Real-time plasma position reflectometry system development and integration on COMPASS tokamak

Abstract

O-mode frequency-modulated continuous wave (FMCW) reflectometry provides an alternative to magnetic measurements in the determination of the plasma separatrix position for plasma position control. This type of measurement proves to be particularly attractive for the control of future fusion reactors where the harsh radiation environment may damage magnetic probes or induce non-compensable measurement drifts. Plasma position reflectometry (PPR), first demonstrated in ASDEX-Upgrade, is a control technique that is increasingly important to validate in diversified experimental devices and relevant plasma regimes. The COMPASS tokamak provides suitable conditions for such advanced demonstrations and regular PPR operation and development, thanks to its O-mode reflectometer and Multi-Threaded Application Real-Time executor (MARTe) based real-time control system. Herein we present the integration of a PPR system on COMPASS, both at hardware and software levels. Reflectometry swept measurements require signals to be acquired in bursts of data and streamed to the corresponding MARTe-PPR node through PCIe$\circledR$ fibre-optic links. The data transferred in real-time is used to reconstruct the radial density profiles from which the outer separatrix position is estimated. This estimate is then delivered to the central MARTe controller node via a dedicated Xilinx$\circledR$ Aurora$\circledR$ link at a rate matching COMPASS's 500 $\mu$s slow control cycle. The implemented system systematically met the required latency specifications, being able to deliver an estimation of the plasma radial position capable of successfully replacing the corresponding magnetic measurements in the plasma position feedback control loops.

Introduction

Microwave based diagnostics present a viable solution in plasma position determination, being able to provide robust measurements even in the presence of turbulence. Microwave reflectometry was proposed for ITER [1] as a diagnostic to measure the plasma boundary position (gap control) and is presently envisaged for DEMO [2]. In particular, Plasma position reflectometry (PPR) is convenient for separatrix position based control applications [3] as it works on ordinary mode (O-mode), thus not depending on knowledge of the magnetic field. The technique demonstration was conducted on ASDEX Upgrade, first by probing the plasma at the low-field side (LFS) [4] and latter at LFS and high-field side (HFS) simultaneously [5], [6]. Considering the available O-mode reflectometry diagnostic, the demanding RTCS implemented in MARTe and the relevant plasma regimes achievable in COMPASS tokamak, this device provides the right environment to validate further PPR as a plasma position control technique, in line with the work being conducted on other fusion devices [7].

The COMPASS tokamak, installed in IPP-CAS Prague, is a compact divertor device with an ITER-relevant geometry capable of H-mode operation and targeting pedestal and edge physics studies for ITER [8]. The real-time control system (RTCS) implementation was done on MARTe [9], controlling vertical and horizontal plasma position, equilibrium, shaping and plasma current using magnetic diagnostics and plasma density based on microwave interferometer [10], [11]. Besides other diagnostics, the device is equipped with a multi-band O-mode reflectometer [12]. The Multi-threaded Application Real-Time executor, known as MARTe, is a real-time (RT) framework based on the BaseLib2 C++ library and is employed into the deployment of control systems for critical applications [13]. This framework, employed on different Controlled Nuclear Fusion devices [11] as a robust solution, is compliant with Portable Operating System Interface (POSIX) standards for real-time application interfaces and was ported to several operating systems [13], including Linux, Linux/RTAI, VxWorks, Solaris, OS X and MS Windows.