We have shown, through current density profile control, that energy confinement in the RFP can be made similar to a tokamak (of equal current and size), while simultaneously maintaining high values of beta. We may now be at the point that magnetic fluctuations have been sufficiently reduced so that transport is becoming dominated by other processes, perhaps by electrostatic fluctuations.

A ~100-fold increase in hard-x-ray bremsstrahlung during PPCD implies that the confinement of high energy electrons is vastly improved. Fokker-Planck transport modeling has been used to reconstruct the x-ray flux, thereby inferring the diffusive properties of the collisionless electrons. The electron diffusion coefficient for standard plasmas is proportional to the parallel velocity of the electrons, characteristic of transport in a stochastic magnetic field. For the PPCD case, the electron diffusion coefficient is independent of the parallel velocity, implying non-stochastic residual transport. A velocity-independent diffusion coefficient is more characteristic of electrostatic turbulent transport, like that observed in tokamak and stellarator plasmas.
We aim to understand transport by electrostatic fluctuations, likely an important contributor to transport in improved confinement RFP plasmas, and perhaps the determinant of the ultimate confinement limit in the RFP. This topic is key to the future of the RFP as a configuration for fusion energy. It provides a unique opportunity to advance the understanding of the fundamentals of electrostatic transport in plasmas with strong magnetic shear and large gyroradii.

With a heavy-ion beam probe, we have obtained the first measurement of electrostatic fluctuation-induced particle transport in the RFP core. The particle flux is obtained from the correlation between the fluctuating electrostatic potential and electron density. (Electrostatic particle (energy) transport is given by the product of electric field fluctuations and density (pressure) fluctuations for the species.) It is found that electrostatic transport is far too small to account for core particle transport in standard plasmas, as expected. With improved confinement, the total particle transport is within the error bars of the electrostatic transport measurement. Determination of the value of electrostatic transport in improved confinement plasmas awaits an increase of the resolution of the beam probe diagnostic. In addition, improvements in the heavy-ion beam probe system will allow measurement from the center to the edge of MST giving a more complete picture of how electrostatic fluctuations contribute to transport. Electron density and temperature fluctuations will be examined in the core with a new fast Thomson Scattering diagnostic. These will enable the first measurements of electron temperature fluctuations in the core of the RFP and, when combined with potential fluctuations from the heavy-ion beam probe, the first estimates of electrostatic fluctuation-induced heat transport.

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