TCV Tokamak Research Articles

Poloidal asymmetry in the narrow heat flux feature in the TCV scrape-off layer

Heat flux profiles inferred from a reciprocating probe at the outer midplane of the TCV tokamak during inner wall limited discharges feature radial fall-off lengths that shorten near the last closed flux surface (LCFS) consistent with the so-called narrow feature. The narrow feature is significantly wider on the outboard side compared with that measured on the inner wall by infrared thermography, so it is difficult to discern from the main scrape-off layer feature. After small shifts were applied for alignment, the fraction of the power contained in the narrow feature matches between inboard and outboard measurements, and they scale together with plasma current Ip, suggesting that we are observing the same phenomenon. The outboard side fall-off length within the narrow feature is found to scale closely with the radial correlation length of the edge turbulence as expected if the narrow feature arises due to radially sheared E × B flows. This is found to hold true even for cases where the narrow feature is weak and the fall-off lengths are approaching that of the far scrape-off layer. After the small shifts for alignment, non-zero floating potential profiles were found to match between inboard and outboard sides. A simple model of polarization and diamagnetic cross-field currents is described, which is consistent with the shape of these floating potential profiles. The model predicts that the floating potential at the LCFS must be negative, which supports the argument to shift the upstream measurements. The predicted currents are also consistent with the E × B flows believed to cause the narrow feature. The model is used to predict the magnitude of the floating potential of the LCFS, and the results are found to match measurements for all values of Ip. This paper therefore demonstrates consistency between the measurements of the narrow feature on the inboard and outboard sides of the plasma, as well as consistency between the measurements, non-linear turbulence simulations, and analytical models of the narrow feature arising from sheared E × B flows.

Integration of a Real-Time Node for Magnetic Perturbations Signal Analysis in the Distributed Digital Control System of the TCV Tokamak

This paper presents the integration of an acquisition and computing unit capable of acquiring and processing fast magnetic signals in real time in the control system of the tokamak a configuration variabile tokamak. All aspects of system integration and testing are reported, leading to testing of the system on plasma discharges. An example of a real-time analysis algorithm designed for detecting and classifying neoclassical tearing modes plasma instabilities is also described.

Commissioning of the heating neutral beam injector on the TCV tokamak

The TCV tokamak mechanical structure has been recently adapted to leave access for a neutral beam injector (NBI) capable of 1MW of neutral power during 2s into the TCV plasma. Budker Institute of Nuclear Physics (BINP) was in charge of design and procurement this equipment, considering the experimental constraints imposed both by the future physics objectives of TCV, and the mechanical requirements complying with the limited space available for installing the material inside the tokamak building. The development, the design phase, as well as the preparation work have been described in Fasel et al. (2015); Karpushov et al. (2015) [1,2].This paper will concentrate on the description of the commissioning of the NBI system at the Swiss Plasma Center (SPC) site. The main results obtained during the first operation phase with TCV are presented in Karpushov et al. (2016) [3]. The stages went through from the installation of the main auxiliaries (HVPS, RF PS, cooling circuits, cryogenics equipment etc.) up to the final acceptance tests on the plasma targets will be detailed. The measurements and the associated protection interlocks acting on the TCV control system will be described, including also the human safety rules imposed to comply with a safe operation of the injector. To conclude, the next stage to finalize the integration of the NBH on TCV will be briefly developed.

Electron cyclotron stray radiation detection and machine protection system proposal for JT-60SA

The JT-60SA tokamak is scheduled to start operations in 2019 to support the ITER experimental programme and to provide key information for the design of DEMO scenarios. The device will count on ECRH and NBI as auxiliary heating. Potentially dangerous situations that could harm the tokamak structures during operations characterized by low EC absorption cannot be a priori excluded. An estimation of the wall load under direct beam exposure and during plasma operations will be given, using the EC antenna main characteristics and launching geometry. A system capable to prevent the risk of damage to machine components for JT-60SA is presented and examples of potential benefits of such a system will be given, based on recent experience gained on the TCV tokamak during EC wall cleaning experiments performed in preparation of JT-60SA operations.

Results from recent detachment experiments in alternative divertor configurations on TCV

Divertor detachment is explored on the TCV tokamak in alternative magnetic geometries. Starting from typical TCV single-null shapes, the poloidal flux expansion at the outer strikepoint is varied by a factor of 10 to investigate the X-divertor characteristics, and the total flux expansion is varied by 70 to study the properties of the super-X divertor. The effect of an additional X-point near the target is investigated in X-point target divertors. Detachment of the outer target is studied in these plasmas during Ohmic density ramps and with the ion B drift away from the primary X-point. The detachment threshold, depth of detachment, and the stability of the radiation location are investigated using target measurements from the wall-embedded Langmuir probes and two-dimensional CIII line emissivity profiles across the divertor region, obtained from inverted, toroidally-integrated camera data. It is found that increasing poloidal flux expansion results in a deeper detachment for a given line-averaged density and a reduction in the radiation location sensitivity to core density, while no large effect on the detachment threshold is observed. The total flux expansion, contrary to expectations, does not show a significant influence on any detachment characteristics in these experiments. In X-point target geometries, no evidence is found for a reduced detachment threshold despite a 2–3 fold increase in connection length. A reduced radiation location sensitivity to core plasma density in the vicinity of the target X-point is suggested by the measurements.

TCV divertor upgrade for alternative magnetic configurations

The Swiss Plasma Center (SPC) is planning a divertor upgrade for the TCV tokamak. The upgrade aims at extending the research of conventional and alternative divertor configurations to operational scenarios and divertor regimes of greater relevance for a fusion reactor. The main elements of the upgrade are the installation of an in-vessel structure to form a divertor chamber of variable closure and enhanced diagnostic capabilities, an increase of the pumping capability of the divertor chamber and the addition of new divertor poloidal field coils. The project follows a staged approach and is carried out in parallel with an upgrade of the TCV heating system. First calculations using the EMC3-Eirene code indicate that realistic baffles together with the planned heating upgrade will allow for a significantly higher compression of neutral particles in the divertor, which is a prerequisite to test the power dissipation potential of various divertor configurations.

The control system of the 1-MW TCV tokamak heating neutral beam injector

The control system of the neutral beam injector is presented that was designed and constructed at the Budker Institute of Nuclear Physics in Novosibirsk, for the TCV tokamak in Lausanne, Switzerland. The system hardware includes an industrial computer with reconfigurable PCI Express input–output boards, as well as interfacing electronics, which ensures galvanic isolation and electrical compatibility with the injector units. The software and firmware that implement the control system algorithm by means of both the industrial computer and the FPGAs of the reconfigurable boards are written using LabVIEW. The injection parameters are calculated using the beam-power model; the graphical representation and storage of the results are carried out in MATLAB. A communication interface with the tokamak control system is also provided for data exchange and synchronization of operation.

Neutral beam heating on the TCV tokamak

The TCV tokamak contributes to physics understanding in fusion reactor research by harnessing a wide experimental tool set: in particular flexible shaping and high power electron cyclotron heating. Plasma regimes with high plasma pressure, a wider range of temperature ratios and significant fast-ion population are now attainable with a TCV heating system upgrade. In a first stage, a 1MW neutral beam was installed (2015) and is reported in this paper.The installation of the NB injector required modifications of the vacuum vessel and considerable work on the machine infrastructure, resulting in a shutdown from late 2013 to mid-2015. TCV is now operating partly as a European Medium-Size Tokamak (MST) facility under the auspices of the EUROfusion consortium. The NBI was intensively operated in the February–July 2016 phase of the MST campaign. Record ion temperatures of 2.0–2.5keV and toroidal rotation velocities up to 160km/s were promptly attained in the first few L-mode discharges with NB injection. Ion temperatures up to 3.5keV were subsequently achieved in ELMy H-mode. The injector produces a focused deuterium neutral beam with 25keV energy, 1MW neutral power and 2s duration.

An arbitrary wavelength solver for global gyrokinetic simulations. Application to the study of fine radial structures on microturbulence due to non-adiabatic passing electron dynamics

The influence of the fine layers of the non-adiabatic passing electron response on electrostatic turbulent transport, previously studied systematically in flux tube geometry [Dominski et al., Phys. Plasmas 22, 062303 (2015)], is pursued in global geometry in conditions relevant for the TCV tokamak with a deuterium plasma (mi/me = 3672). The spectral organization of the passing electron turbulent flux and its dependence on the radial profile of the safety factor are revealed. A radially dependent toroidal spectral analysis of the turbulent fluxes led to the key result that the particle and heat diffusivities of passing-electrons are proportional to the local density of low-order mode rational surfaces. To permit this study of the short radial scales associated with the passing electron dynamics, a new field solver valid at an arbitrary wavelength is implemented in ORB5, for the gyrokinetic quasi-neutrality equation. A benchmark is conducted against the global version of the gyrokinetic code GENE, showing very good agreement.

Experimental observations and modelling of intrinsic rotation reversals in tokamaks

The progress made in understanding spontaneous toroidal rotation reversals in tokamaks is reviewed and current ideas to solve this ten-year-old puzzle are explored. The paper includes a summarial synthesis of the experimental observations in AUG, C-Mod, KSTAR, MAST and TCV tokamaks, reasons why turbulent momentum transport is thought to be responsible for the reversals, a review of the theory of turbulent momentum transport and suggestions for future investigations.

Embedded control systems for gyrotrons applications based on NI solutions

The Swiss Plasma Center is involved in the development and the operation of gyrotrons for fusion application (TCV tokamak, W7-X, ITER, DEMO) as well as for basic science applications such as Nuclear Maggnetic Resonance (NMR) spectroscopy. In this framework, embedded control systems based on National Instrument (NI) compact Reconfigurable Input Output (cRIO) and compact Data AcQuisition (cDAQ) offer versatile solutions for dedicated applications. Three specific developments are briefly presented and discussed here. First, a complete control system based on cRIO material has been implemented in a Modulator Power Supply. Second, the development of a compact system, based on a cDAQ solution used for the characterization of the RF losses on the various gyrotron components. Third, the installation of a whole control system for a Dynamic Nuclear Polarization (DNP/NMR) gyrotron dedicated to non-specialist gyrotron users.The flexibility given by these compact control systems and acquisition could offer multiple solutions in many applications for fusion research.

Recent progress in the upgrade of the TCV EC-system with two 1MW/2s dual-frequency (84/126GHz) gyrotrons

The upgrade of the EC-system of the TCV tokamak has entered in its realization phase and is part of a broader upgrade of TCV. The MW-class dual-frequency gyrotrons (84 or 126GHz/2s/1MW) are presently being manufactured by Thales Electron Devices with the first gyrotron foreseen to be delivered at SPC by the end of 2017. In parallel to the gyrotron development, for extending the level of operational flexibility of the TCV EC-system the integration of the dual-frequency gyrotrons adds a significant complexity in the evacuated 63.5mm-diameter HE 11 transmission line system connected to the various TCV low-field side and top launchers. As discussed in [1], an important part of the present TCV-upgrade consists in inserting a modular closed divertor chamber. This will have an impact on the X3 top-launcher which will have to be reduced in size. For using the new compact launcher we are considering employing a Fast Directional Switch (FADIS), combining the two 1MW/126GHz/2s rf-beams into a single 2MW rf-beam.

Detachment evolution on the TCV tokamak

Divertor detachment in the TCV tokamak has been investigated through experiments and modelling. Density ramp experiments were carried out in ohmic heated L-mode pulses with the ion ∇B drift directed away from the primary X-point, similar to previous studies [1]. Before the roll-over in the ion current to the outer strike point, C III and Dα emission from the outer leg recede slowly from the strike point toward the X-point, at a rate of ∼2.0 × 10−19m/m−3 along the magnetic field as the electron temperature along the leg reduces with increasing density. Around the onset of detachment, the upstream density profile and outer target Dα profiles broaden, possibly leading to an increase in radiation in the SOL by increased interaction between the SOL and the carbon tiles lining the outer wall. The plasma conditions upstream and at various locations along the detached outer divertor leg have been characterised, and the consistency of this data has been checked with the interpretive OSM-EIRENE-DIVIMP suite of codes [2] and are broadly found to be consistent with measured Dγ/Dα emissivity profiles along the detached outer divertor leg.

Role of infernal modes dynamics and plasma rotation on the onset of NTMs in ECH-ECCD TCV plasmas

Experiments in the TCV tokamak show that when high power central electron cyclotron heating (ECH) and current drive (ECCD) are deposited in the central plasma region, tearing-like instabilities can develop. In the present work we address, both analytically and numerically, the problem of tearing mode (TM) generation analysing extensively the behaviour of L-mode and low plasma current TCV discharges when high power heating is deposited in the plasma core region. Three possible mechanisms are discussed. The first is the modification of the equilibrium current density profile due to the power deposition leading to variation of the sign of the classical tearing stability index Δ', where finite β effects could also be taken into account. The second mechanism still the modification of the tearing stability parameter Δ', but this time due to the presence of an equilibrium sheared toroidal plasma rotation. Finally, the third mechanism considered is the role of infernal-type instabilities in driving tearing-like modes when the safety factor becomes sufficiently close to a rational in the core region.

An automated liquid nitrogen refilling system for cryopumps in the 1-MW heating neutral-beam injector of the TCV tokamak

A system for automated liquid nitrogen refilling of cryopumps in the neutral beam injector of the TCV tokamak (Switzerland) is described. The system includes industrially produced temperature and level monitors for cryogenic liquids, as well as an intelligent controller developed by the Budker Institute of Nuclear Physics on the basis of a field programmable gate array. The controller is used to control refilling of two cryopumps connected to a common nitrogen line and prevent emergency situations without operator attendance. Additional protection is ensured by the “timeout” mechanism used in different phases of the refilling cycle. The communication interface is available, and the user software is developed.

Simple predictive electron transport models applied to sawtoothing plasmas

In this work, we introduce two simple transport models to evaluate the time evolution of electron temperature and density profiles during sawtooth cycles (i.e. over a sawtooth period time-scale). Since the aim of these simulations is to estimate reliable profiles within a short calculation time, two simplified ad-hoc models have been developed. The goal for these models is to rely on a few easy-to-check free parameters, such as the confinement time scaling factor and the profiles’ averaged scale-lengths. Due to the simplicity and short calculation time of the models, it is expected that these models can also be applied to real-time transport simulations. We show that it works well for Ohmic and EC heated L- and H-mode plasmas. The differences between these models are discussed and we show that their predictive capabilities are similar. Thus only one model is used to reproduce with simulations the results of sawtooth control experiments on the TCV tokamak. For the sawtooth pacing, the calculated time delays between the EC power off and sawtooth crash time agree well with the experimental results. The map of possible locking range is also well reproduced by the simulation.

Parameter estimation for a nonlinear control-oriented tokamak profile evolution model

A control-oriented tokamak profile evolution model is crucial for the development and testing of control schemes for a fusion plasma. The RAPTOR (RApid Plasma Transport simulatOR) code was developed with this aim in mind (Felici 2011 Nucl. Fusion 51 083052). The performance of the control system strongly depends on the quality of the control-oriented model predictions. In RAPTOR a semi-empirical transport model is used, instead of a first-principles physics model, to describe the electron heat diffusivity in view of computational speed. The structure of the empirical model is given by the physics knowledge, and only some unknown physics of , which is more complicated and less well understood, is captured in its model parameters. Additionally, time-averaged sawtooth behavior is modeled by an ad hoc addition to the neoclassical conductivity and electron heat diffusivity. As a result, RAPTOR contains parameters that need to be estimated for a tokamak plasma to make reliable predictions. In this paper a generic parameter estimation method, based on the nonlinear least-squares theory, was developed to estimate these model parameters. For the TCV tokamak, interpretative transport simulations that used measured profiles were performed and it was shown that the developed method is capable of finding the model parameters such that RAPTOR’s predictions agree within ten percent with the simulated q profile and twenty percent with the measured profile. The newly developed model-parameter estimation procedure now results in a better description of a fusion plasma and allows for a less ad hoc and more automated method to implement RAPTOR on a variety of tokamaks.

Effects of central electron cyclotron power on plasma rotation and on triggerless onset of NTMs in the TCV tokamak

Experiments in the TCV tokamak show that high power central electron cyclotron heating (ECH) and current drive (ECCD) produce significant direct modification of the plasma rotation profile, as well as an effect on the equilibrium current density profile. In a regime of unsteady rotation, these effects contribute to the onset of neoclassical tearing instabilities, in the absence of triggers such as sawteeth, edge localised modes (ELMS) or relevant ‘error’ fields. In turn the growing tearing modes' breaking axisymmetry provides a nonlinear magnetic torque which converts the power absorption in a co-directed rotation with a flattening of the profile at the rational surfaces. The experimental results are presented and discussed in the context of theoretical models of neoclassical toroidal viscosity and ion inertial effects.

Upgrade of a 30 kV/10 mA anode power supply for triode type gyrotron

The RF power of a gyrotron with a triode type magnetron-injection-gun (MIG) can be directly controlled via the voltage applied between its anode and its cathode. Hence, the performance of this type of gyrotron relies directly on the possibilities offered by the power supply controlling the anode to cathode voltage. For a system of gyrotrons connected to the same main high-voltage power supply, with a triode MIG one has the additional advantage of independently controlling each individual gyrotron. This paper presents the modifications brought to the three existing 30 kV/10 mA anode power supplies connected to the 500 kW/118 GHz/2s X3 gyrotrons operated on the TCV Tokamak. The new working principle based on phase-shift modulation (PSM) is described in detail. Experimental results obtained on dummy load are compared to simulations performed during the design phase. With respect to the initial working principle, the modulation frequency capability has been increased by a factor 10 reaching more than 5 kHz, whereas the output voltage ripple as well as the overshoot/undershoot have been significantly reduced. (C) 2015 Elsevier B.V. All rights reserved.

Supply equipment to the new NBH system for the TCV tokamak

The TCV tokamak is equipped with a high power real time-controllable Electron Cyclotron Heating (ECH) system, made of 9 gyrotrons capable of 500kW RF power. One of the major scientific challenge, the TCV upgrades aimed to, will be the access to a wider range of temperature ratios up to Te/Ti∼1. For this purpose, the TCV heating system will be upgraded by the integration of a Neutral Beam Heating (NBH) system impacting directly on the ion temperature (Ti), while gyrotrons are used to vary mainly the electron temperature (Te). This NB Injector (NBI) will provide 1MW of neutral power during 2s into the TCV plasma, at a nominal energy of 30keV, for an electrical power installed of 2.2MVA. This paper describes the infrastructure required to implement successfully this novel NBI at CRPP premises.