Absolute calibration of fast-ion loss detectors

Nuclear fusion is one of the energy options that is virtually inexhaustible and respectful with the environment. In current nuclear fusion devices, the existence of suprathermal particles is essential to transmit momentum and energy to the plasma. Losses due to different mechanisms of these particles affect the heating efficiency of the plasma and can generate excessive heat load on the reactor wall, damaging its integrity. The main diagnosis for the study of the transport mechanisms of suprathermal particles is the Fast-Ions Loss Detectors (FILD).

These detectors provide complete information on the velocity space of the ions escaping from the plasma and thus allowing to identify the type of fluctuation responsible for the losses. To be able to make predictions on future fusion reactors such as ITER and reproduce experimental observations of current devices, orbit following simulation codes (like ASCOT) must be validated. However, given the complexity of the luminescent response of the scintillator materials under extreme radiation conditions (dependence on energy, ionic species, background emission, ...) and temperature at which the diagnosis has to operate during the experiments in a reactor, absolute experimental measurements of particle fluxes impacting the wall of the devices are not available.

The work developed by our team constitutes a pioneering study in the absolute calibration of the Fast-Ions loss detectors based on scintillators materials. At the National Accelerator Center in Seville (CNA), the experimental work was focused on the development of a new vacuum chamber coupled to the Tandem accelerator, which has allowed the application of the ionoluminesce technique for the first time in the history of CNA in a temperature range between room temperature and 500ºC. The absolute yields and kinematics for protons, deuterium and alpha particles in an energy range between 600 keV and 3.5 MeV of several scintillators samples were studied. The material known as TG-Green (SrGa2S4:Eu2+) has proven to be the most suitable material to follow the rapid fluctuations in fusion plasmas.

An instrumental function was presented from the absolute calibration of the scintillator performance, the detector’s optical detection system and the influence of the geometry of the collimator that has been applied to provide, for the first time, the absolute measurements of the fast ion losses in the experimental reactor ASDEX Upgrade (Max Planck institute of plasma Physics, Munich) in plasmas with the simplest admost reproducible conditions. This work concluded with a comparative study between the infrared camera diagnostics installed in the reactor and the losses obtained at the detector head with the ASCOT simulation code.

[1] M. Rodriguez-Ramos et al., First absolute measurements of fast-ion losses in the ASDEX Upgrade Tokamak, Plasma Phys. Control Fusion. 59, 105009 (2017).
[2] J. Galdon-Quiroga et al., Velocity-space sensitivity and tomography of scintillator-based fast-ion loss detectors, Plasma Phys. Control Fusion. 60, 105005 (2018).

The smart FILD

A magnetically driven Fast-Ion Loss Detector (FILD) system has been designed, assembled and will be installed in the ASDEX Upgrade tokamak. The device is feedback controlled to adapt the detector head position to the heat load and physics requirements. To analyze its mechanical behaviour, dynamic simulations have been performed including effects such as friction, coil self-induction and eddy currents. A real time positioning control algorithm to maximize the detector operational window has been developed. This algorithm considers mechanical resistance as well as measured and predicted thermal load.

More details on this work can be found in:
[1] J. Gonzalez-Martin et al., First measurements of a magnetically driven fast-ion loss detector on ASDEX Upgrade, JINST. 14, C11005 (2019).
[2] J. Ayllon-Guerola et al., A fast feedback controlled magnetic drive for the ASDEX Upgrade fast-ion loss detectors, Rev. Sci. Instrum. 87, 11E705 (2016).

Fig. 1. The magnetically driven FILD system at ASDEX Upgrade.

Poloidal distribution of fast-ion losses

The installation of two new Fast Ion Loss Detectors (FILD) will complete the poloidal array. Since 2018, five FILD systems are in operation in ASDEX Upgrade:

FILD1 and FILD2 (covering the same poloidal position), and FILD3 close to the upper divertor. FILD1 is mounted on the midplane manipulator while FILD2 and FILD3 have their own reciprocating system.

FILD4 is installed above the lower PSL tails and is moved by a Magnetically Driven Reciprocation System (MDRS) opening the possibility to a real time adaptive position control.

FILD5 is moved by a mechanical feedthrough installed in port Cu7/1/3. The detector head is located right above the divertor and in-between two B-coils, probing directly the perturbation of the magnetic field in front of the FILD.

[1] J. Gonzalez-Martin et al., First measurements of a scintillator based fast-ion loss detector near the ASDEX Upgrade divertor, Rev. Sci. Instrum. 89, 10I106 (2018).

Fig. 2.

Rotary FILD in MAST-U

The Mega Amp Spherical Tokamak (MAST), in United Kingdom, uses lower magnetic fields and a lower aspect ratio to achieve the same plasma pressure, in comparison to conventional tokamaks. MAST is currently being upgraded (MAST-U) to allow for state-of-the-art divertor physics, current drive and ELM stability research. Fast-ion confinement will be crucial to successfully carry out the research plan in MAST-U. For this reason, in the framework of this upgrade, the first FILD for MAST-U has been designed and installed. It is mounted on an axially and angularly actuated mechanism that makes it possible to independently adapt the orientation and radial position of the probe, thus maximizing the detector velocity-space coverage in a broad range of plasma scenarios. Besides, due to the low magnetic field in MAST-U, the fast-ions gyroradius are in the range of 13 cm, requiring a large probe head of 15 cm diameter, which has made it a challenging mechanical design. The MAST-U operation is scheduled to start in 2021.

For more details about the MAST-U FILD see:
[1] J.F. Rivero-Rodriguez et al., A rotary and reciprocating scintillator based fast-ion loss detector for the MAST-U tokamak, Rev. Sci. Instrum. 89, 10I112 (2018).

The ITER Fast-Ion Loss Detector (FILD)

Although several diagnostics for confined fast-ions are being proposed for ITER, a lost alpha diagnostic has not been approved as of yet. The harsh environment in ITER – a nuclear installation - places a number of constraints on standard fast-ion loss detection techniques unprecedented in present tokamaks with easier access and more tolerable conditions. On the basis of the physics requirements, the ITPA Energetic Particle (EP) Topical Group (TG) has started to undertake a conceptual study of four different and complementary fastion loss detectors for ITER: a reciprocating Fast-Ion Loss Detector (FILD), a fast-ion loss monitor based on edge gamma radiation, an under-the-dome detector, and a dedicated infrared system. Based on the ITPA EP TG prioritization, the Port Plugs and Diagnostics Integration Division at ITER Organization has recently initiated an effort to develop a conceptual design of a reciprocating FILD in ITER in close collaboration with the PSFT group.

More details on this work can be found in:
[1] M. Garcia-Munoz et al., Conceptual design of the ITER fast-ion loss detector, Rev. Sci. Instrum. 87, 11D829 (2016).
[2] J. Ayllon-Guerola et al.,
Dynamic and thermal simulations of a fast-ion loss detector for ITER, Fus. Eng. Des. 123, 807 (2017).
[3] M. Kocan, et al.,
The impact of the fast ion fluxes and thermal plasma loads on the design of the ITER fast ion loss detector, JINST. 12, C12027 (2017).

Fig. 3. Conceptual design of the ITER FILD.