Measurement of electromagnetic waves from runaway electrons.

Electromagnetic waves emitted during a tokamak discharge can be partially ascribed to coupling with plasma waves. In particular, in the presence of runaway electrons, the electromagnetic waves deliver information, otherwise inaccessible, about kinetic instabilities excited by the fast particles. Experiments aimed at studying radio frequency emissions from runaway electron scenarios during different stages of plasma discharge have been carried out at the Frascati Tokamak Upgrade. Frequencies in the range of lower hybrid and whistler waves have been explored, in the presence of relativistic electrons with different energies, ranging from a few to tens of MeV. A pronounced sensitivity of the radio frequency measurements in detecting driven instabilities is observed, providing the possibility to exploit this kind of technique as a monitor of the instability processes and for studies of the fast electron activity. In particular, in this work, we propose a simplified analysis of the frequency scaling of a specific family of kinetic instabilities arising at the lower hybrid frequency range during the current ramp-up stage. The study is performed with respect to the density profile and the wave vector coupling conditions and is aimed at obtaining a rough estimate of the most likely radial location of the interaction between the runaway electron beam and plasma waves at the emission times of the observed signals.

First Intrashot Observation of Runaway-Electron-Driven Instabilities at the Lower-Hybrid Frequency Range under ITER-Relevant Plasma-Wave Dispersion Conditions.

Kinetic instabilities driven by runaway electrons (REs) have recently received attention in the fusion community as a means to control and diagnose REs in a tokamak. Experiments aimed at studying such kinetic instabilities have been performed at the Frascati Tokamak Upgrade (FTU), where different families of waves have been identified, from wide-band bursting emissions to quasi-monochromatic waves and sharp lines, in the presence of REs with energies from a few to tens of MeV. A specific family of waves with intense kinetic drive was directly observed for the first time, during both the early Ohmic plasma start-up and the current ramp-up. A clear wave frequency scaling with respect to the electron density was demonstrated. This scaling, with the complementary analysis of signals observed at different magnetic fields, allowed the identification of these instabilities as lower-hybrid waves. The relevant analysis shown in this Letter is based on a continuous intrashot detection of the RE-driven wave, which is reported for the first time for this kind of instability. We demonstrated that unstable waves are excited already at the very beginning of a tokamak discharge, opening the way to new possible research on the exploitation of this kind of measurement for monitoring seed REs formation at the early plasma stage, while most diagnostics still have limited capabilities. The conditions for plasma wave dispersion at the early phase of the FTU discharge are very similar to the ones expected during the ITER start-up, when analogous instabilities might, hence, come to light, in case of formation of suprathermal populations.

Characterization of signals for a Divertor Tokamak Test facility interferometer/polarimeter system.

In magnetically confined fusion experiments, laser interferometer/polarimeter systems allow one to determine plasma density, give valuable information on the internal magnetic fields, and contribute to the evaluation of the plasma magnetic equilibrium and to the real-time estimation of the q profile to allow feedback configuration control. This work presents an analysis of the interferometric and polarimetric signals of a multi-chord far-infrared interferometer/polarimeter for the divertor tokamak test facility, the new tokamak device currently under construction in Italy. The polarimetric signals are calculated both with approximate formulas and by solving the equation describing the evolution of the laser beam polarization inside the plasma using the Mueller formalism. The latter method correctly accounts for crosstalk between Faraday rotation and the Cotton-Mouton effect. The impact of the plasma birefringence on the interferometric phase shift is also studied, and it is found that a perturbation of the interferometric phase shift is present also in the case of an initial fixed linear polarization of the probe laser beam.

Implementation of the new multichannel X-mode edge density profile reflectometer for the ICRF antenna on ASDEX Upgrade.

A new multichannel frequency modulated continuous-wave reflectometry diagnostic has been successfully installed and commissioned on ASDEX Upgrade to measure the plasma edge electron density profile evolution in front of the Ion Cyclotron Range of Frequencies (ICRF) antenna. The design of the new three-strap ICRF antenna integrates ten pairs (sending and receiving) of microwave reflectometry antennas. The multichannel reflectometer can use three of these to measure the edge electron density profiles up to 2 × 1019 m-3, at different poloidal locations, allowing the direct study of the local plasma layers in front of the ICRF antenna. ICRF power coupling, operational effects, and poloidal variations of the plasma density profile can be consistently studied for the first time. In this work the diagnostic hardware architecture is described and the obtained density profile measurements were used to track outer radial plasma position and plasma shape.

A microwave interferometer for small and tenuous plasma density measurements.

The non-intrusive density measurement of the thin plasma produced by a mini-helicon space thruster (HPH.com project) is a challenge, due to the broad density range (between 10(16) m(-3) and 10(19) m(-3)) and the small size of the plasma source (2 cm of diameter). A microwave interferometer has been developed for this purpose. Due to the small size of plasma, the probing beam wavelength must be small (λ = 4 mm), thus a very high sensitivity interferometer is required in order to observe the lower density values. A low noise digital phase detector with a phase noise of 0.02° has been used, corresponding to a density of 0.5 × 10(16) m(-3).

Current drive at plasma densities required for thermonuclear reactors.

Progress in thermonuclear fusion energy research based on deuterium plasmas magnetically confined in toroidal tokamak devices requires the development of efficient current drive methods. Previous experiments have shown that plasma current can be driven effectively by externally launched radio frequency power coupled to lower hybrid plasma waves. However, at the high plasma densities required for fusion power plants, the coupled radio frequency power does not penetrate into the plasma core, possibly because of strong wave interactions with the plasma edge. Here we show experiments performed on FTU (Frascati Tokamak Upgrade) based on theoretical predictions that nonlinear interactions diminish when the peripheral plasma electron temperature is high, allowing significant wave penetration at high density. The results show that the coupled radio frequency power can penetrate into high-density plasmas due to weaker plasma edge effects, thus extending the effective range of lower hybrid current drive towards the domain relevant for fusion reactors.

Two-color medium-infrared scanning interferometer for the Frascati tokamak upgrade fusion test device.

A scanning beam interferometer installed on the Frascati tokamak upgrade (FTU) experiment is presented. The scanning beam scheme combined with the small dimensions of the beams produces a system with very high spatial resolution: more than 30 adjacent (nonoverlapping) chords sample most of the plasma cross section. A good time resolution is achieved by the use of a proper scanning device, with a scanning frequency >or=8 kHz. Very fast events are measured by three additional fixed lines of sight providing a time resolution >or=100 kHz. The instrument is a two-color medium-infrared-compensated-type interferometer; two wavelengths (colors) are used to measure both the density and the mechanical vibrations of optical components. A CO2 laser (lambda=10.6 microm) is the main light source, and a CO laser (lambda=5.4 microm) is the compensation one. The optical scheme is a double pass Mach-Zehnder type. All the retroreflector mirrors are mounted directly on the FTU mechanical structure thanks to the compensation system that allows for large vibration amplitudes of optical components. Heterodyne detection at 30 and 40 MHz is obtained by frequency shifting the reference beams with two acousto-optic modulators (Bragg cells). Many features are implemented to achieve high measurement accuracy and reliability. A real-time system computes the integral density measured on one of the fixed lines of sight and provides an analog signal for density feedback control. The interferometer was used to measure density profiles both in medium-density discharges (n(e) approximately 10(20) m(-3)) and in high-density pellet injected discharges (n(e) approximately 7-8 x 10(20) m(-3)). The measurement error is approximately 2 x 10(18) m(-2) under optimal conditions but can be higher in some cases, mainly because of the large tilt of the retroreflector mirrors.

Dynamics of tearing modes during strong electron cyclotron heating on the FTU tokamak.

The localized electron cyclotron resonance heating power that can suppress sawteeth reconnection often drives m = 2 tearing modes in a tokamak operating at constant current. The dynamics of mode onset and coupled mode evolution is described in detail and compared with a nonlinear theoretical model that identifies the effects of mode coupling, finite inertia of the rotating islands, and wall braking.