. 10641kA 12732kA 90kA 103kA Ip=1.2MA Ip=0.8MA • Measured off-axis NBCD is spatially localized both at Ip=0.8&1.2MA. • consistent with neutron emission profile • Peak CD location in calc. shifts inward than exp. by dr~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

14. Summary • Real-time control of qmin was demonstrated in high b plasma having bN=1.3-1.6, fBS=0.46, fCD=0.87. • qmin was evaluated by MSE, and current profile was modified by off-axis LHCD at r=0.3-0.5. • qmin followed the given reference rising in time. • Complex interaction between j, p and Te was observed. • NTM (m/n=2/1) was stabilized through qmin increase above 2 during qmin 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 dr~0.2. • Some mechanism is required for the explanation.  further study