On hybrid scenarios in KSTAR

Abstract

We report the status of hybrid scenario experiments in Korea Superconducting Tokamak Advanced Research (KSTAR). The hybrid scenario is defined as stationary discharges with β N ⩾ 2.4 and H 89 ⩾ 2.0 at q 95 < 6.5 without or with very mild sawtooth activities in KSTAR. It is being developed towards reactor-relevant conditions. High performance of β N ≲ 3.0, H 89 ≲ 2.4 and G-factor (≡ β N H 89 /q 2 95 ) ≲ 0.46 has been achieved and sustained for ≳ 40τ E at n e /n GW ~0.7 with heating power of ≲5 MW. Some KSTAR hybrid discharges exhibit a unique feature of a slow transition from conventional H-mode to hybrid mode after the third neutral beam injection. The reason for the confinement enhancement is extensively studied in this transition period of a representative discharge exhibiting a common feature of KSTAR hybrid scenarios. 0D performance analysis with magnetohydrodynamic activities, 1D kinetic profile dynamics, power balance analysis, linear gyro-kinetic analysis and edge pedestal stability analysis were conducted. The enhancement is thought to be from both the core and the pedestal. The improvement in the core region of the ion energy channel is observed from the linear gyro-kinetic analysis considering the electromagnetic, the fast ion, the Shafranov shift, ω E×B , and the magnetic shear effect. The electromagnetic finite β stabilisation plays a role in the inner core region at ρ tor ∼ 0.35 together with the fast ion effect. The alpha stabilisation effect is also found at ρ tor ∼ 0.5. ω E×B , which could reduce the linear growth of the ion temperature gradient mode in the outer core region at ρ tor ∼ 0.5 − 0.7 with the highest contribution from the toroidal rotation. Regarding the improvement in the pedestal, Shafranov shift broadens the stability boundary of the pedestal in support of the diamagnetic effect. The pedestal height and width could be reproduced by the EPED model, while a realistic current profile is used to calculate the internal inductance for Shafranov shift. Based on these findings, a comprehensive confinement enhancement mechanism has been proposed by considering the core-edge interplay.

A sustained high-temperature fusion plasma regime facilitated by fast ions

“…Therefore, one can conjecture that this ITB is strongly correlated with fast ions in the core region of the plasma. As previously reported, fast ions can stabilise or mitigate microturbulence 8,[47][48][49][50] such that the transport of thermal ion energy can be reduced at the core region where fast ions are populated.…”

“…H-mode is considered the reference scenario to produce a fusion power of 500 MW with a fusion gain Q = 10, implying a fusion power ten times higher than the input heating power, in the International Thermonuclear Experimental Reactor (ITER) 4 , which is the world's largest tokamak being built in France under a collaboration between China, the EU, India, Japan, the Republic of Korea, Russia, and the U.S.A. [5][6][7] . Various advanced tokamak operation modes such as hybrid mode 8,9 , high poloidal beta mode 10 , and high q min mode 11 are based on this H-mode. H-modes can satisfy the high con nement and avoid impurity accumulation for a long pulse duration 12,13 , but their large pressure gradient at the edge due to ETBs triggers a signi cant edge plasma instability, the so-called edge localised mode (ELM), which can severely damage the inner wall of the device 14,15 .…”

“…We can evaluate the origin of the enhancement of the FIRE mode by comparing it with another con nement mode that shows comparable performance. A 'hybrid mode' is selected for this comparison 8 , which can be categorised into H-modes but with higher performance. It is an alternative high-performance scenario for substituting the H-mode in ITER for engineering tests of reactor-relevant components 9 and is considered a candidate operation scenario for DEMO 41,42 .…”

“…Notably, the effect of ExB ow shear or zonal ows, breaking turbulence eddies and reducing the amplitude of the turbulence uctuations, is also active in this regime, which can contribute to triggering the ITB [54][55][56][57] , but thorough analyses are needed with nonlinear simulations in the future. The fast ion stabilisation effect also plays a role in the hybrid mode 8,49 , however, the effect is much lower due to the smaller fraction of fast ions though the hybrid mode shows similar fusion performance with the FIRE mode. Since this core improvement is comparable to the edge improvement in the hybrid mode, they show similar fusion performances.…”

A new regime that “FIRE”s fusion plasmas for long sustained and high performance reactor conditions

“…Therefore, one can conjecture that this ITB is strongly correlated with fast ions in the core region of the plasma. As previously reported, fast ions can stabilise or mitigate microturbulence 8,[47][48][49][50] such that the transport of thermal ion energy can be reduced at the core region where fast ions are populated.…”