Overview of L–H power threshold studies in NSTX

Abstract

A summary of results from recent L–H power threshold (P LH) experiments in the National Spherical Torus Experiment is presented. First P LH (normalized linearly by plasma density) was found to be a minimum in double-null configuration, tending to increase as the plasma was shifted more strongly towards lower- or upper-single null configuration with either neutral beam or rf heating. The measured P LH/n e was comparable with neutral beam or rf heating, suggesting that rotation was not playing a dominant role in setting the value of P LH. The role of triangularity (δbot) in setting P LH/n e is less clear: while 50% less auxiliary heating power was required to access H-mode at low δbot than at high δbot, the high δbot discharges had lower ohmic heating and higher dW/dt, leading to comparable loss of power over a range of δbot. In addition, the dependences of P LH on the density, species (helium versus deuterium), plasma current, applied non-axisymmetric error fields and lithium wall conditioning are summarized.

Study on the L–H transition power threshold with RF heating and lithium-wall coating on EAST

“…Note that, in this study, the definition of divertor configuration in terms of small dR sep seems somewhat arbitrary and the presence of a second X-point inside the vacuum vessel and close to the plasma might affect on the edge plasma, wall interaction as well as recycling, which may result in the distinctive findings on EAST. On the other hand, similar results from ASDEX-Upgrade and MAST [19] and studies on NSTX [20] showed a minimum P thr also near balanced DN configuration. These findings show that the occurrence of a minimum in P thr is a universal feature in EAST, regardless of wall materials, pointing to a potential role played by SOL and divertor physics in the L-H transition.…”

“…This observation has been confirmed in a series of dedicated experiments (B × ∇B ↓) conducted in 2012 [7,14]. This is in contrast to other tokamaks [15][16][17][18][19][20]. In addition, all the type-I ELMy H-modes were obtained in the LSN configuration (B × ∇B ↑) in the 2012 campaign, suggesting that LSN with the ion ∇B drift away from the major X-point appears to facilitate achievement of highperformance stable H-mode plasmas on EAST.…”

“…This results in a weak pumping capability which is not so much sufficient to affect the L-H transition power threshold. All of these findings demonstrate that a minimum P thr occurs in the DN configuration, and that a lower power threshold is required with the ion ∇B drift direction away from the major X-point for both the C wall and Mo/C wall on EAST, which is in contrast to other tokamaks [15][16][17][18][19][20]. Note that, in this study, the definition of divertor configuration in terms of small dR sep seems somewhat arbitrary and the presence of a second X-point inside the vacuum vessel and close to the plasma might affect on the edge plasma, wall interaction as well as recycling, which may result in the distinctive findings on EAST.…”

“…Note that the coupling power from LHCD is strongly reduced in DN after the L-H transition, while the H-mode can still be maintained for a long period at a higher density compared with the USN and LSN configuration. In contrast, the plasma remains in L-mode (shot 42022) for the LSN case with dR sep ~ −1.5 cm, suggesting that the power required for the L-H transition is lower with the ion ∇B drift away from the major X-point on EAST, which is in contrast to other tokamaks [15][16][17][18][19][20]. As mentioned above, EAST has only one in-vessel cryopump located underneath the lower divertor for providing the main pumping.…”

Calculation of the Non-Inductive Current Profile in High-Performance NSTX Plasmas

“…Two lithium evaporators deposit lithium vapor on the vessel floor; this lithium primarily coats the divertor, and to a lesser extent, the center column and lower passive plates [83,84]. This conditioning has many beneficial effects, including the elimination of ELMs [85], reduction of electron transport [86] and the lowering of the H--mode threshold [87]. The elimination of ELMs, however, typically leads to an uncontrolled accumulation of impurities [84], including metals and carbon from the plasma facing components.…”