Xuan Sun

Observation of double layer in the separatrix region during magnetic reconnection

We present in situ observation of double layer (DL) and associated electron measurement in the subspin time resolution in the separatrix region during reconnection for the first time. The DL is inferred to propagate away from the X line at a velocity of about ion acoustic speed and the parallel electric field carried by the DL can reach −20 mV/m. The electron displays a beam distribution inside the DL and streams toward the X line with a local electron Alfven velocity. A series of electron holes moving toward the X line are observed in the wake of the DL. The identification of multiple similar DLs indicates that they are persistently produced and therefore might play an important role in energy conversion during reconnection. The observation suggests that energy dissipation during reconnection can occur in any region where the DL can reach.

Microwave Doppler reflectometer system in the Experimental Advanced Superconducting Tokamak.

A Doppler reflectometer system has recently been installed in the Experimental Advanced Superconducting (EAST) Tokamak. It includes two separated systems, one for Q-band (33-50 GHz) and the other for V-band (50-75 GHz). The optical system consists of a flat mirror and a parabolic mirror which are optimized to improve the spectral resolution. A synthesizer is used as the source and a 20 MHz single band frequency modulator is used to get a differential frequency for heterodyne detection. Ray tracing simulations are used to calculate the scattering location and the perpendicular wave number. In EAST last experimental campaign, the Doppler shifted signals have been obtained and the radial profiles of the perpendicular propagation velocity during L-mode and H-mode are calculated.

Ion cyclotron resonance heating (ICRH) systems for the Keda Mirror with AXisymmetry (KMAX)

In this paper, we describe the engineering work involved in constructing two ion cyclotron resonance heating (ICRH) systems for use in the Keda Mirror with AXisymmetry tandem mirror experiment. Because they offer an effective and robust heating method, ICRH systems have been widely used in a variety of plasma experiments. The goal of our system is to heat the hydrogen plasma contained in the central cell using the fundamental ion cyclotron frequency. Both systems can deliver a radiofrequency power of ∼120 kW with adjustable operating frequencies that are tuned to be slightly lower than their local ion cyclotron frequencies. Two types of antennas are installed in the central cell in an attempt to launch both slow and fast waves. The heating mechanism is reliant on the magnetic beach effect for slow waves.

Field-reversed configuration formed by in-vessel θ-pinch in a tandem mirror device

We describe a field reversed configuration (FRC) experiment featuring in-vessel θ-pinch coils and open-field-line plasmas confined in a tandem mirror. Two FRCs, formed near the west and the east mirror throats of a central cell, are ejected toward the mid-plane for colliding and merging. Each FRC consists of four groups of pulsed power supplies and four groups of coils, having diameters 35, 35, 40, and 45 cm. The rise time of the main reversal field is 7.15 μs, and the maximum voltage is 40 kV with total currents of 416 kA, corresponding to a magnetic field of 1690 G. The total capacitive stored energy is 115.2 kJ. A fast pulse gas injection system was designed and tested to inject neutral gas into the FRC formation region with controlled directions. The successful installation of the θ-pinch coils inside the vacuum vessel offers greater freedom for diagnostics and control instruments as well as preserving magnetic tandem mirror configuration. The magnetic field reversal is confirmed by internal magnetic field measurements. The plasma temperature, density, and lifetime are, respectively, ∼100 eV, ∼3.0 × 1018 m-3, and ∼300 μs for the current operating conditions.

Electrode Biasing Experiment in KMAX Tandem Mirror

Abstract An electrode biasing system has been installed on the KMAX (Keda Mirror with AXisymmetricity) tandem mirror machine to control the rotation speed. It consists of a metal disk-type electrode and a concentric ring-shaped electrode. On each of them are 12 embedded single probes distributed uniformly in the azimuthal direction plus a single probe on the center. An adjustable power supply provides the biasing voltage from −1 kV to 1 kV, and a silicon controlled rectifier with rising time ~5 ~s and maximum current up to 3000 A is used to switch on the circuit. While most of applied voltages are inevitably lost on the sheath as confirmed by the experiments, the plasma potentials have been found to change substantially.

Characterization of a medium-sized washer-gun for an axisymmetric mirror

A new medium-sized washer gun is developed for a plasma start-up in a fully axisymmetric mirror. The gun is positioned at the east end of the Keda Mirror with AXisymmetricity facility and operated in the pulsed mode with an arc discharging time of 1.2 ms and a typical arc current of 8.5 kA with 1.5 kV discharge voltage. To optimize the operation, a systematic scan of the neutral pressure, the arc voltage, the bias voltage on a mesh grid 6 cm in front of the gun and an end electrode located on the west end of mirror, and the mirror ratio was performed. The streaming plasma was measured with triple probes in the three mirror cells and a diamagnetic loop in the central cell. Floating potential measurements suggest that the plasma could be divided into streaming and mirror-confined plasmas. The floating potential for the streaming plasma is negative, with an electric field pointing inwards. The mirror-confined plasma has a typical lifetime of 0.5 ms.

Laser-induced fluorescence diagnostic via pulsed lasers in an argon plasma

Laser-induced fluorescence (LIF) using a pulsed laser is successfully applied in an argon plasma. The laser system consists of a pumping pulse laser fixed at 532 nm and a tunable dye laser. Using a homemade Fabry-Perot interferometer, the large linewidth of the original output is reduced by one order from 4 GHz to 340 MHz. The measured ion temperature is 0.15 eV with a velocity resolution about 200 m/s. It provides great possibility for the combination of LIF and planar LIF using the same pulsed laser system.

Formation of field-reversed configuration using an in-vessel odd-parity rotating magnetic field antenna in a linear device

A Field-Reversed Configuration (FRC) is formed by an in-vessel odd-parity rotating magnetic field (RMF) antenna in a tandem mirror device, Keda mirror with axisymmetricity. The 40-cm diameter antenna is fed independently by four IGBT-based power units with an output current of 1500 A each at 84 kHz, and their phases are adjustable to launch odd or even parity RMFs. A medium-sized washer gun is utilized to generate a highly ionized seed hydrogen plasma. Driven by RMF, the resultant FRC is formed with a separatrix radius of ∼17 cm, external field of ∼50 G, and trapped poloidal flux of ∼0.15 mWb. The formation process of FRCs is evidenced by the measurement of an array of internal two-dimensional probes; a comparison with the rigid rotor model is presented in this paper. In addition, substantial toroidal electron current is found to be driven, despite the partial RMF penetration. Moreover, the dependence of the driven current on the antenna current is reported and is found to be consistent with the RMF current driving model.

Ion cyclotron resonant heating in the central cell of the Keda Mirror with AXisymmetricity (KMAX)

In this paper, we report the results of ion cyclotron resonance frequency (ICRF) heating in the central cell of a fully axisymmetric tandem mirror. With a total power of 100 kW radiated by double half-turn and half-turn antennas, the plasma diamagnetism increases by 15-fold, with a corresponding peak β ⊥ ∼2%, density ∼ 1.5 × 10 18 m − 3, and total temperature ∼60 eV. The effects of the magnetic configuration on resonance heating and wave emission are studied by varying the magnetic fields at the midplane and at the location of the antennas, respectively; the results confirm that the magnetic beach configuration is key to successful ICRF heating. The axial phase speed measurements suggest that the excited wave is a slow wave in the plasma core and a fast wave at the edge.In this paper, we report the results of ion cyclotron resonance frequency (ICRF) heating in the central cell of a fully axisymmetric tandem mirror. With a total power of 100 kW radiated by double half-turn and half-turn antennas, the plasma diamagnetism increases by 15-fold, with a corresponding peak β ⊥ ∼2%, density ∼ 1.5 × 10 18 m − 3, and total temperature ∼60 eV. The effects of the magnetic configuration on resonance heating and wave emission are studied by varying the magnetic fields at the midplane and at the location of the antennas, respectively; the results confirm that the magnetic beach configuration is key to successful ICRF heating. The axial phase speed measurements suggest that the excited wave is a slow wave in the plasma core and a fast wave at the edge.

A high voltage pulse generator based on silicon-controlled rectifier for field-reversed configuration experiment

A high voltage pulse generator based on a silicon-controlled rectifier has been designed and implemented for a field reversed configuration experiment. A critical damping circuit is used in the generator to produce the desired pulse waveform. Depending on the load, the rise time of the output trigger signal can be less than 1 μs, and the peak amplitudes of trigger voltage and current are up to 8 kV and 85 A in a single output. The output voltage can be easily adjusted by changing the voltage on a capacitor of the generator. In addition, the generator integrates an electrically floating heater circuit so it is capable of triggering either pseudosparks (TDI-type hydrogen thyratron) or ignitrons. Details of the circuits and their implementation are described in the paper. The trigger generator has successfully controlled the discharging sequence of the pulsed power supply for a field reversed configuration experiment.