Development of FPGA controlled diagnostics on the MAST fusion reactor

Field Programmable Gate Array technology (FPGA) is very useful for implementing high performance digital signal processing algorithms, data acquisition and real-time control on nuclear fusion devices. This thesis presents the work done using FPGAs to develop powerful diagnostics. This has been achieved by developing embedded Linux and running it on the FPGA to enhance diagnostic capabilities such as remote management, PLC communications over the ModBus protocol and UDP based ethernet streaming. A closed loop real-time feedback prototype has been developed for combining laser beams onto a single beam path, for improving overall repetition rates of Thomson Scattering systems used for plasma electron temperature and density radial profile measurements. A controllable frequency sweep generator is used to drive the Toroidal Alfven Eigenmode (TAE) antenna system and results are presented indicating successful TAE resonance detection. A fast data acquisition system has been developed for the Electron Bernstein Wave (EBW) Synthetic Aperture Microwave Imaging system and an active probing microwave source where the FPGA clock rate has been pushed to the maximum. Propagation delays on the order of 2 nanoseconds in the FPGA have been finely tuned with careful placement of FPGA logic using a custom logic placement tool. Intensity interferometry results are presented on the EBW system with a suggestion for phase insensitive pitch angle measurement

Calibration of the high-frequency magnetic fluctuation diagnostic in plasma devices

The increasing reservoirs of energetic particles which drive high-frequency modes, together with advances in the understanding of magnetohydrodynamics, have led to a need for higher-frequency (50 kHz to >20MHz) measurements of magnetic field fluctuations in magnetic fusion devices such as tokamaks. This article uses transmission line equations to derive the voltage response of a Mirnov coil at the digitizer end of a transmission line of length ℓ. It is shown that, depending on the terminations of the line, resonances can occur even for ℓ/λ⪡1, with λ the wavelength of a fluctuation in the transmission line. A lumped-circuit model based on the approach of Heeter et al. [R. F. Heeter, A. F. Fasoli, S. Ali-Arshad, and J. M. Moret. Rev. Sci. Instrum.71, 4092 (2000)] is extended to enable the inclusion simultaneously of both serial resistance and parallel conductance elements. As originally proposed by Heeter et al. the lumped-circuit model offers the advantage of remote calibration; this may be of particular value when upgrading existing systems to operate at frequencies above the original design specification. It is formally shown that the transmission line equations for the transfer function and measured impedance reduce to those of the lumped circuit model of Heeter et al. under specific conditions. The result extends the use of the lumped-circuit model of Heeter et al., which can be used to extract the transfer function from measurement of the impedance, beyond the case of an open-circuit termination. Although the numerical procedure does exhibit some problems associated with non-uniqueness, it provides a simple calibration method for systems that are not well defined. Using typical parameters for a high-frequency Mirnov coil installed on the Joint European Torus (JET) tokamak, the lumped-circuit approximation agrees with the steady-state transmission line model to within 0.015° in phase and 22% in amplitude for frequencies up to 1 MHz. A matched termination, though eliminating line resonances and reducing the length of time for the system to reach steady state, is inappropriate for the JET-type coils which exhibit significant temperature-dependent resistance. Finally, for fluctuations of finite duration, a method of computing the discrepancy due to the simplifying assumption of Fourier-stationary conditions is described.This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by EURATOM

Multi-machine scaling of the main SOL parallel heat flux width in tokamak limiter plasmas

As in many of today’s tokamaks, plasma start-up in ITER will be performed in limiter configuration on either the inner or outer midplane first wall (FW). The massive, beryllium armored ITER FW panels are toroidally shaped to protect panel-to-panel misalignments, increasing the deposited power flux density compared with a purely cylindrical surface. The chosen shaping should thus be optimized for a given radial profile of parallel heat flux, q in the scrape-off layer (SOL) to ensure optimal power spreading. For plasmas limited on the outer wall in tokamaks, this profile is commonly observed to decay exponentially as q q = − exp ( / r λ ) 0 q omp , or, for inner wall limiter plasmas with the double exponential decay comprising a sharp near-SOL feature and a broader main SOL width, λq omp. The initial choice of λq omp , which is critical in ensuring that current ramp-up or down will be possible as planned in the ITER scenario design, was made on the basis of an extremely restricted L-mode divertor dataset, using infra-red thermography measurements on the outer divertor target to extrapolate to a heat flux width at the main plasma midplane. This unsatisfactory situation has now been significantly improved by a dedicated multi-machine ohmic and L-mode limiter plasma study, conducted under the auspices of the International Tokamak Physics Activity, involving 11 tokamaks covering a wide parameter range with R = = 0.4–2.8 m, 1 B I 0 p .2–7.5 T, = 9–2500 kA. Measurements of λq omp in the database are made exclusively on all devices using a variety of fast reciprocating Langmuir probes entering the plasma at a variety of poloidal locations, but with the majority being on the low field side. Statistical analysis of the database reveals nine reasonable engineering and dimensionless scalings. All yield, however, similar predicted values of λq omp mapped to the outside midplane. The engineering scaling with the highest statistical significance, λ = ( / ( )) ( / /κ) − − q 10 P V W m a R omp tot 3 0.38 1.3 , dependent on input power density, aspect ratio and elongation, yields λq omp = [7, 4, 5] cm for Ip = [2.5, 5.0, 7.5] MA, the three reference limiter plasma currents specified in the ITER heat and nuclear load specifications. Mapped to the inboard midplane, the worst case (7.5 MA) corresponds to λq ~ 57 1 ± 4 imp mm, thus consolidating the 50mm width used to optimize the FW panel toroidal shape.EURATOM 633053Czech Science Foundation GA CR P205/12/2327, GA15-10723S, MSMT LM2011021US Department of Energy DE-FG02- 07ER54917, DE-AC02-09CH11466, DE-FC02-04ER5469

A Doppler Coherence Imaging Diagnostic for the Mega-Amp Spherical Tokamak

Developing a plasma exhaust solution suitable for future high power tokamaks is one of the major challenges facing the development of magnetic confinement fusion as a terrestrial energy source. In order to improve our understanding of the relevant physics, high quality experimental measurements of plasma dynamics in the scrape-off-layer (SOL) and divertor plasma regions are required. This thesis is concerned with the development of diagnostic instrumentation for measuring exhaust plasma flow: an important phenomenon with implications for the control of exhaust particles and heat as well as unwanted impurities. Coherence imaging spectroscopy (CIS) is a relatively new diagnostic technique which can be used to obtain time resolved 2D imaging of flows using the Doppler shifts of visible ion emission lines. The technique makes use of an imaging polarization interferometer and is based on the concept of Fourier transform spectroscopy. The principle advantages of this over other flow measurement techniques are the very large amount of spatial information collected, and the simple relationship between the measured quantities and spatially varying flows in the plasma. This thesis presents the development of, and first results from, a CIS ion flow diagnostic for the UK's Mega Amp Spherical Tokamak (MAST). The diagnostic can image flows of intrinsic C II, C III and He II impurity ions over fields of view between 10 - 45 degrees, at frame rates between 50Hz - 1kHz and with flow resolution typically around 1km/s (compared with measured flows of typically 5 - 30km/s). Spatial resolution is better than ~4.5 cm over a 1.4 x 1.4m area of the plasma cross-section. After reviewing the principles and theory of the coherence imaging technique, the design of a coherence imaging flow diagnostic for MAST is presented in detail. Results of careful laboratory characterization and calibration of the instrument are presented, and the instrument performance is compared to the design calculations. The diagnostic was used successfully for flow measurements on MAST during an experimental campaign in May - September 2013. On-plasma validation of the instrument performance is presented, as well as examples of novel flow observations made with the diagnostic. These include field-aligned flow structures associated with high field side gas fuelling of the plasma, and the first measurements of spatial flow structure in the divertor associated with the application of resonant magnetic perturbations (RMPs). Possible future improvements to the instrument design and extensions of the present work are suggested

Ion cyclotron emission on ASDEX upgrade

This works deals with the Ion Cyclotron Emission (ICE), a plasma instability that takes place both in astrophysical plasmas and in fusion energy facilities like Tokamaks and Stellarators, when a population of high energetic ions is present. These fast ions can interact with the waves which propagate in the background thermal plasma and excite instabilities in the Mega-Hertz range. This emission can be measured in a non-intrusive way with radio-frequency probes and provide information on the characteristics of the fast ions. The hope of a new diagnostic sparked many studies in the years 1992-2002 but, in spite of the theoretical and experimental progresses, no practical instrumentation was achieved. There are indeed two main difficulties: first, the ICE involves many different types of plasma phenomena: waves propagation, resonances, conversion and absorption in complex geometries, core and edge plasma modelling, fast ion creation and trajectories; all these aspects are entangled. Therefore, accurate data both in time and frequency domains and a theory that covers these physics fields are necessary to distinguish the impact of these different phenomena. Second, there are technical difficulties in measuring high-frequency signals with a sufficient Signal-to-Noise Ratio to discriminate it from the background noise. The purpose of this study is to address these issues with the use of the latest acquisition technologies and an improved ICE theory, which can relate in a new light the properties of the fast ions to the characteristics of the emission