Energy confinement comparable with tokamak quality is achieved in the Madison Symmetric Torus (MST) reversed field pinch (RFP) at a high beta and low toroidal magnetic field. Magnetic fluctuations normally present in the RFP are reduced via parallel current drive in the outer region of the plasma. In response, the electron temperature nearly triples and beta doubles. The confinement time increases tenfold (to ∼10 ms), which is comparable with Land H-mode scaling values for a tokamak with the same plasma current, density, heating power, size and shape. Runaway electron confinement is evidenced by a 100-fold increase in hard x-ray bremsstrahlung. Fokker-Planck modelling of the x-ray energy spectrum reveals that the high energy electron diffusion is independent of the parallel velocity, uncharacteristic of magnetic transport and more like that for electrostatic turbulence. The high core electron temperature correlates strongly with a broadband reduction of resonant modes at mid-radius where the stochasticity is normally most intense. To extend profile control and add auxiliary heating, rf current drive and neutral beam heating are in development. Low power lower-hybrid and electron Bernstein wave injection experiments are underway. Dc current sustainment via ac helicity injection (sinusoidal inductive loop voltages) is also being tested. Low power neutral beam injection shows that fast ions are well-confined, even in the presence of relatively large magnetic fluctuations.

Coupling to the electron Bernstein wave (EBW) via a phased array of waveguides is experimentally investigated in the Madison symmetric torus reversed field pinch (RFP). EBWs promise to provide localized heating and current drive in overdense plasmas such as those in the RFP, provided a technique can be developed to efficiently couple power from an externally launched electromagnetic wave to the electrostatic EBW. The choice of antenna structure and launched wave polarization are important factors in coupling waves with frequencies in the electron cyclotron range to EBW, especially on the RFP where the mode conversion to the EBW can take place in the near-field of the antenna. In this paper, a recently developed theory of coupling to EBW from a phased array of waveguides is tested against experiment. The theory predicts that coupling efficiency will vary with launch angle and that coupling depends sensitively upon the edge density profile. Amplitudes and phases of reflected power in each of the waveguides are measured experimentally and compared with predictions based upon density profiles measured by a Langmuir probe in the edge of the plasma. The parametric dependence of reflection has been studied for different polarizations, different launch angles, time varying density profiles and several frequencies of the launched wave. An asymmetry in reflection versus the launch angle predicted by theory was found experimentally for the X-mode, while the O-mode was symmetric. The dependence on density scale length predicted by theory was confirmed in the experiment. The phase of the reflected signal is shown to contain reflectometry-based information about the edge density profile. For appropriate phasing, measurements show that the total power reflection coefficient can be lower than 15%.

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.

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