This paper summarizes the characteristics of the spheromak .lasmas obtained during the past five-year operation period of S-1 experiments. The S-1 Spheromak device, which began operation in 1983, generates a compact toroid in which the self-generated toroidal field in the plasma is comparable to the poloidal field. The S-1 experiment is unique in that spheromak plasmas are formed by inductive transfer of magnetic flux from a toroidal-shaped "flux core", and plasma stability is maintained by shaping of the externally applied equilibrium field and using loose-fitting passive conductors. Without stabilizer coils, the plasma was unstable to gross MHD modes, with the tilt or shift being dominant. Significant improvements in plasma stability and parameters occurred after the installation of passive Figure-8 stabilization coils or funr.el-shaped conductor plates. Stable spheromaks with up to 550 kA
Abstract. The National Spherical Torus Experiment Upgrade (NSTX-U) will operate at axial toroidal fields of ≤ 1 T and plasma currents, I p ≤ 2 MA. The development of non-inductive (NI) plasmas is a major long-term research goal for NSTX-U. Time dependent numerical simulations of 28 GHz electron cyclotron (EC) heating of low density NI start-up plasmas generated by Coaxial Helicity Injection (CHI) in NSTX-U predict a significant and rapid increase of the central electron temperature (T e (0)) before the plasma becomes overdense. The increased T e (0) will significantly reduce the I p decay rate of CHI plasmas, allowing the coupling of fast wave heating and neutral beam injection. A megawatt-level, 28 GHz electron heating system is planned for heating NI start-up plasmas in NSTX-U. In addition to EC heating of CHI start-up discharges, this system will be used for electron Bernstein wave (EBW) plasma start-up, and eventually for EBW heating and current drive during the I p flattop.
PPPLElectron Bernstein waves (EBW) are being investigated for plasma heating and current drive of overdense plasmas on NSTX [1]. EB emission studies on NSTX have shown promising coupling results during some plasma edge conditions and are supportive of further work in high power heating and current drive. Additionally Electron Cyclotron heating (ECH) has several uses during under dense conditions such as plasma startup assist and coaxial helicity injection (CHI) start up assist. A 3 MW EBW system has been proposed for NSTX and is awaiting funding due to the high cost of new gyrotrons. A medium power 350 kW 28 GHz system has been approved recently that will utilize existing gyrotrons and HV modulator/regulator equipment at ORNL and a HV power supply at PPPL. These gyrotrons may also be capable of operating at reduced power at 15.3GHz in a lower order TEO1 cavity mode that will provide additional EBW coupling experimental capability. The launcher is a difficult area to optimize for EBW coupling with limited port area available on NSTX. Low power EBE launcher studies have provided good data on coupling efficiency when good antenna beam quality is available [2]. A preliminary design for the medium power generation, transmission and launching system will be presented in addition to results for gyrotron tests at 15.3 GHz.
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