Towards steady state tokamak operation with double transport barriers

Abstract

Core confinement and the overall performance of the tokamak have been considerably enhanced with the combination of two improved confinement regimes. Internal transport barriers characteristic of the optimized shear regime and an edge transport barrier of the high confinement H mode regime have been superposed in the double barrier (DB) mode. In DT discharges the DB mode has resulted in a fusion gain Q a factor of 2 higher than in conventional sawtoothing steady state ELMy H mode plasmas. The DB mode has been routinely established in the new Gas Box divertor configuration on JET. Off-axis lower hybrid current drive and edge radiation cooling through impurity seeding have been used for current and pressure profile control. This has made it possible to increase the performance up to βN = 2.4 and H 89 = 2.7 and to extend flat-top phases up to a duration of three energy confinement times, approaching steady state conditions. Modelling studies of DB mode DT operation on JET predict a fusion power in the range 20-30 MW. In ITER transport code modelling, steady state operation with a fusion power output of 1.3 GW is obtained for advanced tokamak scenarios in the DB mode.

Internal transport barriers in tokamak plasmas*

“…Correspondingly, Q eq DT between, for instance, 0.3 in DIII-D and 1.2 in JT-60U have been achieved transiently. In TFTR (Bell et al 1997) and JET (Gormezano et al 1998) ITBs have been produced using the relevant fuel mixture of deuterium and tritium for a burning fusion plasma, resulting in actual Q DT values of 0.4 in JET (Söldner et al 1999).…”

“…In JET, ITBs have been combined with L-and H-mode edge conditions. The latter include edge transport barriers with both Type-III and Type-I ELMs and have been termed double transport barriers or double barrier (DB) modes (Söldner et al 1999, JET Team (presented by F X Söldner) 1999). However, long sustainment of ITBs to date was only possible with the smaller Type-III ELMs, but at the expense of a lower pedestal pressure.…”

Dynamics of the formation, sustainment and destruction of transport barriers in magnetically contained fusion plasmas*

“…Multiple barriers can form in the plasma as a result of the co-existence of the edge barrier with various forms of ITBs, e.g., quiescent double barrier (QDB) plasmas in DIII-D [9], multiple barrier plasmas in JET [10], high _p H-mode and RS H-mode plasmas in JT-60U [11,12]. Also, ITBs may form for only certain combinations of ions, electrons, toroidal momentum and particle barriers in contrast to edge transport barriers (ETBs) which typically have clear and well-formed barriers for all the above quantities simultaneously.…”

Real-time control of internal transport barriers in JET*

“…A previously developed optimized shear regime with low or weakly negative magnetic shear in the plasma centre [3,12,13] has been extended to noticeably negative central magnetic shear configurations [8,[14][15][16][17]. The q-profile has been varied using mainly the LHCD during the initial current ramp-up phase of the plasma discharge, prior to the application of the high-power heating.…”