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Abstract
A novel, non-inductive current drive technique has been developed for initiating and maintaining tokamak discharges in CDX-U: the current drive experiment-upgrade, a low-aspect-ratio tokamak facility. The new method utilizes naturally occurring internally generated currents which are present in toroidal plasmas. On CDX-U, electron cyclotron heating (ECH) was used to provide the heating power necessary to create and maintain a high-$\beta\sb{pol}$ plasma, the plasma for which self-generated currents are significant. A novel poloidal field configuration provided initial confinement for an ECH produced, trapped electron population. The ECH power, injected through a simple (non-phased) waveguide, was well suited to produce a hot, low-collisionality electrons needed for current generation. With application of ECH, internal plasma generated currents occurred spontaneously and increased with applied ECH power. The generated current scaled inversely with neutral particle density, showing the importance of reducing the plasma collisionality. The current direction depended only on the poloidal field direction, not on the toroidal field direction. The currents flowing into segmented limiters were found to be very small, confirming that the currents were internally generated. With application of $\sim$8 kW of ECH power, a toroidal plasma current of up to 1200 A was generated. At this plasma current level, the poloidal fields from the plasma currents were sufficiently large to form a low-aspect-ratio tokamak plasma, demonstrated unambiguously by poloidal field reversal on the inner wall of the vessel. The $\beta\sb{pol}$ in this experiment was high, $\epsilon\beta\sb{pol}\simeq 1$, consistent with the observed pressure driven currents. Furthermore, the normalized collisionality--the ratio of the electron bounce period to collision time--was less than one in regions of strong current density; thus, the observed currents were consistent with theoretical predictions of trapped particle generated currents. This result demonstrated that it is possible to generate plasma currents, even when the magnetic topology is changing. In CDX-U, the equilibrium evolved from an open field line configuration to a closed field line tokamak configuration. The result opens up the possibility of creating and maintaining tokamak plasmas solely by internally generated currents.
