Polarimetry measurements of the Doppler-shifted H fl emission from a neutral hydrogen beam on the PBX-M tokamak have been employed in a novel technique for obtaining q{r) and magnetic field pitchangle profiles using the Stark effect. The resulting q(r) profile is very broad and its central value, #(0), is significantly below 1, which has important implications for theoretical models of sawteeth. PACS numbers: 52.70.Ds, 52.70.Kz The current-density profile and the safety factor, q(r), are important in the theoretical modeling of plasma equilibrium, stability, and confinement. Many observed phenomena, however, are not well understood due to the lack of consistent and detailed experimental data. For example, current-driven instabilities have been observed or predicted under various conditions, and are known to affect plasma confinement and transport. Since such discharges generally involve auxiliary heating, q(r) measurements in the past have not been available in the resulting high-density plasmas. In addition, when #(0) < 1, models predict the plasma to be unstable to resistive kink modes, which are believed to be responsible for sawtooth oscillations in the central temperature. Several theoretical models have been proposed to explain these observations, but to distinguish between them, a detailed knowledge of the safety-factor profile evolution is needed, including evidence beyond experimental uncertainty for a drop in the central #(0) below unity. There are many other instability issues related to the current or q(r) profile, such as the understanding of ballooning or fishbone modes, that could be resolved with detailed profile information.Techniques for active control of the current density, and the q(r) profile by using rf, neutral beams, or other means are presently underway or being planned. To understand the effects of modifying current-density profiles on stability, confinement, and transport, a credible means of measuring the poloidal field distribution is necessary. Here again, such techniques must be operable in high-density, auxiliary-heated plasmas that are at or near the reactorlike parameters of the present generation of fusion devices.To date, methods for obtaining current-density and safety-factor profiles have suffered from several problems. Frequently, the measurement is line integrated, which requires a numerical inversion process to obtain spatial information. 1,2 This introduces some uncertainty, particularly for noncircular plasma shapes. Neutralbeam probe techniques have shown good spatial resolution, but so far suffered from beam attenuation for densities above -lxl0 13 cm~3. 3 ' 4 In this Letter we report the first measurements of the central rotational transform, #(0), using the motional Stark effect (MSE) to polarize the spectral emission from a 55-keV neutral-hydrogen beam. One of the principal advantages of this technique is that it can provide a local (~l-2-cm resolution) and highly accurate measurement of the pitch angle of the magnetic field, Y p (r) =tan~1[5 p (/')/ J 5r(r)],...

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Bean-shaped configurations have been successfully formed in the PBX tokamak and /? values of over 5% have been obtained. These discharges still lie in the first stability regime for ballooning modes, and magnetohydrodynamics stability analysis implicates the external kink as responsible for the present fi limit.

The highly indented plasmas of the PBX-M tokamak experiment [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989), Vol. 1, p. 97] have reached plasma regimes of both high volume-averaged beta (βt), and high-beta poloidal (βp), and show evidence of the suppression of external surface modes by the passive stabilizing system. Values of βt up to 4.0 I/aB (% MA/m T) with Ti(0)≊4 keV have been obtained. A magnetohydrodynamic analysis of plasmas with βp=2.0 indicates that these plasmas are near the threshold of the second stability regime. A value of βt of 6.8% has been reached with Ti(0)&gt;5 keV and an indentation of 28%. Control of plasma shape is accomplished with a feedback system that uses a moment expansion about a single equilibrium and is augmented by time-dependent waveforms to redefine plasma shape. Diagnostics to measure the safety factor q have been developed and used to make accurate measurements of q(r) and to verify changes made in q(0).