Experimental program and plans for HSX upgrade

The HSX stellarator is currently undergoing a major upgrade. A new 70 GHz gyrotron will be installed that will provide up to 300 kW of power for up to 100 ms. This upgrade will allow operation at three times higher plasma densities (up to 2.0x 10¹⁹/m³) with an ECH absorption efficiency of ~ 90%. As a result of increased ion-electron coupling, the core ion temperature is expected to increase from the present value of ~50 eV to more than 150 eV, moving closer to a low collisionality regime. The radial electric field is calculated to be more negative in the core. The higher plasma density operation is expected to reduce the density of background neutrals. Because of the reduced flow damping due to neutrals, there should be higher flows in the direction of symmetry. The post-upgrade experimental program of HSX emphasizes areas that exploit these new capabilities: 1) exploring higher ion temperature and lower neutral density regimes, 2) optimizing the magnetic geometry to vary turbulent transport, 3) measuring plasma flows and electric fields at higher ion temperatures, more negative electric fields, and lower neutral damping and 4) measuring and understanding impurity transport.