Compact Torus Injection Research Articles

Vacuum-Ultraviolet spectrometer system for impurity emission measurement on a Compact Torus Injection System of EAST

Recently, a compact torus injection (CTI) system was developed for central fueling on experimental advanced superconducting tokamak (EAST). As impurity generated by the interaction between CTI plasma and the electrode material can dilute the fuel particle, it is important to measure impurity content in CTI and evaluate the effect of impurity on the plasma before the application of CTI to EAST. A vacuum-ultraviolet (VUV) spectrometer that utilizes a concave holographic grating with 1200 groove mm-1 was developed and installed on EAST-CTI for impurity emission measurement and transport study. The mainly parts of the spectrometer are an entrance slit, a concave holographic grating with 1200 groove mm-1 and a charge coupled device (CCD). The spectrometer is designed to image the spectra of 50–460 nm by turning the grating. Preliminary experimental results were obtained from the recent EAST-CTI campaign. Impurity line spectra from 50 to 460 nm wavelength range were measured and identified for several impurity species, such as iron, copper, chromium, oxygen, carbon and fluorine. For example, spectra in the intervals of 150 nm to 300 nm contained strong Fe lines. Helium spectra are also measured in the form of He I and He II spectral lines as helium is the main component of the plasma. The results show that the VUV spectrometer is capable of measuring impurity radiations on EAST-CTI and will be a useful tool for impurity behavior study.

Theory and simulation of the force-free equilibria in the EAST-CTI

Compact torus injection (CTI) is a novel approach to fueling the core of a fusion reactor. In this paper, the force-free equilibrium based on the EAST-CTI is calculated. It is found that the , which is proportional to the ratio of the toroidal current to the toroidal magnetic field, plays a key role for the force-free equilibrium. When it is smaller than the eigenvalue, the open poloidal magnetic flux decreases while the magnetic field strength increases as its value increases. However, once equals the eigenvalue, the amplitude of the magnetic field tends to infinity and the boundary magnetic field is tangential to the wall. In addition, in this work we found that the eigenvalue is mainly determined by the width of the formation region, enlarging the width will significantly reduce the eigenvalue and further affecting the compact torus (CT) formation. Another interesting phenomenon in this work is that the poloidal magnetic field plays a hindering role in the formation of CT, and theoretically, CT is difficult to detach from the forming region when is less than the threshold . In experiments, we show that a discharge with leads to an increased delay between the onset of the current in the acceleration and formation regions. These findings provide some guidance for the carrying out of the experiments and future modifications of the device.

Design and simulation of 1.5 T conduction-cooled superconducting magnet with flip-over capability

The Compact Torus Injection (CTI) system is a plasma physics device used to create and study compact torus plasmas, which are dense and highly energetic structures with potential applications in fusion energy research and space propulsion. The Experimental Advanced Superconducting Tokamak compact torus injection (EAST-CTI) system is a device used for plasma heating and fueling in fusion experiments. In this paper, a 1.5 T cryocooled superconducting magnet was developed for the EAST-CTI system, which requests the magnet system can work vertically or horizontally to meet different working conditions. The design of the superconducting magnet and cryogenic system is based on the analysis of the thermal and mechanical characteristics of the magnet system. Thermal and mechanical finite element simulation were used to support the design. The 1.5 T magnet superconducting magnet was manufactured according to the design and simulation works. After 35 h cooling, the superconducting magnet stabilized at 3.87 K, and successfully provided a horizontal magnetic field environment for the experiments on EAST-CTI system.

Development of a compact torus injection system for the Keda Torus eXperiment

A compact torus injection system, KTX-CTI, has been developed for the planned injection experiments on the Keda Torus eXperiment (KTX) reversed field pinch (RFP) device to investigate the physics and engineering issues associated with interaction between a compact torus (CT) and RFP. The key interests include fueling directly into the reactor center, confinement improvement, and the injection of momentum and helicity into the RFP discharges. The CT velocity and mass have been measured using a multichannel optical fiber interferometer, and for the first time the time evolution of the CT density profile during CT propagation is obtained. The effects of discharge parameters on the number of injected particles, CT velocity and CT density have been characterized: the maximum hydrogen CT plasma mass, is 50 μg, corresponding to 30% of the mass in a typical KTX plasma; the CT velocity exceeds 120 km s−1. It is observed for the first time that multiple CTs can be produced and emitted during a very short period (<100 μs) in one discharge, which is significant for the future study of repetitive CT injection, even with an ultra-high frequency.

An overview of diagnostic upgrade and experimental progress in the KTX

The Keda Torus eXperiment (KTX) is still operated in the commissioning phase, and preparation for the operation capability of the KTX phase II upgrade is underway. The diagnostics in the KTX have been greatly developed: (1) the terahertz interferometer has been upgraded to seven chords for electron density profile inversion; (2) a Thomson scattering system with a 5 Joule laser has been installed and commissioning is in progress; (3) a 3D movable probe system has been developed for the electromagnetic turbulence measurement; (4) double-foil soft x-ray imaging diagnostics have been set up and a bench test has been completed; (5) an edge capacitive probe has been installed for the radial electrical field measurement; (6) a multi-channel spectrograph system has been built for detecting impurities of carbon and oxygen. In addition, the design of a new compact torus injection system has been completed for feeding and momentum driving. Pilot research, such as the 3D reversed field pinch physics and electromagnetic turbulence, etc, have been conducted in the discharge status of the KTX. The 3D spectra characters of electromagnetic turbulence are firstly measured using a classical two-point technique by Langmuir probe arrays set on the 3D movable probe system and edge magnetic sensors. The forward scattering is collected by the interferometer system, which shows the potential for turbulence research. The electromagnetic turbulence is tentatively investigated in the KTX. The formation of a quasi-single-helicity state in the KTX regime is also preliminarily explored in simulation.

Effects of edge safety factor on the toroidal flow velocities during the resonant magnetic perturbations and compact torus injection experiments on the STOR-M tokamak

ABSTRACTThe effects of the edge safety factor on the toroidal flow velocity modification due to resonant magnetic perturbation (RMP) and tangential compact torus injection (CTI) have been investigated on the Saskatchewan Torus-Modified (STOR-M) tokamak. The (l = 2, n = 1) RMP coils are wound against the outer surface of the vacuum vessel. The safety factor has been varied by changing the plasma current at a fixed toroidal magnetic field. As the edge safety factor is reduced, the magnetic perturbations on the q = 2 surface are expected to shift radially outwards, moving closer towards the RMP coils. The evolution of the plasma flow velocity was measured through ion Doppler spectroscopy (IDS). It has been found that RMP changes the flow velocity towards the direction of the plasma current direction and this effect is enhanced when the edge safety factor is reduced. When CT was injected tangentially along the plasma current direction, similar changes in the flow velocity have been found. However, the change is not sensitive to the edge safety factor in the CT injection case.

Modification of toroidal flow velocity through momentum Injection by compact torus injection into the STOR-M tokamak

In the Saskatchewan torus-modified (STOR-M) tokamak, tangential compact torus injection (CTI) experiments have been performed with normal (counter-clockwise, CCW, top view) and reversed (clockwise, CW, top view) plasma current directions while the compact torus (CT) injection direction remains in the CCW direction. The intrinsic toroidal flow direction reverses when the discharge current is reversed. However, the change in the toroidal flow direction is always toward the CTI direction (CCW). It has been determined that the momentum in high density and high velocity CT is more than ten times larger than the intrinsic toroidal rotation momentum in the typical STOR-M plasma. Therefore, the modification of the plasma toroidal rotation velocity is attributed to momentum transfer from CT to the tokamak discharge.

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.

Bench test and preliminary results of vertical compact torus injection experiments on the STOR-M tokamak

The University of Saskatchewan compact torus injector has been modified to allow vertical injection of the compact torus (CT) into the STOR-M tokamak. The injector stayed at the horizontal position and the CT trajectory was bent by 90° using a curved conducting drift tube. The curved drift tube did not significantly slow the CT velocity down or change the CT magnetic field topology. Preliminary vertical CT injection experiments have revealed a prompt increase in the line averaged electron density and in the soft x-ray radiation level in the tokamak discharge immediately following vertical CT injection. Suppression of the m = 2 Mirnov oscillation amplitude has also been observed after CT injection.

Tangential and Vertical Compact Torus Injection Experiments on the STOR-M Tokamak

This paper describes the setup and results of compact torus (CT) injection experiments on the STOR-M tokamak. Tangential CT injection into STOR-M induced H-mode-like phenomena including doubling the electron density, reduction in the Hα radiation level, suppression of the floating potential fluctuations, suppression of the m = 2 Mirnov oscillations, and increase in the global energy confinement time. Experimental setup, bench-test results, and some preliminary injection data for vertical CT injection experiments on STOR-M will be shown. In addition, numerical simulations of the CT trajectories in tokamak discharges for both tangential and vertical injection geometries will be discussed.

Improved confinement induced by tangential injection of compact torus into the Saskatchewan Torus-Modified (STOR-M) tokamak

Compact torus injection into the Saskatchewan Torus-Modified [Phys. Fluids B 4, 3277 (1992)] tokamak discharges has triggered improved confinement characterized by an increase in the electron density by more than twofold, 30% reduction in the Hα radiation level, significant suppression of floating potential fluctuations and m=2 Mirnov oscillations. In this paper, we present detailed experimental setup and results, as well as an extended theory explaining the mechanism for triggering improved confinement in a tokamak by compact torus injection.

Role of parallel flow in the improved mode on the STOR-M tokamak.

A novel feature of the H-mode induced by compact torus injection on the STOR-M tokamak is observed. There is almost no change in the radial electric field profiles during and after the L-H transition. The usual hypothesis of the E x B shear stabilization mechanism is therefore unlikely to play a role in this transition. A new mechanism of the stabilization of microinstabilities by parallel flow is suggested as the plausible cause for the transition to this improved regime.

The Role of Plasma Biasing in the H‐Modes in the STOR‐M Tokamak

H-modes have been induced in the STOR-M tokamak with various techniques, including short current pulses, electrode/limiter biasing, and compact torus injection. The common features of the H-modes in STOR-M include a sudden drop in the H α radiation level, a significant increase in the electron density, and modification of the edge electron density and floating potential profiles. Other phenomena specific to certain types of H-modes in STOR-M will also be presented.