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10-15 May 2021
Virtual Event
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

Latest results on quiescent and post-disruption runaway electrons mitigation experiments at Frascati Tokamak Upgrade

13 May 2021, 08:30
4h
Virtual Event

Virtual Event

Regular Poster Magnetic Fusion Experiments P5 Posters 5

Speaker

daniele carnevale (University of Rome Tor Vergata)

Description

Runaway electrons (RE) are one of the major concerns for ITER operations. In tokamak devices where fusion reactions take place at high rate and with large total current, the loss of plasma confinement can lead to runaway electrons formation (possibly up to 12 MA in ITER) via primary and secondary generation mechanisms [1,2]. The major issue with runaway electrons that form after the disruption is their high energy, mainly increased by the large electrical field at current quench (CQ), and the small pitch angle that could deposit unsustainable power on the plasma facing components causing deep melting in the tokamak structure. The main strategies on RE mitigation rely on increasing collisionality to avoid RE beam formation or to quickly dissipate its energy if already formed [1,8]. The latter action is unavoidably associated with undesirable fast-growing vertical displacement events, i.e. quick RE energy dissipation by heavy-Z material injection is linked with fast current decay leading to uncontrollable VDEs: it is a race among vertical displacement, energy dissipation and electromechanical loads that might not lead to ITER feasible solutions. Researchers are providing further techniques that might be used in combination with MGI/SPI such as a dedicated control strategy [3,4], 3D stochastic fields by resonant magnetic perturbation [7] and further instabilities [9]. Recent results obtained at DIII-D and further investigated at JET show that large RE currents, driven by the central solenoid after deuterium injection (SPI) to quickly reduce the drag, induce current-driven (low safety factor) kink instabilities with extremely fast RE beam loss and no sign of localized energy deposition, opening the path for an alternative RE mitigation strategy [3]. Continuing the studies of past years at FTU [4,5] we tested a number of alternative solutions for RE mitigation. In FTU large population of quiescent REs in steady state current ohmic discharges have been created and interaction with (multiple) deuterium pellets and Laser Blow Off (LBO) injection have been studied. In quiescent scenarios, it has been observed with a significative number of tests that D2 pellet on quiescent RE population might lead to REs fast growth up to the RE beam formation, if RE population or MHD activity is above a given threshold, meanwhile multiple pellets injection fairly close enough (1-20 ms) can produce quick REs total expulsion and or dissipation restoring a “safe” steady-state discharge with no REs as shown in Fig. 1 and Fig. 2. Different ablation rates of pellets with different size (1-2E20) depending on the REs quiescent population and inter-time pellet injection have been registered by fast H-alpha acquisition channels and the fast CO2 scanning interferometer. Such data can be important for pellet ablation models with REs that can be useful for ITER predictions. It is the worth to mention that pellet injections are as well the subject of studies at FTU for large MHD stabilization, possibly providing a solution to discharge recovery and RE preemptive dissipation at once. There have been also few cases in which pellets have been launched at CQ and, interestingly and somehow expected, the REs formed have lower energy. Indeed, one possible dissipation methodology would consist into providing a large number of electrons (deuterium injections) that could absorb at least part of the electrical field produced at the CQ preventing high level energy increase of RE seeding meanwhile flashing out all high-Z species at CQ would decrease the current drop and increasing then the RE beam controllability. It has also been observed, for the first time at FTU, density increase after D2 pellet injection as well as LBO ionization (and drag effects) on a post disruption RE beam with clear signs of increased background plasma temperature: fan-like instabilities seem to play an important role on such temperature increase. Modulated ECRH has been used in order to further increase background plasma density and temperature and a surprisingly synchronization with fan-like and MHD driven instabilities has been found. Data on RE energy evolution have been acquired with the new REIS [5,6] during RE quiescent flat-top as well as Ip ramp and post-disruption RE beam to provide data for RE energy model validation.
**Fig. 1**(left): Effect of RE mitigation on quiescent scenario obtained injecting D2 pellets of 1E20@0.6s plus 2E20@0.62s: neu213 lies on BF3 at 0.72s meaning that there are no more runaways left. **Fig. 2** (right): single 2E20 D2 pellets are injected at 0.5s: RE suppression is achieved if quiescent REs and/or MHD activity are below a given threshold.
To conclude, the FTU last experimental campaign provided data on D2 pellet
effectiveness for preemptive RE mitigation technique, pellet ablation studies for ITER, as well as its effect on post-disruption RE beams and data for RE model validation.
Acknowledgement: This work has been carried out within the framework of the EUROfusion Consortium. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
References:

1 Lehnen M et al, J. Nucl. Mater. 463 39 (2015)
[2] Connor J W and Hastie R J, NF, 15 415 (1975).
[3] C. Paz-Soldan et al, A novel path to runaway electron mitigation via current-driven kink instability, submitted to IAEA 2020.
[4] D. Carnevale et al., Plasma Phys. Control. Fusion 61 014036 (2019).
[5] Esposito B. et al., PPCF, vol. 59, ISSN: 0741-3335 (2016).
[6] F. Causa et al, Review of Scientific Instruments 90, 073501 (2019).
[7] M. Gobbin et al, PPCF 60 1 (2017).
[8] G. Papp et al, RE generation and mitigation on the European medium sized tokamaks ASDEX Upgrade and TCV,26th IAEA (2016).
[9] F. Causa, NF 59 4 (2019).

Country or International Organization Italy
Affiliation University of Rome Tor Vergata

Primary author

daniele carnevale (University of Rome Tor Vergata)

Co-authors

Matteo Baruzzo (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) William Bin (Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Milano, Italy) Francesca Bombarda (1ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Luca Boncagni (ENEA, Fusion and Nuclear Safety Department) Onofrio Tudisco (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) B. Tilia (ENEA, Fusion and Nuclear Safety Department, Frascati (Rome), Italy) Mr mario sassano (University of Rome Tor Vergata) A. Sibio (ENEA, Fusion and Nuclear Safety Department, Frascati (Rome), Italy) Afra Romano (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Giuseppe Ramogida (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Mr marco passeri (university of Rome Tor Vergata) V. Piergotti (ENEA, Fusion and Nuclear Safety Department, Frascati (Rome), Italy) Gianluca Pucella (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Luigi Panaccione (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Eric Nardon (CEA) Francesco Napoli (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Cristina Mazzotta (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Mr francesco martinelli (University of Rome Tor vergata) Mr simone magagnino (ENEA) Davide Liuzza (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Michael Lehnen (ITER Organization) Paolo Buratti (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Matteo Iafrati (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) A. Grosso (ENEA, Fusion and Nuclear Safety Department, Frascati (Rome), Italy) GUSTAVO GRANUCCI (ISTITUTO DI FISICA DEL PLASMA - CNR) Mr sergio galeani (University of rome tor vergata) Saul Garavaglia (ISTP-CNR, Istituto di Fisica del Plasma, Milano, Italy) L. Gabellieri (ENEA, Fusion and Nuclear Safety Department, Frascati (Rome), Italy) Basilio Esposito (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Ocleto D'Arcangelo (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Francesco Cordella (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Cesidio Cianfarani (ENEA) C. Centioli (ENEA, Fusion and Nuclear Safety Department, Frascati (Rome), Italy) Carmine Castaldo (ENEA) Mauro Cappelli (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)) Mr luca calacci (University of Rome Tor vergata) Silvio Ceccuzzi (ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy))

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