Imaging of turbulent structures and tomographic reconstruction of GyM plasma emissivity

In GyM, a linear magnetized plasma device, low-frequency electrostatic fluctuations are measured by means of Langmuir probes. To complement electrostatic probe measurements of plasma turbulence and the study of plasma structures, a nonperturbative direct imaging system has been used on GyM, including a fast-framing Photron APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute time-resolved emissivity profiles of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. The validity and robustness of the tomographic reconstruction technique are examined with respect to noise and wall reflection effects. The tomographic reconstruction is applied to fast camera movies acquired with a frame rate of 75 kHz and 4 is of exposure time to obtain the temporal evolution of the emissivity fluctuations. Plasma structures can be detected and tracked in the reconstructed emissivity movies with a finest spatial resolution of 2 cm. Conditional average sampling is used to determine the size and speed of turbulent structures.

Exploration of candidate fusion reactor regimes by real time control of tokamak plasma shape

Himank Anand

The potential of nuclear fusion to provide a practically inexhaustible source of energy has motivated scientists to work towards developing nuclear fusion tokamak power plants. Stable operation of a tokamak at high performance requires simultaneous treatment of several plasma control problems. Moreover, the complex physics which governs the tokamak plasma evolution must be studied and understood to make correct choices in controller design. This mutual inter-dependence has informed this thesis, using control solutions as an experimental tool for physics studies, and using physics knowledge for developing new advanced control solutions. The TCV tokamak at SPC-EPFL is ideally placed to explore issues at the interface between plasma physics and plasma control, by combining a state-of-the-art digital real time control system with a flexible and diverse set of actuators including a full set of independently powered shaping coils. The recent deployment of the real time version of the Grad-Shafranov equilibrium reconstruction code LIUQE, with a sub-ms cycle time in the digital control system, has facilitated the design of a new generalised plasma position and shape controller, based on the information on poloidal flux and magnetic field provided by the real-time Grad-Shafranov solver. The first issue addressed in the thesis is the development and experimental testing of a new real time control strategy to construct a generalised control algorithm for not only controlling the position of the plasma but also to aid in the precise control of higher order shape moments, X-points and strike points, particularly in advanced plasma configurations such as negative-triangularity plasmas, snowflake and super-X divertors, and doublets. A controller formulation ensuring flexibility through an ordering of controlled variables from the most easily to the least easily controlled, while respecting the hardware limits on the poloidal field coil currents, is developed. The successful experimental implementation of the control algorithm has been demonstrated for both fixed and time varying plasma position and shape for limiter and divertor plasma discharges. In addition, the controller has provided satisfactory performance with respect to plasma scenarios involving complex changes in the plasma shape and position. The second issue addressed in the thesis is the application of the generalised plasma position and shape controller to a snowflake plasma configuration. A comparison between the optimised generalised plasma position and shape controller with the performance of the TCV hybrid controller for a given reference snowflake plasma discharge showed a marked improvement in various geometrical properties of the snowflake plasma configuration in the vicinity of the null point. However, strong control of the poloidal magnetic field at the two X-points resulted in a tradeoff on the upper part of the plasma boundary, where the overall precision was comparable to that of the legacy controller. In the experimental time available, the snowflake shots developed exhibited a boundary that was too close to the inner wall of the vessel, modifying the edge plasma behaviour (studied with infrared cameras and Langmuir probes) and making it difficult to study the physics properties of the exact snowflake. However, further optimisation should well be possible.

EPFL2017

Imaging of turbulent structures and tomographic reconstruction of TORPEX plasma emissivity

Ambrogio Fasoli, Ivo Furno, Davoud Iraji, Christian Gabriel Theiler

In the TORPEX [ A. Fasoli et al., Phys. Plasmas 13, 055902 (2006) ], a simple magnetized plasma device, low frequency electrostatic fluctuations associated with interchange waves, are routinely measured by means of extensive sets of Langmuir probes. To complement the electrostatic probe measurements of plasma turbulence and study of plasma structures smaller than the spatial resolution of probes array, a nonperturbative direct imaging system has been developed on TORPEX, including a fast framing Photron-APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute the poloidal emissivity profile of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. This allows comparing statistical, spectral, and spatial properties of visible light radiation with electrostatic fluctuations. The shape and position of the time-averaged reconstructed plasma emissivity are observed to be similar to those of the ion saturation current profile. In the core plasma, excluding the electron cyclotron and upper hybrid resonant layers, the mean value of the plasma emissivity is observed to vary with (Te)^α(ne)^β, in which α = 0.25–0.7 and β = 0.8–1.4, in agreement with collisional radiative model. The tomographic reconstruction is applied to the fast camera movie acquired with 50 kframes/s rate and 2 μs of exposure time to obtain the temporal evolutions of the emissivity fluctuations. Conditional average sampling is also applied to visualize and measure sizes of structures associated with the interchange mode. The ω-time and the two-dimensional k-space Fourier analysis of the reconstructed emissivity fluctuations show the same interchange mode that is detected in the ω and k spectra of the ion saturation current fluctuations measured by probes. Small scale turbulent plasma structures can be detected and tracked in the reconstructed emissivity movies with the spatial resolution down to 2 cm, well beyond the spatial resolution of the probe array.

American Institute of Physics2010

Fast Imaging of Turbulent Plasmas in the TORPEX Device

Davoud Iraji

This thesis addresses the issue of turbulence generated modes and intermittent structures using a fast visible imaging system on TORPEX. The plasma radiation mechanisms and experimental setup of the imaging system are discussed together with an optimized tomographic reconstruction method. In TORPEX typical plasmas, radiative ionization is the dominant radiation mechanism. The photon flux emitted by hydrogen plasmas is a linear function of √Te and ne for Te ≥ 4eV. A Photron Ultima APX-RS fast visible camera was used to acquire the light emission of TORPEX plasmas. The camera equipped with the image intensifier is able to image TORPEX plasmas up to 200 kframes/s of framing rate and down to 1µs of exposure time, which results in clear images of plasma structures. The time-resolved poloidal emissivity of TORPEX plasma emission is tomographically reconstructed from tangentially viewed images using a pixel method and singular value decomposition approach. The spatial resolution in the reconstructed emissivity profiles, is 2cm. The plasma emissivity and ion saturation current profiles are compared using the conditional average sampling (CAS) technique and statistical analysis of fluctuations. The CAS is performed to visualize the mode structures and to estimate their sizes. The resulting fast imaging of the plasma, non-perturbative and high spatio-temporal resolution diagnostics, can visualize small scale turbulent plasma structures, well beyond the typical spatial resolution of probe arrays. A two-dimensional gas puff (2D-GPI) imaging system, including a 2D movable gas puff nozzle and a fast framing camera, has been developed in the TORPEX device. Direct local measurements from a large number of probes, distributed across the plasma column, are used to establish a correlation between the plasma parameters and the locally measured visible light emission. GPI and ion saturation current signals are well correlated, with correlation coefficients higher than 0.75. Fourier analysis of the GPI and internal probes show that the GPI can detect the dominant interchange mode. Quasi-coherent interchange modes and intermittent, turbulence generated blobs can be detected and studied using fast visible imaging in TORPEX. For an increasing vertical magnetic field, the principal interchange mode is damped and a sharp transition occurs between low-k and high-k harmonics. The scaling of the radial width of different interchange harmonics with respect to the vertical wave-number (kz) shows good agreement between the experimental and theoretical radial-width-kz scaling in TORPEX plasmas. The gradient-removal mechanism, describing the level of turbulent fluctuations, was tested using experimental data from the visible camera. The saturation level of the interchange instabilities predicted by the gradient-removal mechanism is in reasonable agreement with the experimental measurements. Tomographically inverted data from the fast camera were used to study the propagation of intermittent blobs. The experimental speed-versus-size scaling of blobs in the camera data is in good agreement with theoretical predictions. This thesis confirms the possibility of using fast imaging to estimate saturation level of fluctuations associated with ideal interchange instability and to reconstruct the blob speed-size scaling law in tokamak plasmas non-perturbatively. This can be used to non-perturbatively estimate the transport levels in the SOL of fusion devices.

EPFL2011