Shattered pellet penetration in low and high energy plasmas on DIII-D

Abstract

Shattered pellet injection (SPI) has been adopted as the baseline disruption mitigation system for ITER, as the radiative payload penetration into DIII-D plasmas from SPI is superior to those using the massive gas injection (MGI) method. Because of the substantial differences in the energy content of ITER plasma and those in present experiments, reliable 3D MHD modeling, benchmarked against present experiments is needed to project to ITER plasmas. In support of these needs, the depth of SPI fragment penetration in DIII-D plasmas was investigated by injecting SPI into two discharges with vastly different energy content and pedestal height. 400 Torr-L pure Ne fragmented pellets at a velocity of about 200 m s −1 were injected into a 0.2 MJ L-mode discharge and a 2 MJ super H-mode discharge. Results show deep penetration of SPI fragments into low-energy plasmas in DIII-D. SPI fragment penetration is reduced as the plasma energy content increases, with some discharges exhibiting penetration that is confined to the outer regions of the plasma. The injected SPI fragments are also spread out over a distance of about 20 cm, which results in some fragments arriving near the end of or after the thermal quench is over.

Triggering of tearing instability by impurity radiation through resistive interchange reversal in a tokamak

“…The local concentration of impurities in the absence of an island can take place in the pellet injection process, such as the shattered or shell pellet injection (SPI or ShPI) experiments. In particular, for examples, the hollow pressure profile has been observed in the SPI simulations [29] and experiments [30]. Thus the simulation study in this work is mainly motivated by the on-going research on the most effective disruption mitigation schemes.…”

Observation of non-thermal electron formation during the thermal quench of shattered pellet injection shutdowns in DIII-D

“…The physics of SPI shutdowns is complex and the dynamics is being actively studied in SPI experiments at DIII-D [11], JET [12], and KSTAR [13]. The SPI injected material consists of a mixture frozen shards and gas which hit the plasma edge and begin to cause edge cooling.…”

Stability impacts from the current and pressure profile modifications within finite sized island

“…[2] To avoid such damages and safely guard the continued operation of future high performance devices, various disruption avoidance systems (DAS) are designed, such as the electron cyclotron current drive (ECCD) or Heating (ECRH), [3][4][5][6] to suppress the instability of the dominant magnetic islands by modifying the current and pressure profile within. Facing this, disruption mitigation systems (DMS) such as the shattered pellet injection (SPI) [7][8][9][10][11][12][13][14][15][16][17] or other massive material injection (MMI) [18][19][20][21][22][23][24][25] techniques are used to deplete the majority of thermal energy via radiation loss during the TQ, and raise the density to help with runaway electron suppression during the CQ phase.…”

“…There have been a lot of recent numerical explorations [11,[26][27][28][29][30][31][32][33][34][35] on how the DMS would interplay with global MHD modes. Such interplay could strongly impact the mitigation efficiency and these studies provided valuable insight into the disruption mitigation dynamics.…”