7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Mixed hydrogen-neon pellet injection in toroidal plasmas– theory and observation

Akinobu Matsuyama1,*1
National Institutes for Quantum Science and Technology, Rokkasho Institute*Graduate School of Energy Science, Kyoto University (Present address)e-mail (speaker): matsuyama.akinobu.3p@kyoto-u.ac.jp
Cryogenic solid pellet injection is the most promisingtechnique for fueling magnetically confined fusion
reactors. The ablation and assimilation process of
injected pellets in dilute high-temperature plasmas iscomplicated and is an interesting topic not only for its
practical aspect of controlling toroidal discharges but
also for the physics aspect of plasma-matter interaction.
In the last decade, cryogenic solid pellet injection has
been applied to a new research area in the field –machineload mitigation during major disruptions in tokamak
fusion reactors [1]. Disruptions are a large-scale collapse
event that is abruptly triggered when the discharge
and/or control system becomes unstable. The present
tokamak experiments have demonstrated the possibility
that the injection of large-sized pellets can intentionallytrigger a disruption while minimizing the thermal impacton
theplasma-facing
components
andtheelectromagnetic force on the vessel wall. Such a‘disruption mitigation’ scheme constitutes a way for theemergency shutdown of future reactors. In ITER, theinjection of mixed hydrogen-neon (H2+Ne) pellets was
selected for this purpose. This paper reviews recent
developments in theory and experiments on the ablation
and assimilation processes of mixed pellets to understandthe plasma-matter interactions underlying the ITER
disruption mitigation scheme. The key points of thisstudy are summarized as follows.
 Ablation of mixed pellets: The ablation cloudself-adopts its thickness to shield the electron particle
flux from the ambient plasma. Such self-shielding
mechanisms are affected by the introduction of neons
to low-Z hydrogen clouds by enhancing elastic
scattering. The ablation rate is difficult to measure
directly, but experimental data for pellet penetration
depths have been well reproduced using the neutral
gas and plasma shielding (NGPS) model [2].
 Plasmoid drift mechanism: The ablated material
forms a plasmoid with an intermediate atom density(1022-1024) between that of solid hydrogen and thebackground plasma. The plasmoid experiences
expansion along the magnetic field and cross-field
drift motion. The pellet injection experiment in theLHD steollarator/heliotron [3] demonstrated that thecross-field drift can be controlled by changing theneon concentration in the mixed pellets (see Figure1), which is now a concept that needs to be developed
for application in ITER.
 Disruption trigger: Hydrogen-neon mixed pellets
trigger a disruption within a few milliseconds afterinjection into tokamaks. Trigger time is crucial fordisruption mitigation control. A key process is‘dilution cooling [4]’: Hot background electrons are

Figure 1. Comparison of the electron density profiles
measured by Fast Thomson Scattering (FTS) after
pellet injection between pure hydrogen (red) andhydrogen-neon mixed (blue) pellets. The observed
deeper material assimilation can be explained by thesuppression of the outward drift motion of plasmoids[2,3].
cooled due to equipartition with delivering many cold
electrons. Ideally, pure hydrogen injection allows thehot plasma to be diluted to several hundred eV; ifsuch a plasma is maintained at the center of thechamber over the slowing down time of fast electrons,
the generation of fast electrons during the disruption
can be avoided. The simulation indicated [5] thatITER may realize such a situation by injecting singlehydrogen-neon pellets when carefully controlling theneon concentration, injection quantities, and fragment
sizes for Shattered Pellet Injection (SPI), which is thebaseline scheme for material delivery in ITER.
Experimental validation is ongoing with focusedefforts.
References[1] M. Lehnen, et al., “R&D for reliable disruption
mitigation in ITER”, Proceedings for the 27th IAEA
FEC (Gandhinagar, 2018).
[2] A. Matsuyama, et al., Phys. Plasmas 29, 042501(2022).
[3] A. Matsuyama, et al., Phys. Rev. Lett. 129, 255001(2022).
[4] E. Nardon, et al., Nucl. Fusion 60, 126040 (2022).
[5] A. Matsuyama, et al., Plasma Phys. Control. Fusion
64, 105018 (2022).