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Physics related to fast ions in magnetically confined fusion plasmas is a very important issue, since these particles will play an important role in future burning plasmas. Indeed, they will act as primary heating source and will sustain the self-ignited condition. To measure the fast ion slowing-down times in a magnetohydrodynamic-quiescent plasmas in different scenarios, very short pulses of a deuterium neutral beam, so-called 'blip,' with duration of about 5 ms were tangentially co-injected into a deuterium plasmas at the HuanLiuqi-2A (commonly referred to as HL-2A) tokamak [L. W. Yan, Nucl. Fusion 51, 094016 (2011)]. The decay rate of 2.45 MeV D-D fusion neutrons produced by beam-plasma reactions following neutral beam termination was measured by means of a 235U fission chamber. Experimental results were compared with those predicted by a classical slowing-down model. These results show that the fast ions are well confined with a peaked profile and the ions are slowed down classically without significant loss in the HL-2A tokamak. Moreover, it has been observed that during electron cyclotron resonance heating the fast ions have a longer slowing-down time and the neutron emission rate decay time becomes longer. [ABSTRACT FROM AUTHOR]

In an experimental study of the runaway electron generation during major disruptions in the HuanLiuqi-2A (commonly referred to as HL-2A) [L. W. Yan, Nucl. Fusion 51, 094016 (2011)] tokamak, detailed time and space resolved x-ray images of the long-lived runaway electron beam in flight have been observed and these allow a detailed analysis of the generation and evolution of the disruption produced runaway electron beam in a major disruption, where the conversion efficiency of pre-disruption plasma current into runaway current is up to 55% on HL-2A tokamak. Moreover, a delay of about 7 ms between the start of the disruption and the formation of runaway electron beam has been found. With the aid of the Equilibrium FIT (EFIT) code, magnetic configuration reconstruction has made possible a detailed observation of the magnetic flux geometry evolution during major disruptions. The EFIT magnetic configuration reconstructions show that the delay is due to the transient strong deformation of the magnetic configuration in the initial stage of the current quench, which may provide a possibility of suppressing or mitigating the runaway electron beam during this period by massive gas injection or other methods. [ABSTRACT FROM AUTHOR]

Interactions among pedestal shear flows, turbulence, and the formation of the edge transport barrier have been studied in H-mode plasmas of the HL-2A tokamak by multi-channel Doppler reflectometry with high spatiotemporal resolution. Geodesic acoustic mode (GAM) has been observed during the L-I-H transition. It has been observed that the plasma transits into the I-phase when the mean E × B shear flow reaches a critical value. The bi-spectrum analysis has shown that there is a strong interaction between GAM and limit cycle oscillation (LCO), and the energy transfer is from GAM to LCO, suggesting that GAM can assist the L-I transition. The regulation of the edge turbulence by LCOs helps to build the steep pedestal and initialize the confinement improvement of the plasma. It has been found that the mean E × B shear flow is further increased just before the I-H transition, accompanied by the turbulence suppression, leading to the edge transport reduction and the pedestal formation. It has been demonstrated that the increase of the mean E × B shear flow prior to the L-I and I-H transitions is due to the ion diamagnetic component of E r . These results corroborate that the mean E × B shear flow plays a key role in the L-I and I-H transitions. [ABSTRACT FROM AUTHOR]