N. Dominguez

Bootstrap current control in stellarators

The poloidal coil system of a stellarator can be used to modify the ‖B‖ spectrum and, as a consequence, change the magnitude of the bootstrap current. The addition of a small l=1 field component to a stellarator field can cancel or reverse the direction of the bootstrap current. It is shown that currentless operation in the collisionless regime is not impaired by bootstrap current, because the effects of bootstrap current can be eliminated by an appropriate external coil system. The advanced toroidal facility (ATF) [Fusion Technol. 10, 179 (1986)] can be used to test some of these results.

Low-aspect-ratio torsatron configurations

Low-aspect-ratio stellarator configurations can be realized by using torsatron windings. Plasmas with aspect ratios in the range of 3.5 to 5 can be confined by these low-aspect-ratio torsatron configurations. Stable operation at high beta should be possible in these devices if a vertical field coil system is adequately designed to avoid breaking of the magnetic surfaces at finite beta.

Dissipative trapped electron modes in l=2 torsatrons

Trapped electron modes in stellarators can be more unstable than those in tokamaks. They could be easier to detect in a stellarator and may be responsible for anomalous losses in the low collisionality regime. 5 refs., 1 fig.

Second stability in the ATF torsatron—Experiment and theory

Access to the magnetohydrodynamic (MHD) second stability regime has been achieved in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Operation with a field error that reduced the plasma radius and edge rotational transform resulted in peaked pressure profiles and increased Shafranov shift that lowered the theoretical transition to ideal MHD second stability to β0≊1.3%; the experimental β values (β0≤3%) are well above this transition. The measured magnetic fluctuations decrease with increasing β, and the pressure profile broadens, consistent with the theoretical expectations for self‐stabilization of resistive interchange modes. Initial results from experiments with the field error removed show that the pressure profile is now broader. These later discharges are characterized by a transition to improved (×2–3) confinement and a marked change in the edge density fluctuation spectrum, but the causal relationship of these changes is not yet clear.

Recent results from the ATF torsatron

Recent experiments in the Advanced Toroidal Facility (ATF) torsatron [Plasma Physics and Controlled Nuclear Fusion Research 1990 (IAEA, Vienna, in press)] have emphasized the role of magnetic configuration control in transport studies. Long‐pulse plasma operation up to 20 sec has been achieved with electron cyclotron heating (ECH). With neutral beam injection (NBI) power of ≥1 MW, global energy confinement times of 30 msec have been obtained with line‐average densities up to 1.3×1020 m−3. The energy confinement and the operational space in ATF are roughly the same as those in tokamaks of similar size and field. The empirical scaling observed is similar to gyro‐reduced Bohm scaling with favorable dependences on density and field offsetting an unfavorable power dependence. The toroidal current measured during ECH is identified as the bootstrap current. The observed currents agree well with predictions of neoclassical theory in magnitude and in parametric dependence. Variations of the magnetic configuration ...

Ideal low-n and Mercier mode stability boundaries for ℓ = 2 torsatrons

The relationship between the stability properties of ideal low-n internal modes and the threedimensional (3-D) ideal Mercier criterion for l = 2 torsatron configurations is discussed. A broad range of aspect ratios and pressure profiles are considered. It is found that the low-n modes with resonant surfaces lying in a Mercier unstable region are unstable in most of the cases studied and that the critical beta given by the Mercier criterion is always lower than the critical beta for the lowest-n unstable mode. This is verified even in the cases where global n = 1 modes are unstable. Therefore, the 3-D Mercier criterion is a useful guide in mapping the ideal stability beta limits for these torsatron configurations.

Fluctuation and modulation transport studies in the Advanced Toroidal Facility (ATF) torsatron

The Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] has completed experiments focusing on microwave scattering measurements of density fluctuations and transport studies utilizing the modulation of dimensionless parameters. Microwave scattering measurements of electron density fluctuations in the core of low‐collisionality electron cyclotron heated (ECH) plasmas show features that might be evidence of trapped electron instabilities. Starting from gyro‐Bohm scaling, the additional dependence of confinement on the dimensionless parameters ν* and β (collisionality and beta) has been investigated by modulating each of these parameters separately, revealing the additional favorable dependence, τE∝τgBν*−0.18β+0.3.

Low‐n stability calculations for three‐dimensional stellarator configurations

A set of reduced magnetohydrodynamic (MHD) equations to study linear stability in general toroidal configurations is presented. The equations are derived by applying the averaging method to an equilibrium represented in a straight‐field‐line coordinate system. They permit the calculation of the low‐n stability of fully three‐dimensional (3‐D) equilibria for stellerator configurations with either a planar or a helical magnetic axis. Numerical solutions are shown to agree well with marginal stability boundaries given by the classical stellerator expansion when average stellarator equilibria with a planar magnetic axis are used as input.

Ideal Mercier stability for the TJ-II flexible Heliac

The configuration flexibility provided to the TJ-II Heliac by the helical hard core permits a significant change in the stability properties of the plasma. In this numerical analysis of full three-dimensional equilibria it is found that, by exploiting the flexibility of the machine, the stability beta limit given by the Mercier criterion can be varied almost continuously over values of average beta from zero to more than 6%.

Alpha destabilization of the TAE mode using a reduced gyrofluid model with Landau closure

A reduced MHD fluid model for the unstable toroidicity-induced shear Alfven eigenmode (TAE) is described. This consists of four coupled time evolution equations for the poloidal magnetic flux, toroidal component of vorticity, energetic particle density and parallel flow velocity, which are solved numerically using the three-dimensional initial value code FAR in toroidal geometry. The TAE mode is readily excited and exhibits similar scalings as have been predicted analytically.