MHD Events Research Articles

Confinement and stability of DIII-D negative central shear discharges

Negative central magnetic shear (NCS) discharges with and up to 80% of the current non-inductively driven are reproducibly produced in the DIII-D tokamak. Strong peaking of , plasma rotation and, in some cases, are observed inside the NCS region. Transport analysis shows that the core ion thermal diffusivity is substantially reduced and near the neoclassical value after the formation of the internal transport barrier. The negative central shear is necessary but not sufficient for the formation of this transport barrier. The power required for the formation appears to increase with the toroidal magnetic field. The high performance phase of H-mode NCS discharges often ends with an ELM-like collapse initiated from the edge whereas the L-mode discharges which have a more peaked pressure profile tend to end with a more global n = 1 MHD event.

Characterization of alpha particle loss during disruptions in TFTR during deuterium-tritium operation

The timing of alpha losses, with respect to the various phases of a disruption, and the impact location of the losses are characterized during high fusion power operation of TFTR with deuterium and tritium fuels. Characterization of alpha losses is important for the design of future fusion devices such as ITER. In addition, characterization of the alpha losses with respect to the disruption evolution helps in the understanding of the dynamics of the disruption process and related MHD events such as sawtooth crashes. Disruptions are characterized as having several standard phases, applicable to most disruptions in all tokamaks: precursor, thermal quench(es) and current quench. Most of the losses are observed to occur during the thermal quench phase(s). In high beta disruptions, alpha losses start abruptly during the growth of MHD precursors, just before the onset of the thermal quench. The initial burst of losses, lasting as little as 100 mu s, can release a significant fraction (one third) of the total disruption induced losses during the thermal quench. An inventory of alpha particles suggests that the alpha loss distribution during disruptions might be quite different from that expected during non-disruptive discharges. There are no obvious differences between fast fusion product losses during deuterium-deuterium (DD) and deuterium-tritium (DT) disruptions, aside from the large alpha loss component in DT discharges

ELM studies on DIII-D and a comparison with ASDEX results

In previous analysis of edge localized modes (ELMs), three different types (types I, II and III) of ELM were observed in DIII-D, whereas only one type of ELM has been found in ASDEX. The type III ELMs observed on DIII-D are similar to the ELMs observed on ASDEX, whereas the type I ELMs did not occur on ASDEX. ELMs are found to be MHD events of short duration (1 to 2 ms), which, on DIII-D, may be followed by a transient L mode phase. This compound event was not observed on ASDEX. A stability analysis of DIII-D discharges indicates that in the standard DIII-D configuration, type III ELMs occur well below the ideal ballooning limit, whereas Type I ELMs generally occur close to this limit. In DIII-D discharges where the plasma parameters are close to the ASDEX values, type I ELMs are not observed, even at the beta limit. This suggests that the ELM behaviour in DIII-D is not uniquely governed by ballooning stability

Deuterium and tritium experiments on TFTR

Three campaigns, prior to July 1994, attempted to increase the fusion power in DT plasmas on the Tokamak Fusion Test Reactor (TFTR). The first campaign was dedicated to obtaining >5 MW of fusion power while avoiding MHD events similar to the JET X-event. The second was aimed at producing maximum fusion power irrespective of proximity to MHD limits, and achieved 9 MW limited by a disruption. The third campaign increased the energy confinement time using lithium pellet conditioning while raising the ratio of alpha heating to beam heating.

Periods of enhanced transport during H-mode in PBX-M

In PBX-M, periods of enhanced transport are often observed in high beta polH-mode discharges. They can reduce the energy confinement time by a factor of two. Their duration is typically between 2 to 20 ms. They are followed by periods of normal transport of a similar duration. These periods of enhanced transport consist of a series of turbulent m=even MHD events, starting several cm inside the plasma edge. They propagate into the confinement region in 600 mu s, causing a progressive erosion of the profile. Each m=even event can also cause an energy loss of 1% or less, mainly from the confinement region. These m=even events are reminiscent of grassy ELMs.

Investigations of VH-mode in DIII-D and JET

The VH-mode regime of high confinement has been observed in both DIII-D and JET. VH-mode is characterized by thermal confinement twice that seen in H-mode, with the edge transport barrier penetrating deeper into the plasma. Two mechanisms have been identified as important in achieving this high level of confinement. Expansion of the E*B velocity shear turbulence suppression zone is important in allowing reductions in local transport, while access to the second ballooning stability regime in the edge allows avoidance or elimination of ELMs which impede the confinement improvement. The high performance phase of these discharges is usually terminated by an MHD event which removes energy from a large portion of the plasma cross-section, and is followed by an H-mode phase.

Shear reversal and MHD activity during pellet enhanced performance pulses in JET

Analysis of MHD activity in pellet enhanced performance (PEP) pulses is used to determine the position of rational surfaces associated with the safety factor q. This gives evidence for negative shear in the central region of the plasma. The plasma equilibrium calculated from the measured q values yields a Shafranov shift in reasonable agreement with the experimental value of about 0.2 m. The corresponding current profile has two large off-axis maxima in agreement with the bootstrap current calculated from the electron temperature and density measurements. A transport simulation shows that the bootstrap current is driven by the steep density gradient, which results from improved confinement in the plasma core where the shear is negative. During the PEP phase (m,n)=(1,1) fast MHD events are correlated with collapses in the neutron rate. The dominant mode preceding these events usually is n=3, whereas the mode following them is dominantly n=2. Toroidal linear MHD stability calculations assuming a non-monotonic q-profile with an off-axis minimum decreasing from above 1 to below 1 describe this sequence of modes (n=3,1,2), but always give a larger growth rate for the n=1 mode than for the n=2 mode. This large growth rate is due to the high central poloidal beta of 1.5 observed in the PEP pulses. Finally, a rotating (m,n)=(1,1) mode is observed as a hot spot with a ballooning character on the low field side. The hot spot has some of the properties of a 'hot' island consistent with the presence of a region of negative shear