CORDIS - EU research results

Development Of Scintillator-Based Fast-Ion Loss Detectors For Fusion Devices Using Low-Energy Particle Accelerators

Final Report Summary - FILDDEV (Development Of Scintillator-Based Fast-Ion Loss Detectors For Fusion Devices Using Low-Energy Particle Accelerators)

Magnetohydrodynamic (MHD) instabilities are ubiquitous in laboratory as well as in astrophysical plasmas. Their interplay with energetic (suprathermal) ions is of crucial importance to understand the energy and particle exchange in astrophysical plasmas as well as to obtain a viable energy source in magnetically confined fusion devices such as ITER. Experimentally, a detailed knowledge of the wave-particle interaction can be gained from direct measurements of MHD induced fast-ion losses in fusion plasmas. Although fast-ion loss detectors (FILD) systems are working on virtually all large fusion devices worldwide, they have currently some limitations that prevent the community to make any assessment on the total fast-ion losses caused by any MHD fluctuation. This is, indeed, of crucial importance to validate codes and to be able to make predictions towards ITER and related solar events. The main goal of this project is to quantify the fast-ion losses induced by a broad spectra of MHD perturbations using advanced and absolutely calibrated FILD systems. For this purpose, several actions have been taken during the first two years of the project.

An Ion Beam Analysis (IBA) vacuum chamber for ionoluminescence studies has been installed at the CNA Tandem accelerator. The new IBA chamber has been equipped with absolutely calibrated light detectors (photomultiplier and spectrometer) as well as with a sample holder that allows changing the sample temperature up to 450ºC. This new facility has been used to characterize the response of several scintillator screens including those used currently in the Fast-Ion Loss Detectors (FILD) of the ASDEX Upgrade (AUG) tokamak. The degradation of the scintillator efficiency as function of operational temperature as well as of impinging ion fluence has been characterised for hydrogen, deuterium and alpha particles with fusion-relevant energies and fluxes. The scintillator response as well as the absolute light transmission of the optical systems of the AUG FILD detectors have been used to estimate the absolute fluxes of escaping ions in the AUG tokamak in the presence of a broad spectra of MHD perturbations.

In MHD quiescent plasmas, absolute fluxes of fast-ion losses has been used to validate full orbit codes and so improving our abilities to make predictions towards ITER. Indeed, experimentally validated full orbit simulations and calculated scintillator efficiencies have helped to design and estimate the signal of a reciprocating FILD system in ITER.

Absolute fluxes of fast-ion losses induced by mitigated and unmitigated Alfvén Eigenmodes (AEs) have been obtained in discharges heated with Neutral Beam Injection (NBI) during the current ramp-up phase. The AE mitigation has been obtained using localized ECRH at different plasma locations. Consistently with internal measurements of the fast-ion profiles, fast-ions losses become negligible as AEs are mitigated.

Phase-space time-resolved measurements of fast-ion losses induced by edge localized modes (ELMs) and ELM mitigation coils have been obtained in AUG H-mode discharges with multiple FILD detectors. Filament-like bursts of fast-ion losses are measured during ELMs by several FILDs at different toroidal and poloidal positions. Externally applied magnetic perturbations (MPs) have little effect on plasma profiles, including fast-ions, in high collisionality plasmas with mitigated ELMs. A strong impact on plasma density, rotation and fast-ions is observed, however, in low density/collisionality and q95 plasmas with externally applied MPs. During the mitigation/suppression of type-I ELMs by externally applied MPs, the large fast-ion bursts observed during ELMs are replaced by a steady loss of fast-ions with a broad-band frequency and an amplitude of up to an order of magnitude higher than the NBI prompt loss signal without MPs. Multiple FILD measurements at different positions, indicate that the fast-ion losses due to static 3D fields are localized on certain parts of the first wall rather than being toroidally/poloidally homogeneously distributed.

Measurements show that the escaping ions have a broad energy and pitch-angle range and are typically on banana orbits that explore the entire pedestal/scrape-off-layer (SOL). Infra-red measurements are used to estimate the heat load associated with the MP-induced fast-ion losses. The heat load on the FILD detector head and surrounding wall can be up to six times higher with MPs than without 3D fields. Orbit simulations are used to test different models for 3D field equilibrium reconstruction including vacuum representation, the free boundary NEMEC code and the one fluid MARS-F and two-fluid M3D-C1 codes which account for the plasma response.

Recent experiments in the AUG tokamak have revealed the existence of an Edge Resonant Transport (ERTL) layer responsible for the observed fast-ion losses induced by externally applied MPs. The amplitude and velocity-space of the measured fast-ion losses depends on the 3D field poloidal spectrum, the magnetic background helicity (q95) and the plasma collisionality. Full orbit simulations in 3D fields including plasma response reproduce key aspects of the experimental observations including poloidal spectrum, q95 and collisionality dependency. Simulations also show that sideband harmonics of the main toroidal perturbation can modify significantly the overall plasma response and associated fast-ion losses. An edge peeling amplification leads to 20% enhancement of the total losses, with respect to the vacuum fields, for a certain poloidal spectrum range while an internal kink amplification leads to a local enhancement of the losses but not to a significant contribution to the total losses. In general, the fast-ion transport is caused by rational orbital resonances with the applied 3D fields. While the fast-ion redistribution by the internal kink is caused by isolated and well localized resonances, the peeling induced losses are caused by an overlapping of multiple rational resonances in the ERTL. The ERTL fast-ion fractional resonances depend strongly on the particle pitch angle but not on the particle energy suggesting that similar fractional resonances may also exist for thermal ions.

Future work will focus on the active control of the fast-ion distribution function in tokamaks through external actuators such as radio frequency beatwaves and externally applied MPs.