D. McColl

Alternating current plasma operation in the STOR-M tokamak

One cycle alternating current (AC) plasma operation without a dwell time has been achieved in the STOR-M tokamak with good reproducibility using a newly developed ohmic heating circuit. The plasma current of +24 kA is smoothly ramped down in 10 ms with a rampdown rate of around 2.0 kA/ms and then ramped up to between -20 and -24 kA directly without a dwell time. The plasma density of up to (3.7+or-0.6)*1018 m-3 remains at the current reversal as observed in recent soft landing experiments. The key to a successful, reproducible and direct transition in AC tokamak operations on STOR-M is to control both the total vertical field by a feedback control system and the plasma density by careful gas puffing during the current reversal phase. This experiment has demonstrated that the initial loop voltage for the second negative current is minimized when the dwell time approaches zero, and the AC operation without dwelling is possible whenever the plasma current can be softly terminated with a finite residual plasma density

Feedback control experiments on 1.0 and 1.5 cycle ac operations in the STOR-M tokamak

Complete one cycle and 1.5 cycle ac operations are performed in the STOR-M tokamak with the plasma current of ∼20 kA using newly developed feedback control and Ohmic heating circuits. Bias voltage adjustment is installed in the plasma position circuit to optimize the plasma position in the second negative plasma current phase for multicycle ac operation. The key to successful, reproducible multicycle ac tokamak operations on STOR-M is to control both the total vertical field by the feedback control system and the plasma position by application of the bias voltage.

Effects of compact torus injection on toroidal flow in the STOR-M tokamak

In compact torus injection (CTI) experiments on the STOR-M tokamak, an ion Doppler spectrometer is installed to observe the effects of CTI on toroidal plasma flows. The intrinsic toroidal flow in ohmic discharges without CTI is sheared with counter plasma current flow in the core region and co-current direction at the periphery. With tangential CTI along the co-current direction, the flow velocity in the core region decreases by more than 5 km s−1, while in the periphery the flow velocity increases by 3–4 km s−1. These data indicate that the observed flow change is due to the injection of toroidal momentum. Density increase and high soft x-ray emission after CTI are observed during the changes in the toroidal flow.

Development Toward a Repetitive Compact Torus Injector

A compact torus (CT) injector operated at high repetition rates has been developed. A system consisting of a stack of insulated-gate bipolar transistors and storage capacitor banks (slow banks) for repetitive operation has been installed on the circuit of both CT formation and acceleration. Following the installation of silicon-controlled rectifiers for the purpose of triggering ignitrons at CT discharge circuits, a repetition rate of 10 Hz was achieved on the University of Saskatchewan CT Injector.

Plasma current start-up by the outer ohmic heating coils in the Saskatchewan TORus Modified (STOR-M) iron core tokamak

A plasma current up to 15 kA has been driven with outer ohmic heating (OH) coils in the STOR-M iron core tokamak. Even when the inner OH coil is disconnected, the outer OH coils alone can induce the plasma current as primary windings and initial breakdown are even easier in this coil layout. This result suggests a possibility to use an iron core in a spherical tokamak to start up the plasma current without a central solenoid. The effect of the iron core saturation on the extension of the discharge pulse length has been estimated for further experiments in the STOR-M tokamak.

Plasma current start-up experiments without a central solenoid in the iron core STOR-M tokamak

Reproducible plasma current start-up without a central solenoid (CS) has been demonstrated using the outer ohmic heating (OH) coils in the iron core STOR-M tokamak (Mitarai et al 2014 Fusion Eng. Des. 89 2467–71). Although the outer OH coil current saturates the iron core eventually, it has been demonstrated that the plasma current can be maintained during the iron core saturation phase. In this work, further studies have been conducted to investigate the effects of the turn number of the outer OH coils (N = 4 or N = 6) in the CS-less discharges and to evaluate the plasma stability with respect to the n-decay index of the vertical magnetic field. For the loose coupling of the iron core with N = 4 turns, the plasma current can be sustained after the additional third capacitor bank is applied near the iron core saturation phase, showing the slow transition from the unsaturated to the partially saturated phase. For the case of stronger coupling of N = 6 turns, the plasma current is increased at the same fast bank voltage, but the main discharge is shortened from 35 to 20 ms. As the magnetizing current is smaller due to stronger coupling between the OH coils and the plasma current, the transition from the unsaturated to the saturated phase is slightly difficult at present. The present experimental results suggest a feasible operation scenario in a future spherical tokamak (ST) at least using loose iron core coupling for smoother transition from the unsaturated to the saturated iron core phase. Thus, a reliable plasma current start-up by the outer OH coils and the current ramp-up to a steady state by additional heating power and vertical field coils could be considered as an operation scenario for future ST reactors with an iron core transformer.

Effect of atomic number of the gas on ion beam emission and hard x-ary radiation in a plsma focus device

Summary form only given. Effect of atomic number of operating gas on hard x-ray and ion beam emission from plasma focus have been studied. Circuit analyses have been done quantitatively to measure plasma voltage and inductance of focusing plasma. The results show increasing the atomic number of gas, increase the energy of generated ions, intensify the radiation of hard x-r a y, and enhancing the amount of injected energy in to plasma during the pinching time. Based on the measured plasma inductance and injected energy in to plasma it is speculated that increasing atomic number (Z≥18) might drive the condition of radiation-enhanced compression during pinch phase.

Plasma-assisted syntheses of diamond films and nanotubes

Summary form only given, as follows. Experimental investigation of plasma-assisted syntheses of carbon based material, including diamond films and carbon nanotubes, has been carried out. The diamond films were deposited on silicon substrates using a hot filament device with an optional glow discharge capability. The optional glow discharge current was about 0.3 A. The substrate temperature was in the range 700-800/spl deg/C. The diamond films were grown at a pressure around 20 Torr in a vessel flushed with a mixture of hydrogen and methane gases.