The effect of the secondary x-point on the scrape-off layer transport in the TCV snowflake minus divertor

The snowflake (SF) magnetic configuration is investigated as a potential power exhaust solution for a fusion reactor, but also to improve our understanding of divertor physics in general. Unlike a conventional single null (SN), it features an additional nearby x-point, which deeply modifies the magnetic field in the scrape-off layer (SOL) and can thereby affect parallel and cross-field transport of heat and particles. This paper investigates the power exhaust properties of the snowflake minus (SF-) configuration on the TCV tokamak, for Ohmically heated, L-mode, low-density, attached plasmas for a range of x-point separations, magnetic field directions and locations of the secondary x-point (low-field-side (LFS) or high-field-side (HFS)). Due to the relatively large x-point separation in physical space, this study probes x-point transport features in general, rather than reactor relevant aspects of the SF configuration. The target heat fluxes at all strike points are simultaneously monitored with an infrared (IR) thermography system and the kinetic profiles of the SOL at the outer mid-plane with a reciprocating probe (RCP). The placement of the additional x-point is seen to affect the inner-outer divertor power balance. An effective heat flux width lambda(eff)(q,u) for the SOL in the low poloidal field region is inferred from the measured power repartition between the two SOL manifolds created by the secondary x-point. For the HFS SF- configuration (secondary null in the LFS SOL), the lambda(eff)(q,u) is two times larger than that measured by the RCP at the outer mid-plane and by IR at the outer target of a comparable SN, while the outer mid-plane SOL profiles are similar to the SN. This is interpreted as the effect of increased effective cross-field diffusivity chi(null)(perpendicular to) in the intra-null region relative to the rest of the SOL. For the HFS SF-configuration (secondary null in the HFS SOL), no such increased transport is observed. The pressure-driven plasma convection expected near the primary null cannot explain the increased chi(null)(perpendicular to), which is instead consistent with interchange ballooning-like turbulence enhanced by the low poloidal field.

Basil Duval, Benoît Labit, Roberto Maurizio, Holger Reimerdes, Umar Sheikh, Christian Gabriel Theiler, Cedric Kar-Wai Tsui

2019

Scrape-Off Layer physics in limited plasmas in TCV

Federico Nespoli

Controlled nuclear fusion is the most promising candidate for being an inexhaustible, clean and intrinsically safe energy source. In the tokamak fusion reactor concept, a high temperature plasma is confined by magnetic fields. Turbulence diffuses the confined plasma to the tokamak outermost region, the Scrape-Off Layer (SOL), featuring open field lines. In the SOL, the plasma is convected along the field lines and deposited on the solid surfaces of the tokamak wall. The plasma-wall interaction through the SOL strongly affects the reactor performances, and the elevated heat loads on the solid surfaces are one of the most limiting factors for fusion. SOL physics is not completely understood, not even in the simple

limited'' configuration, where the main plasma touches the reactor wall, and the contact point defines the Last Closed Flux Surface (LCFS). In this thesis, we advance the understanding of SOL physics in limited plasmas, combining experiments and numerical simulations. In particular, two topics are addressed. First, the separation between the

near'' and ``far'' SOL is investigated. The near SOL extends typically a few mm from the LCFS, features steep radial profiles of parallel heat flux and is responsible for the peak of heat deposition on the tokamak first wall. The far SOL, typically a few cm wide, features flatter heat flux profiles, and accounts for the majority of the heat deposited on the first wall. Secondly, blob dynamics is investigated. Blobs are high density plasma filaments generated by turbulence, and are an ubiquitous feature of plasmas in open magnetic field lines. The blobs travel outwards to the reactor walls, increasing the cross field transport. Dedicated experiments have been performed on the TCV tokamak. Inboard-limited Deuterium (D) and Helium (He) plasma discharges are performed, varying the main plasma parameters (current, density and shaping). The parallel heat flux radial profiles are determined with infrared thermography. For the first time, the presence of a near SOL in TCV limited plasmas is reported, both for D and He discharges. The near SOL is found to disappear for high plasma resistivity. Non-ambipolar currents are measured to flow to the wall in the near SOL using Langmuir probes, and their presence is found to correlate with the strength of the near SOL heat fluxes. A simple interpretation is given. The heat fluxes and electric potentials are also measured on the low field side using a reciprocating Langmuir probe, and compared with the ones measured on the tokamak wall. A method for the mitigation and suppression of the near SOL heat fluxes through impurity seeding is proposed, and first experimental evidences are presented. The experiments are compared with numerical simulations of the TCV SOL, performed with the GBS code. The simulated parallel heat flux profiles qualitatively agree with the experimental ones, showing the presence of a near and far SOL. Also, non-ambipolar currents are observed to flow to the wall. The effect of resistivity is investigated through a second simulation with a 40 times higher resistivity. The blob dynamics in TCV is investigated using a conditional average sampling technique on the reciprocating Langmuir probe data. The results for two discharges, for low and high resistivity respectively, are discussed. A blob detection and tracking algorithm is applied to the numerical simulation outputs, and the results are discussed.

EPFL2017

The role of the sheath in magnetized plasma turbulence and flows

Joaquim Loizu Cisquella

Controlled nuclear fusion could provide our society with a clean, safe, and virtually inexhaustible source of electric power production. The tokamak has proven to be capable of producing large amounts of fusion reactions by confining magnetically the fusion fuel at sufficiently high density and temperature, thus in the plasma state. Because of turbulence, however, high temperature plasma reaches the outermost region of the tokamak, the Scrape-Off Layer (SOL), which features open magnetic field lines that channel particles and heat into a dedicated region of the vacuum vessel. The plasma dynamics in the SOL is crucial in determining the performance of tokamak devices, and constitutes one of the greatest uncertainties in the success of the fusion program. In the last few years, the development of numerical codes based on reduced fluid models has provided a tool to study turbulence in open field line configurations. In particular, the GBS (Global Braginskii Solver) code has been developed at CRPP and is used to perform global, three-dimensional, full-n, flux-driven simulations of plasma turbulence in open field lines. Reaching predictive capabilities is an outstanding challenge that involves a proper treatment of the plasma-wall interactions at the end of the field lines, to well describe the particle and energy losses. This involves the study of plasma sheaths, namely the layers forming at the interface between plasmas and solid surfaces, where the drift and quasineutrality approximations break down. This is an investigation of general interest, as sheaths are present in all laboratory plasmas. This thesis presents progress in the understanding of plasma sheaths and their coupling with the turbulence in the main plasma. A kinetic code is developed to study the magnetized plasma-wall transition region and derive a complete set of analytical boundary conditions that supply the sheath physics to fluid codes. These boundary conditions are implemented in the GBS code and simulations of SOL turbulence are carried out to investigate the importance of the sheath in determining the equilibrium electric fields, intrinsic toroidal rotation, and SOL width, in different limited configurations. For each study carried out in this thesis, simple analytical models are developed to interpret the simulation results and reveal the fundamental mechanisms underlying the system dynamics. The electrostatic potential appears to be determined by a combined effect of sheath physics and electron adiabaticity. Intrinsic flows are driven by the sheath, while turbulence provides the mechanism for radial momentum transport. The position of the limiter can modify the turbulence properties in the SOL, thus playing an important role in setting the SOL width.

EPFL2013

The role of the sheath in magnetized plasma turbulence and flows

Joaquim Loizu Cisquella

EPFL2013

Scrape-Off Layer physics in limited plasmas in TCV

Federico Nespoli

limited'' configuration, where the main plasma touches the reactor wall, and the contact point defines the Last Closed Flux Surface (LCFS). In this thesis, we advance the understanding of SOL physics in limited plasmas, combining experiments and numerical simulations. In particular, two topics are addressed. First, the separation between the

EPFL2017

The effect of the secondary x-point on the scrape-off layer transport in the TCV snowflake minus divertor

Basil Duval, Benoît Labit, Roberto Maurizio, Holger Reimerdes, Umar Sheikh, Christian Gabriel Theiler, Cedric Kar-Wai Tsui

2019