Optically isolated millimeter-wave detector for the Toroidal Plasma Experiment

We have designed and built an optically isolated millimeter-wave detection system to prevent interference from a nearby, powerful, 2.45 GHz microwave source in millimeter-wave propagation experiments in the TORoroidal Plasma EXperiment (TORPEX). A series of tests demonstrates excellent system noise immunity and the ability to observe effects that cannot be resolved in a setup using a bare Schottky diode detector

Langmuir probe electronics upgrade on the tokamak a configuration variable

A detailed description of the Langmuir probe electronics upgrade for TCV (Tokamak a Configuration Variable) is presented. The number of amplifiers and corresponding electronics has been increased from 48 to 120 in order to simultaneously connect all of the 114 Langmuir probes currently mounted in the TCV divertor and main-wall tiles. Another set of 108 amplifiers is ready to be installed in order to connect 80 new probes, built in the frame of the TCV divertor upgrade. Technical details of the amplifier circuitry are discussed as well as improvements over the first generation of amplifiers developed at SPC (formerly CRPP) in 1993/1994 and over the second generation developed in 2012/2013. While the new amplifiers have been operated successfully for over a year, it was found that their silicon power transistors can be damaged during some off-normal plasma events. Possible solutions are discussed. (C) 2019 Author(s)

Real-time plasma state monitoring and supervisory control on TCV

In ITER and DEMO, various control objectives related to plasma control must be simultaneously achieved by the plasma control system (PCS), in both normal operation as well as off-normal conditions. The PCS must act on off-normal events and deviations from the target scenario, since certain sequences (chains) of events can precede disruptions. It is important that these decisions are made while maintaining a coherent prioritization between the real-time control tasks to ensure high-performance operation. In this paper, a generic architecture for task-based integrated plasma control is proposed. The architecture is characterized by the separation of state estimation, event detection, decisions and task execution among different algorithms, with standardized signal interfaces. Central to the architecture are a plasma state monitor and supervisory controller. In the plasma state monitor, discrete events in the continuous-valued plasma state are modeled using finite state machines. This provides a high-level representation of the plasma state. The supervisory controller coordinates the execution of multiple plasma control tasks by assigning task priorities, based on the finite states of the plasma and the pulse schedule. These algorithms were implemented on the TCV digital control system and integrated with actuator resource management and existing state estimation algorithms and controllers. The plasma state monitor on TCV can track a multitude of plasma events, related to plasma current, rotating and locked neoclassical tearing modes, and position displacements. In TCV experiments on simultaneous control of plasma pressure, safety factor profile and NTMs using electron cyclotron heating (ECH) and current drive (ECCD), the supervisory controller assigns priorities to the relevant control tasks. The tasks are then executed by feedback controllers and actuator allocation management. This work forms a significant step forward in the ongoing integration of control capabilities in experiments on TCV, in support of tokamak reactor operation

Operating a full tungsten actively cooled tokamak: overview of WEST first phase of operation

WEST is an MA class superconducting, actively cooled, full tungsten (W) tokamak, designed to operate in long pulses up to 1000 s. In support of ITER operation and DEMO conceptual activities, key missions of WEST are: (i) qualification of high heat flux plasma-facing components in integrating both technological and physics aspects in relevant heat and particle exhaust conditions, particularly for the tungsten monoblocks foreseen in ITER divertor; (ii) integrated steady-state operation at high confinement, with a focus on power exhaust issues. During the phase 1 of operation (2017–2020), a set of actively cooled ITER-grade plasma facing unit prototypes was integrated into the inertially cooled W coated startup lower divertor. Up to 8.8 MW of RF power has been coupled to the plasma and divertor heat flux of up to 6 MW m−2 were reached. Long pulse operation was started, using the upper actively cooled divertor, with a discharge of about 1 min achieved. This paper gives an overview of the results achieved in phase 1. Perspectives for phase 2, operating with the full capability of the device with the complete ITER-grade actively cooled lower divertor, are also described

Truncated Levy motion through path integrals and applications to nondiffusive suprathermal ion transport

Fractional Levy motion has been derived from its generalized Langevin equation via path integrals in earlier works and has since proven to be a useful model for nonlocal and non-Markovian processes, especially in the context of nondiffusive transport. Here, we generalize the approach to treat tempered Levy distributions and derive the propagator and diffusion equation of truncated asymmetrical fractional Levy motion via path integrals. The model now recovers exponentially tempered tails above a chosen scale in the propagator, and therefore finite moments at all orders. Concise analytical expressions for its variance, skewness, and kurtosis are derived as a function of time. We then illustrate the versatility of this model by applying it to simulations of the turbulent transport of fast ions in the TORPEX basic plasma device

Characterizing time intermittency in non-diffusive fast ion transport through plasma turbulence

Turbulent fast ion transport has been investigated in astrophisycal, laboratory and fusion plasmas. When gyro- and drift-averaging across plasma structures manifest as non-markovian and non-local effects, this results in generally non-diffusive transport. The intermittency of the formation of these plasma structures, such as blobs, can potentially be reflected in the transport of the fast ions as well. In the TORPEX basic plasma device, a toroidal beam of suprathermal Li-6 ions is injected into electrostatic plasma turbulence. Conditional sampling techniques confirm turbulent E x B-drifts as physical driving mechanism of radial fast ion transport, which features sub-, super- or quasi-diffusive regimes depending on the fast ion energy and propagation time. To address the question of how far local intermittency is associated with each regime, we analyze characteristics of time-intermittency on an extensive set of local fast ion time-series across all observed regimes. Modeling the time-average fast ion profiles as the result of a meandering smaller instantaneous beam allows us to predict the skewness of such time-series based on their time-average. Comparisons with the skewness of simple two-valued time-series can yield relative indications towards certain transport regimes in our specific system, based on the differences in the size of the instantaneous fast ion beam

Time intermittency in nondiffusive transport regimes of suprathermal ions in turbulent plasmas

Intermittent phenomena have long been studied in the context of nondiffusive transport across a variety of fields. In the TORPEX device, the cross-field spreading of an injected suprathermal ion beam by electrostatic plasma turbulence can access different nondiffusive transport regimes. A comprehensive set of suprathermal ion time series has been acquired, and time intermittency quantified by their skewness. Values distinctly above background level are found across all observed transport regimes. Intermittency tends to increase toward quasi-and superdiffusion and for longer propagation times of the suprathermal ions. The specific prevalence of intermittency is determined by the meandering motion of the instantaneous ion beam. We demonstrate the effectiveness of an analytical model developed to predict local intermittency from the time-average beam. This model might thus be of direct interest for similar systems, e.g., in beam physics, or meandering flux-rope models for solar energetic particle propagation. More generally, it illustrates the importance of identifying the system-specific sources of time-intermittent behavior when analyzing nondiffusive transport