Changxuan Yu

Experimental investigation of m/n = 1/1 and high-order harmonic modes during the sawtooth oscillation in a low β tokamak plasma

Sawtooth oscillations were investigated using an electron cyclotron emission imaging diagnostic technique on the HT-7 tokamak. High-order harmonic modes are observed in sawtooth precursors. They cause sharp pressure points, leading to the occurrence of reconnection events at more than one place, and which are not preferential on the low field side of the q ~ 1 radius at low density. The reconnection proceeds in two stages. In the first stage, a weak reconnection happens which is characterized by a slow emergence of a small amount of heat and particles through the finite openings. Subsequently, in the second stage, most of the remaining heat and particles in the core escape outward. The observations at low density indicate that high-order harmonic modes are responsible for the secondary reconnection stage, which has seldom been mentioned in reconnection model. The characteristics of the modes are then described by investigating a large number of sawteeth oscillations with different densities and qa values. It is found that when the density increases above a threshold at qa = 3.3, the m/n = 1/1 mode amplitude significantly exceeds its harmonics. The threshold decreases as qa increases.

Design of interferometer system for Keda Torus eXperiment using terahertz solid-state diode sources.

A solid-state source based terahertz (THz) interferometer diagnostic system has been designed and characterized for the Keda Torus eXperiment (KTX). The THz interferometer utilizes the planar diodes based frequency multiplier (X48) to provide the probing beam at fixed frequency 0.650 THz, and local oscillator is provided by an independent solid-state diode source with tunable frequency (0.650 THz +/- 10 MHz). Both solid-state sources have approximately 1 mW power. The planar-diode mixers optimized for high sensitivity, ∼750 mV/mW, are used in the heterodyne detection system, which permits multichannel interferometer on KTX with a low phase noise. A sensitivity of ⟨nel⟩min = 4.5 × 10(16) m(-2) and a temporal resolution of 0.2 μs have been achieved during the initial bench test.

Compatible operation of the power system for steady state and pulse modes in a magnetic torus KT-5D

Compatible operation of steady state mode and pulse mode is realized in the KT-5D device. New power supplies with the operation control systems for the steady state toroidal magnetic field as well as for the vertical field are added, and the rf wave injection systems for sustaining steady state plasmas are upgraded. After the modification, the device now can work not only as a tokomak with pulsed plasma currents as it was but also as a simple magnetized torus with steady state plasma discharges. It allows more flexible and efficient experimental researches on the magnetically confined plasmas to be carried on in the same device.

Fast mega pixels video imaging of a toroidal plasma in KT5D device

A direct imaging system, viewing visible light emission from plasmas tangentially or perpendicularly, has been set up on the KT5D toroidal device to monitor the real two-dimensional profiles of purely ECR generated plasmas. This system has a typical spatial resolution of 0.2mm (1280×1024pixels) when imaging the whole cross section. Interesting features of ECR plasmas have been found. Different from what classical theories have expected, a resonance layer with two or three bright spots, rather than an even vertical band, has been observed. In addition, images also indicate an intermittent splitting and drifting character of the plasmas.

Electron cyclotron heating program and electron cyclotron emission diagnostics on the east and HT-7 superconducting tokamaks

Abstract A program of electron cyclotron heating (ECH) with 4 MW at 140 GHz has been launched for developing scenarios of stable high performance through control of the pressure and current density profiles on the EAST tokamak. Several electron cyclotron emission (ECE) diagnostics are under development as important components of the research program on EAST. The smaller HT-7 tokamak is equipped with a multichannel superheterodyne radiometer and an ECE imaging system. Physics issues including fluctuations driven by electron and ion modes, low frequency zonal flows, magnetic reconnection mechanisms, etc. were investigated on HT-7 using these two systems, which have been moved to EAST after some modifications. New systems, including a 32-channel ECE system and an ECE imaging system of 24(radial) × 16(vertical) channels, are under development. These new systems are designed for the ECH plasma regimes and provide long-range correlation measurements of plasma turbulence. A grating polychromator ECE system has been installed for measurement of the Te profile covering the whole operational range of toroidal magnetic field on EAST.

Magnetically and optoelectronically isolated trigger for pulse-power applications

In this article the design of a magnetically and optically isolated trigger is discussed. Critical issues for trigger design are presented together with some experimental usages. In this trigger, an optical coupler is used to cut off the ground loop between the circuits of the preceding control system and the power supplies of the double functional device KT-5D (as a simple magnetic torus or a tokamak). A magnetic coupler is used to provide a pulse-power output for the silicon controlled rectifier. The output is a 230 mus transistor-transistor logic (TTL) with an amplitude of 3.0 V. The rising time and the trailing time are no more than 4.0 mus. The delay time between the input and the output of the trigger is 6.8+/-0.2 mus. A resistance-capacity branch is integrated into the trigger to provide an adjustable delay time of up to 72 ms. The zero quiescent dissipation character endows the trigger with a long lifetime of years dispensing with any charging or replacing batteries. It is observed that the trigger has a good stability even in a high electromagnetic circumstance (at the order of 1 T). Using it as a trigger for the silicon controlled rectifier, we realized the compatible operation of the steady state mode and the pulse mode in KT-5D.

Investigation of the Effects of the Radial Electric Field by Electrode Biasing in a Toroidal Plasma

The electrode biasing experiments were carried out on the KT‐5C tokamak to investigate the effects of the radial electric field Er on turbulence in a toroidal plasma. It is observed the radial electric field is mainly contributed by the poloidal flow both in Ohmic and in electrode biasing discharges. The changes of Er is led by the changes of poloidal flow, and it is the Reynolds stress driving the poloidal flow in Ohmic discharges in the device, but the Reynolds stress is not so important in the electrode biasing discharges. A modestly enhanced E×B shear layer is formed at the plasma edge by the electrode biasing. In the sheared E×B layer, reductions in the fluctuation amplitude, and the radial correlation length as well as the turbulent particle flux, are observed, Indicating that the turbulence suppression by de‐correlation is due to the modestly enhanced E×B shear layer.

Density fluctuation in HT-6M tokamak by CO2 laser scattering

The small scale density fluctuations in the interior of HT-6M Ohmic plasma have been studied by CO2 laser collective scattering system in deuterium discharges covering a wide range of nqa (chord-average density times safety factor at the limiter) and energy confinement time. The relative density fluctuation level in the interior is inversely proportional to the toroidal magnetic field and average density, and the energy confinement time (tau) E decreases with the fluctuation level increasing in the region where (tau) E linearly increases with nq0.5a and satisfies the Goldston scaling law. It is suggested that the microturbulence in the interior zone is responsible for anomalous transport in tokamaks.

Dual laser acousto‐optical diffraction technique for the calibration of a CO2 laser scattering apparatus

In this paper a dual laser acousto‐optical diffraction technique is described that provides an absolute calibration of a CO2 laser coherent scattering apparatus and other heterodyne receivers. Rather than directly detecting the sound pressure of the acoustic field or the power diffracted from the CO2 laser beam, the sound pressure is measured with the heterodyne detection of acousto‐optical diffraction from a He‐Ne laser beam. Noise equivalent power (NEP) measurement of the heterodyne receiver for the CO2 laser scattering apparatus and the studies of the detector bias and local oscillator power dependence of NEP values are performed by this technique. The measured minimum NEP value (6.8×10−20 W/Hz) is very close to the quantum noise limit.