S203
Tokamak Operation Without the Use of Center Solenoid CoilYuichi Takase
Graduate School of Frontier Sciences, The University of Tokyo
In conventional tokamak operation, plasma current is ramped up and maintained by the centersolenoid (CS) coil located on the inboard side of the torus. If operation without the use of the CS coil
could be realized, economic competitiveness of the tokamak fusion reactor could be improved byenabling a more compact and higher field design. In particular, this is a crucial requirement for the low
aspect ratio spherical tokamak (ST).
It has long been proven that it is possible to ramp up the plasma current by noninductive current drive.
However, plasmas created in such a way has so far been limited to low density plasmas, and there is adrawback that the time scale for current ramp up is very long, determined by the L/R time. This year, ajoint team consisting of university and JAERI scientists has successfully demonstrated plasma current
ramp-up and achievement of high performance plasma without the use of the CS coil in JT-60U (Fig. 1).
This was accomplished by a novel operation consisting of induction by vertical field and shaping coils,
noninductive current drive (mainly by LHCD), and self-driven (bootstrap) current. This plasma hadfeatures of the advanced tokamak, with high β (poloidal beta βp = 3.6, normalized beta βN = 1.6), highconfinement (enhancement factor over the H-mode scaling HH98y2 = 1.6), and high bootstrap fraction (fBS≥ 90%) at a plasma current of 0.6 MA. The safety factor (q) profile is deeply reversed (characteristic ofa high-bootstrap-fraction plasma with a “current hole”), and there are steep gradient regions near theminimum of q at r/a = 0.7 and at the plasma edge (r/a = 1), indicating the presence of both the internaltransport barrier (ITB) and the edge transport barrier (H-mode). Stability analysis indicates that thisplasma is marginally stable against the n = 1 kink-ballooning mode. This result, combined withpreviously obtained results on JT-60U, demonstrates that it is possible to ramp up the plasma, maintainhigh performance plasma in steady state, and ramp down the plasma in a controlled manner without using
the CS coil.
In this experiment, noninductive ramp-up wasE041711
0.7
1V0 V (V)limited in time by the coil operation limit, but in aI (MA)I-102020
reactor it should be possible to ramp up to higherII
I (kA)I (kA)
0I (kA) 0I
currents by extending the duration of noninductive-10-102(MW)PPcurrent ramp-up. Problems remaining to be solved PP (MW)0include establishment of control methods applicable
4P
15βP(MW)
βto autonomic burning plasmas with high bootstrap P (MW)0P
010TT (keV)current fraction. In particular, it is crucial to developT (keV)
T0a way to restore the burning condition after the10n l (10 m )plasma has lost stored energy due to a β collapse. It02
3
456
789
10is also important to increase βN further by profile
time (s)control and active control, without compromising the Fig. 1. Integrated scenario from plasma start-up
to achievement of advanced tokamak plasmabootstrap fraction or good confinement.
llpp

VT

VRVRFVTFECLHECLHPNBPNBNNBpp

NNBcoreie

coreie19e-2

tangential chordl = 6.3 m (LH config.)
l = 5.3 m (NB config.)

without the use of CS coil.