Control Of Vertical Instability Research Articles

WEST plasma control system status

This paper presents the current status of the WEST Plasma Control System (PCS). From the first campaign at the end of 2016 to the latest campaign at the end of 2022, significant upgrades have been deployed to improve several aspects of the machine performance. The vertical instability control loop has been upgraded in order to increase the operation domain (i.e. plasma elongation). This upgrade optimizes the delay at each stage of the control loop using raw magnetic data coupled to a real-time vertical displacement observer. This allows for different shape control modes that address various operation issues such as X-point formation and gap control. The control of the plasma density uses generic feedforward and/or feedback controllers to control the 21 piezo-electric valves of the gas injection system and/or the pellet injection system. The heating systems of WEST (ICRH and LHCD) are also driven by the PCS, which has the ability to control both the power and the phase of the antennas. More recently, feedback control of the loop voltage using the LHCD power has been fully implemented in order to perform plasma discharges of up to 1000 s. Also in the most recent campaigns, the entirety of the lower divertor has been actively cooled. Operating a fully actively cooled metallic machine requires real-time machine protection strategies to avoid water-leaks or other damage to the first wall. On WEST, such protections are based on spectroscopic density measurements of impurities and on the infrared monitoring of critical components. These strategies trigger either minor adjustments of the reference scenario (e.g. decreasing of the requested heating power) or a soft plasma termination.

Controllability study of plasma vertical instability in HL-2M

Plasma vertical instability control is achieved by passive stabilization from induced currents flowing in surrounding structures and by active control using magnetic coils. Both aspects are numerically investigated for the medium-size copper tokamak HL-2 M, presently under construction in China. The double-shell vessel can provide some passive stabilization, with the longest wall penetration time of about 4 ms. We design an out-vessel active control system based on a single null divertor equilibria most demanding for vertical control and the maximum controllable vertical displacement dZmax ≥3.0 cm is required. Robust active control of the vertical instability can be achieved in HL-2 M, by the PF7U and PF7L coils connected in anti-phase and powered by an upgraded power supply with Imax = 4.0 kA, Vmax =2 kV and f = 5 kHz. Safe control is possible for representative plasma configurations with growth rate ranging from 200 s−1 to 400 s−1. Efficiency of each PF coil location for VDE control is investigated, PF7U/L is most capable for single null and double null equilibria, but control capability of PF7U/L is not as good as PF2U/L for snowflake divertor plasma, PF7U/L and PF2U/L will be combined together to provide safe control of low βp ‘close nulls’ advanced divertor plasma in the future.

Non-inductive vertical position measurements by Faraday-effect polarimetry on EAST tokamak.

Vertical instability control in an elongated plasma is highly desirable for a tokamak reactor. A multi-channel 694 GHz far-infrared laser-based polarimeter-interferometer system has been used to provide a non-inductive vertical position measurement in the long-pulse EAST tokamak. A detailed comparison of vertical position measurements by polarimetry and external inductive flux loops has been used to validate Faraday-effect polarimetry as an accurate high-time response vertical position sensor.

Enhancement of EAST plasma control capabilities

In order to improve the plasma control performance and enhance the capability for advanced plasma control, new algorithms such as PEFIT/ISOFLUX plasma shape feedback control, quasi-snowflake plasma shape development and vertical control under new vertical control power supply, have been implemented and experimentally tested and verified in EAST 2014 campaign. P-EFIT is a rewritten version of EFIT aiming at fast real-time equilibrium reconstruction by using GPU for parallelized computation. Successful control using PEFIT/ISOFLUX was established in dedicated experiment. Snowfldivertor plasma shape has the advantage of spreading heat over the divertor target and a quasi-snowflake (QSF) configuration was achieved in discharges with Ip=0.25 MA and Bt=1.8T, κ∼1.9, by plasma position feedback control. The shape feedback control to achieve QSF shape has been preliminary implemented by using PEFIT and the initial experimental test has been done. For more robust vertical instability control, the inner coil (IC) and its power supply have been upgraded. A new control algorithm with the combination of Bang-bang and PID controllers has been developed. It is shown that new vertical control power supply together with the new control algorithms results in higher vertical controllability.

Bangbang controller design and implementation for EAST vertical instability control

In the EAST 2014 campaign, a new internal coil (IC) power supply was used in order to enhance the control over the plasma’s vertical instabilities. The IC power supply now allows for current and voltage working modes with much higher peak voltages and currents and faster response time. In comparison the previous power supply only allowed for the current mode. A Bangbang and PID composite controller has been designed for the voltage mode based on optimal control theory and the RZIP rigid plasma response model. This paper will demonstrate that faster and enhanced controllability are realized with the combination of Bangbang and PID controller. For the large z position drift, the Bangbang controller will export the maximum voltage to achieve much faster power supply response and slow the vertical displacement events (VDEs). The PID controller is used for the small z drifts which will finally stabilize the VDEs with minimum z position oscillation. Furthermore, to evaluate the time latency of this control system and power supply, the stability and performance of the closed loop were simulated and analysed. This controller was finally implementation and test on EAST using the Quasi-snowflake shape which achieved growth rates of 500s−1. This paper shows that the new power supply using the bangbang+PID controller can significantly enhance the control over vertical instabilities.

Concept Design and Analysis of CFETR Vertical Instability Control Using Passive Plates and In-Vessel Coils

The Chinese Fusion Engineering Test Reactor (CFETR) is under conceptual design. In this paper, we address the key issues related to the design of the plasma vertical instability control in CFETR. Based on the linear rigid model, the open loop vertical growth rates of typical CFETR equilibria are calculated. The results show that passive plates are necessary for CFETR passive stability. This paper also investigates the possibility of using in-vessel coils to improve the control of vertical stabilization.

Electromagnetic Modeling of the Passive Stabilization Loop at EAST

A passive stabilization loop (PSL) has been designed and manufactured in order to enhance the control of vertical instability and accommodate the new stage for high-performance plasma at EAST. Eddy currents are induced by vertical displacement events (VDEs) and disruption, which can produce a magnetic field to control the vertical instability of the plasma in a short timescale. A finite element model is created and meshed using ANSYS software. Based on the simulation of plasma VDEs and disruption, the distribution and decay curve of the eddy currents on the PSL are obtained. The largest eddy current is 200 kA and the stress is 68 MPa at the outer current bridge, which is the weakest point of the PSL because of the eddy currents and the magnetic fields. The analysis results provide the supporting data for the structural design.

Control of the vertical instability in advanced tokamak

The control of vertical instability is studied for DN divertor plasmas in a tokamak with both passive and active control coils. The characteristic parameter of the vertical instability and a second damp stable operating region of the feedback gain are found. The favorable effects of higher triangularity and hollow profile on the critical elongation are demonstrated.

Electromagnetic analysis of the twin loops structure for the control of the vertical instability in NET/ITER fusion reactor

The problem of the vertical instability of the plasma column in the NET/ITER (International Thermonuclear Experimental Reactor) reactor is analyzed via a suitable low-frequency electromagnetic modeling of the structure. For this purpose a 3-D eddy current code is coupled with techniques belonging to MHD (magnetohydrodynamics) and control theory domains. In particular, the specific design concept called twin loops, which has recently been proposed for the passive control of the plasma vertical instability in NET/ITER, is analyzed. The results show good agreement among models of different natures adopted to solve this kind of coupled problem. >