Electron and Ion Heating Characteristics during Magnetic Reconnection in the MAST Spherical Tokamak.

Electron and ion heating characteristics during merging reconnection start-up on the MAST spherical tokamak have been revealed in detail using a 130 channel yttrium aluminum garnet (YAG) and a 300 channel Ruby-Thomson scattering system and a new 32 chord ion Doppler tomography diagnostic. Detailed 2D profile measurements of electron and ion temperature together with electron density have been achieved for the first time and it is found that electron temperature forms a highly localized hot spot at the X point and ion temperature globally increases downstream. For the push merging experiment when the guide field is more than 3 times the reconnecting field, a thick layer of a closed flux surface form by the reconnected field sustains the temperature profile for longer than the electron and ion energy relaxation time ~4-10 ms, both characteristic profiles finally forming a triple peak structure at the X point and downstream. An increase in the toroidal guide field results in a more peaked electron temperature profile at the X point, and also produces higher ion temperatures at this point, but the ion temperature profile in the downstream region is unaffected.

Poloidal mode analysis of magnetic probe data in a spherical tokamak configuration.

A method to determine the poloidal mode number m in a spherical tokamak based on magnetic probe data was developed. Perturbed magnetic fields at Mirnov coils are calculated for distributed helical filamentary currents on rational surfaces assuming the maximum current amplitude, m and n (toroidal mode number), and the toroidal location of the filaments. These free parameters were determined from the best fit to the measured signals. The residual error was reduced by a factor of 2 by introducing helical filaments instead of toroidal filaments. Using this method, m/n=2/1 and 3/2 modes were identified in Mega-Ampere Spherical Tokamak discharges, and the time evolution of the tearing modes was derived.

Active control of type-I edge-localized modes with n=1 perturbation fields in the JET tokamak.

Type-I edge-localized modes (ELMs) have been mitigated at the JET tokamak using a static external n=1 perturbation field generated by four error field correction coils located far from the plasma. During the application of the n=1 field the ELM frequency increased by a factor of 4 and the amplitude of the D(alpha) signal decreased. The energy loss per ELM normalized to the total stored energy, DeltaW/W, dropped to values below 2%. Transport analyses shows no or only a moderate (up to 20%) degradation of energy confinement time during the ELM mitigation phase.

Formation of advanced tokamak plasmas without the use of an ohmic-heating solenoid.

A new operational scenario of advanced tokamak formation was demonstrated in the JT-60U tokamak. This was accomplished by electron cyclotron and lower hybrid waves, neutral beam injection, and the loop voltage supplied by the vertical field and shaping coils. The Ohmic heating (OH) solenoid was not used but a small inboard coil (part of the shaping coil), providing less than 20% of total poloidal flux, was used. The plasma thus obtained had both internal and edge transport barriers, with an energy confinement time of 1.6 times H-mode scaling, a poloidal beta of 3.6, and a normalized beta of 1.6, and a large bootstrap current fraction (>90%). This result opens up a possibility to reduce, and eventually eliminate, the OH solenoid from a tokamak reactor, which will greatly improve its economic competitiveness.

Neoclassical tearing physics in the spherical tokamak MAST.

Results from MAST provide a first test of neoclassical tearing mode physics in the spherical tokamak (ST). The mode accounts for the main performance limit in conventional tokamaks. Its behavior in the ST is remarkably well described by existing theoretical models, although it is more readily seeded by sawtooth events in these scenarios. Modeling confirms the significance of stabilizing field-curvature effects. This provides good grounds for optimism that with suitable control of profiles, it may be possible to avoid these modes in the ST.

L-H transition in the mega-amp spherical tokamak.

H-mode plasmas have been achieved on the MAST spherical tokamak at input power considerably higher than predicted by conventional threshold scalings. Following L- H transition, a clear improvement in energy confinement is obtained, exceeding recent international scalings even at densities approaching the Greenwald density limit. Transition is accompanied by an order-of-magnitude increase in edge-density gradient, a marked decrease in turbulence, the efficient conversion of internal electron Bernstein waves into free space waves, and the onset and saturation of edge poloidal rotation.

H-Mode operation in the START spherical tokamak

H-mode operation has been achieved in high current (I(p)>200 kA) plasmas in the START spherical tokamak for both neutral-beam-injection-heated and Ohmic discharges. The transition to H mode features the development of well-defined edge pedestals in density and temperature, which signifies the formation of an edge-transport barrier, and associated edge-localized modes. Recent operation at plasma currents exceeding 250 kA shows that these features are accompanied by increases in energy confinement time. This is the first clear demonstration of the H-mode regime in a spherical tokamak.