Generation and sustainment of the reversed field pinch ͑RFP͒ magnetic configuration normally relies on dynamo activity. The externally applied electric field tends to drive the equilibrium away from the relaxed, minimum energy state which is roughly described by a flat normalized parallel current density profile and is at marginal stability to tearing modes. Correlated fluctuations of magnetic field and velocity create a dynamo electric field which broadens the parallel current density profile, supplying the necessary edge current drive. These pervasive magnetic fluctuations are also responsible for destruction of flux surfaces, relegating the standard RFP to a stochastic-magnetic transport-limited device. Application of a tailored electric field profile ͑which matches the relaxed current density profile͒ allows sustainment of the RFP configuration without dynamo-driven edge current. The method used to ascertain that a dynamo-free RFP plasma has been created is reported here in detail. Several confinement improvements during the accompanying periods of low magnetic fluctuations are observed. Namely, the magnetic fluctuation level is reduced to the point where stochastic-magnetic transport is no longer the dominant process in the core and nested flux surfaces are restored in the core of the dynamo-free RFP.
An overview of MST results is presented. Substantial advances have been achieved in RFP confinement, in the development of auxiliary sources for heating and current drive, and in studies of fluctuations, transport, and magnetic self-organization physics. The density in improved confinement plasmas has been increased above the empirical limit, n/n G =1.5, using pellet injection. A record density of 0.7×10 20 m -3 has been achieved in high current (0.5 MA) plasmas. Maximum energy confinement is attained at 0.5 MA and lower n/n G =0.13; this establishes confinement comparable to a same-size, same-current tokamak over MST's full range of plasma current. Experiments using a new 1 MW, 25 keV neutral beam injector are underway, to explore beta limits, energetic particle confinement, momentum transport, and current profile control. Analysis of the d-d neutron production evidences good fast ion confinement. Results for oscillating field current drive are in good agreement with nonlinear resistive MHD computation, bolstering the OFCD physics basis. Energy confinement with OFCD is measured comparable to that for standard induction. Non-collisional ion heating generates transient temperatures T i =2-3 keV. The heating efficiency is measured to scale with a fractional power of the ion mass, and to be anisotropic depending on the plasma density. Broadband magnetic turbulence exhibits a dissipative nonlinear cascade that could be connected to the ion heating mechanism. A new high rep-rate Thomson scattering diagnostic measures electron temperature fluctuations associated with residual magnetic islands and helical mode structure. The equilibrium and fluctuations of quasi-single-helicity plasmas are investigated with this and MST's other advanced diagnostics. Lower hybrid and electron Bernstein wave injection (~100 kW absorbed power for each) are in development for current drive and heating.
We are designing a new programmable polo ida I field power supply for the MST reversed-field pinch. By providing flexible waveform control, the new supply will expand capabilities in oscillating field current drive, inductive current profile control, and other inductive techniques. The new power supply will allow use of the full flux swing of MST's poloidal transformer core without a separate core bias supply and its isolating inductor. It will also eliminate the risk of harmful surge currents due to core saturation on unplanned termination of the plasma discharge. Used together with the newly-installed toroidal field programmable power supply, the new poloidal supply will allow maximum use ofMST's inductive capabilities.The existing toroidal field programmable supply is based on series-parallel high-power IGBT "-bridges and has an output of about 40 MV A. The poloidal supply will also use high-power IGBTs and will provide about 120 MV A capability. The supply will drive the existing poloidal primary windings, connected with a 10:1 turns ratio. "-bridges will be series-connected in groups of three to yield up to +/-2700 V output; these groups will then be connected in parallel to allow output currents up to +/-80 kA.The power supply units are modular, enabling future expansion for more output capability, and are based on low-inductance laminated heavy copper buses. Waveform generation will be controlled using seven-level Pulse Width Modulation, to reduce IGBT heating and minimize output distortion and noise generation. Overall control of the new poloidal supply will be via General Atomics' Plasma Control System.
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