Electrodeless radiofrequency discharges exhibit two modes of operation: a low-density mode in which the power is capacitively coupled to the plasma and which is known as the E-mode, and a higher density mode which is an inductive discharge known as the H-mode. The transition between these modes exhibits hysteresis, i.e. the E to H transition occurs at a different coil current than the reverse H to E transition. Recent theoretical results show that the hysteresis can be qualitatively understood in terms of electron power balance assuming that either the power dissipated or the power absorbed by the plasma electrons has a nonlinear dependence on the electron density. Experiments have been carried out to examine this hypothesis, both by characterizing steady-state E-and H-mode plasmas with a Langmuir probe, and by using a new approach consisting of measuring the internal plasma parameters in a pulsed discharge. In the latter case, the power is time modulated with increasing and decreasing power ramps. This approach allows us to investigate the hysteresis in detail and to study the dynamics of the transition. A number of time-resolved diagnostics including Langmuir probes, current and voltage sensors, optical emission and B-dot probes have been used.

A Fast Ion Deuterium Alpha (FIDA) spectrometer was installed on MAST to measure radially resolved information about the fast ion density and its distribution in energy and pitch angle.Toroidally and vertically-directed collection lenses are employed, to detect both passing and trapped particle dynamics, and reference views are installed to subtract the background. This background is found to contain a substantial amount of passive FIDA emission driven by edge neutrals, and to depend delicately on viewing geometry. Results are compared with theoretical expectations based on the codes NUBEAM (for fast ion distributions) and FIDASIM. Calibrating via the measured beam emission peaks, the toroidal FIDA signal profile agrees with classical simulations in MHD quiescent discharges where the neutron rate is also classical. Long-lived modes (LLM) and chirping modes decrease the core FIDA signal significantly, and the profile can be matched closely to simulations using anomalous diffusive transport; a spatially uniform diffusion coefficient is sufficient for chirping modes, while a core localized diffusion is better for a LLM. Analysis of a discharge with chirping mode activity shows a dramatic drop in the core FIDA signal and rapid increase in the edge passive signal at the onset of the burst indicating a very rapid redistribution towards the edge. Vertical viewing measurements show a discrepancy with simulations at higher Doppler shifts when the neutron rate is classical, which, combined with the fact that the toroidal signals agree, means that the difference must be occurring for pitch angles near the trapped-passing boundary.Further evidence of an anomalous transport mechanism for these particles is provided by the fact that an increase of beam power does not increase the higher energy vertical FIDA signals, while the toroidal signals do increase.

Local electron and ion heating characteristics during merging reconnection startup on the MAST spherical tokamak have been revealed for the first time using a 130 channel YAG-TS system and a new 32 chord ion Doppler tomography diagnostic. 2D local profile measurement of Te, ne and Ti detect highly localized electron heating at the X point and bulk ion heating downstream. For the push merging experiment under high guide field condition, thick layer of closed flux surface formed by reconnected field sustains the heating profile for more than electron and ion energy relaxation time τ E ei ∼ 4 − 10ms, both heating profiles finally form triple peak structure at the X point and downstream. Toroidal guide field mostly contributes the formation of peaked electron heating profile at the X point. The localized heating increases with higher guide field, while bulk downstream ion heating is unaffected by the change in the guide field under MAST conditions (Bt > 3Brec).PACS numbers: 52.35. Vd, 52.55.Fa, 52.72.+v Magnetic reconnection is a fundamental process which converts the magnetic energy of reconnecting fields to kinetic and thermal energy of plasma through the breaking and topological rearrangement of magnetic field lines. Recent satellite observations of solar flares revealed several important signatures of reconnection heating. In the solar flares, hard X-ray spots appear at loop-tops of coronas together with another two foot-point spots on the photosphere. The loop-top hot spots are considered to be caused by fast shocks formed in the down-stream of reconnection outflow [3]. The two-dimensional (2D) measurements of the Hinode spectrometer documented a significant broadening of Ca line-width downstream of reconnection [4]. These phenomena strongly suggest direct ion heating by reconnection outflow. On the other hand, the V-shape high electron temperature region was found around X-line of reconnection as an possible evidence of slow shock structure [5]. However, those heating characteristics of reconnection are still under serious discussion, indicating that direct evidence for the reconnection heating mechanisms should be provided by a proper laboratory experiment. Since 1986 the merging of two toroidal plasmas (flux tubes) has been studied in a number of experiments: TS-3

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