Advanced Tokamak Research in JT-60U and JT-60SA

Abstract

Results of experiment in JT-60U and design study in JT-60SA (Super Advanced) are described focusing on the development of advanced tokamak. In JT-60U, a high-integrated performance plasma with the normalized beta β N = 2.6, confinement enhancement factor H H98(y,2) = 1.0-1.1 and bootstrap current fraction f BS = 0.4 has been sustained for 25 s (14 times current diffusion time (τ R )). Neoclassical tearing mode (NTM) with the poloidal mode number m = 2 and the toroidal mode number n = 1 has been stabilized with modulated electron cyclotron current drive (ECCD) in synchronization with the mode frequency (∼5 kHz). A high-beta plasma exceeding the ideal MHD limit without conducting wall has been sustained for 5 s (∼3τ R ) by suppressing resistive wall mode (RWM). In addition, two new instabilities in the high-beta regime, Energetic particle driven Wall Mode (EWM) and RWM precursor, have been observed. In JT-60SA, exploration of full non-inductive steady-state operation with current drive by neutral beams and electron cyclotron waves is planned. In addition, NTM control with ECCD and RWM suppression with external coils are planned.

Controlling sawtooth oscillations in tokamak plasmas

“…Provided that the magnetic shear at q = 1 is altered sufficiently to overcome the stabilizing terms in δW in equation ( 9), then the ECCD can affect sawtooth behaviour even when the driven current is small. ECCD has been shown to be a successful sawtooth control actuator in a number of devices including T-10 [138], ASDEX Upgrade [133,[139][140][141], TCV [122,135,142,143], JT-60U [130,144], DIII-D [134], TEXTOR [20,129], Tore Supra [145][146][147], FTU [148] and WT-3 [149]. Sawtooth control can be enhanced by maximizing the local ECCD current density rather than the total driven current at the expense of narrowing the deposition width [139].…”

Power requirements for electron cyclotron current drive and ion cyclotron resonance heating for sawtooth control in ITER

“…Sawtooth destabilization of long period sawteeth induced by ICRH generated core fast ions with energies 0.5 MeV was achieved in Tore Supra, even with modest levels of ECCD power [48,49]. Similarly, ECCD destabilization has also been achieved in the presence of ICRH accelerated NBI ions in ASDEX Upgrade [50] as well as with normal NBI fast ions in ASDEX Upgrade [51] and JT-60U [52]. More recently sawtooth control using ECCD has even been demonstrated in ITER-like plasmas with a large fast ion fraction, wide q = 1 radius and long uncontrolled sawtooth periods in DIII-D [53].…”

Sawtooth control using electron cyclotron current drive in ITER demonstration plasmas in DIII-D

“…Sawtooth destabilization of long period sawteeth induced by ICRH generated core fast ions with energies 0.5 MeV was achieved in Tore Supra, even with modest levels of ECCD power [30,31]. Similarly, ECCD destabilization has also been achieved in the presence of ICRH accelerated NBI ions in ASDEX Upgrade [32] as well as with normal NBI fast ions in ASDEX Upgrade [7] and JT-60U [33]. Despite these promising results, destabilization of so-called monster sawteeth-that is to say sawteeth with periods longer than the energy confinement time, and hence saturated central plasma temperature-in the presence of a significant population of highly energetic particles at high β h (where β h is the fast ion pressure divided by the magnetic pressure) has yet to be demonstrated in ITER-like conditions.…”