A novel path to runaway electron mitigation via deuterium injection and current-driven MHD instability

Relativistic electron (RE) beams at high current density (low safety factor, q a ) yet very low free-electron density accessed with D2 secondary injection in the DIII-D and JET tokamak are found to exhibit large-scale MHD instabilities that benignly terminate the RE beam. In JET, this technique has enabled termination of MA-level RE currents without measurable first-wall heating. This scenario thus offers an unexpected alternate pathway to achieve RE mitigation without collisional dissipation. Benign termination is explained by two synergistic effects. First, during the MHD-driven RE loss events both experiment and MHD orbit-loss modeling supports a significant increase in the wetted area of the RE loss. Second, as previously identified at JET and DIII-D, the fast kink loss timescale precludes RE beam regeneration and the resulting dangerous conversion of magnetic to RE kinetic energy. During the termination, the RE kinetic energy is lost to the wall, but the current fully transfers to the cold bulk thus enabling benign Ohmic dissipation of the magnetic energy on longer timescales via a conventional current quench. Hydrogenic (D2) secondary injection is found to be the only injected species that enables access to the benign termination. D2 injection: (1) facilitates access to low q a in existing devices (via reduced collisionality & resistivity), (2) minimizes the RE avalanche by ‘purging’ the high-Z atoms from the RE beam, (3) drives recombination of the background plasma, reducing the density and Alfven time, thus accelerating the MHD growth. This phenomenon is found to be accessible when crossing the low q a stability boundary with rising current, falling toroidal field, or contracting minor radius—the latter being the expected scenario for vertically unstable RE beams in ITER. While unexpected, this path scales favorably to fusion-grade tokamaks and offers a novel RE mitigation scenario in principle accessible with the day-one disruption mitigation system of ITER.

[1]  J. R. Martin-Solis,et al.  Formation and termination of runaway beams during vertical displacement events in tokamak disruptions , 2022, Nuclear Fusion.

[2]  Jet Contributors,et al.  Demonstration of Safe Termination of Megaampere Relativistic Electron Beams in Tokamaks. , 2021, Physical review letters.

[3]  S. Silburn,et al.  Magnetohydrodynamic simulations of runaway electron beam termination in JET , 2021 .

[4]  P. Merkel,et al.  The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas , 2020, Nuclear Fusion.

[5]  L. Baylor,et al.  Spatially dependent modeling and simulation of runaway electron mitigation in DIII-D , 2020, 2007.15785.

[6]  S. Jardin,et al.  Simulation of MHD instabilities with fluid runaway electron model in M3D-C1 , 2020, Nuclear Fusion.

[7]  P. Parks,et al.  Study of argon expulsion from the post-disruption runaway electron plateau following low-Z massive gas injection in DIII-D , 2020 .

[8]  B. Duval,et al.  Runaway electron synchrotron radiation in a vertically translated plasma , 2020, Nuclear Fusion.

[9]  A. Loarte,et al.  Understanding the reduction of the edge safety factor during hot VDEs and fast edge cooling events , 2020, 2003.04064.

[10]  C. Paz-Soldan,et al.  Runaway electron seed formation at reactor-relevant temperature , 2020, Nuclear Fusion.

[11]  Charlson C. Kim,et al.  MARS-F modeling of post-disruption runaway beam loss by magnetohydrodynamic instabilities in DIII-D , 2019, Nuclear Fusion.

[12]  P. Parks,et al.  Study of argon assimilation into the post-disruption runaway electron plateau in DIII-D and comparison with a 1D diffusion model , 2019, Nuclear Fusion.

[13]  P. Aleynikov,et al.  Physics of runaway electrons in tokamaks , 2019, Nuclear Fusion.

[14]  P. Parks,et al.  Recent DIII-D advances in runaway electron measurement and model validation , 2019, Nuclear Fusion.

[15]  Daisuke Shiraki,et al.  Shattered pellet injection technology design and characterization for disruption mitigation experiments , 2019, Nuclear Fusion.

[16]  O. Embreus,et al.  Influence of massive material injection on avalanche runaway generation during tokamak disruptions , 2019, Nuclear Fusion.

[17]  A. Boozer,et al.  Kink instabilities of the post-disruption runaway electron beam at low safety factor , 2019, Plasma Physics and Controlled Fusion.

[18]  P. Parks,et al.  Demonstration of Tokamak Discharge Shutdown with Shell Pellet Payload Impurity Dispersal. , 2019, Physical review letters.

[19]  M. Hoppe,et al.  Spatiotemporal evolution of runaway electrons from synchrotron images in Alcator C-Mod , 2018, Plasma Physics and Controlled Fusion.

[20]  B. Breizman,et al.  Force-free motion of a cold plasma during the current quench , 2018, Physics of Plasmas.

[21]  M. Hoppe,et al.  Interpretation of runaway electron synchrotron and bremsstrahlung images , 2017, Nuclear Fusion.

[22]  C. Reux.  Physics of the interaction between runaway electrons and the background plasma of the current quench in tokamak disruptions , 2017 .

[23]  B. Breizman,et al.  Model of vertical plasma motion during the current quench , 2017 .

[24]  J. R. Martin-Solis,et al.  Study of Z scaling of runaway electron plateau final loss energy deposition into wall of DIII-D , 2017 .

[25]  A. Stahl,et al.  Effect of Partially Screened Nuclei on Fast-Electron Dynamics. , 2017, Physical review letters.

[26]  Sudip K. Seal,et al.  Space dependent, full orbit effects on runaway electron dynamics in tokamak plasmas , 2017 .

[27]  P. Aleynikov,et al.  Generation of runaway electrons during the thermal quench in tokamaks , 2017 .

[28]  N. W. Eidietis,et al.  Dissipation of post-disruption runaway electron plateaus by shattered pellet injection in DIII-D , 2016 .

[29]  A. Boozer,et al.  Loss of relativistic electrons when magnetic surfaces are broken , 2016 .

[30]  Gta Guido Huijsmans,et al.  Linear MHD stability analysis of post-disruption plasmas in ITER , 2016 .

[31]  D. Alves,et al.  Runaway electron beam generation and mitigation during disruptions at JET-ILW , 2015 .

[32]  S. H. Kim,et al.  Disruptions in ITER and strategies for their control and mitigation , 2015 .

[33]  Jet Efda Contributors,et al.  Runaway beam studies during disruptions at JET-ILW , 2015 .

[34]  Allen H. Boozer,et al.  Theory of runaway electrons in ITER: Equations, important parameters, and implications for mitigation , 2015 .

[35]  D. A. Humphreys,et al.  Status of research toward the ITER disruption mitigation system , 2015 .

[36]  L. Lao,et al.  The external kink mode in diverted tokamaks , 2014, Journal of Plasma Physics.

[37]  A. Hyatt,et al.  Tokamak operation with safety factor q95 < 2 via control of MHD stability. , 2014, Physical review letters.

[38]  Ftu,et al.  Inter-machine comparison of the termination phase and energy conversion in tokamak disruptions with runaway current plateau formation and implications for ITER , 2014 .

[39]  A. R. Raffray,et al.  The ITER blanket system design challenge , 2014 .

[40]  J. R. Martin-Solis,et al.  Magnetic energy flows during the current quench and termination of disruptions with runaway current plateau formation in JET and implications for ITER , 2011 .

[41]  S. Bozhenkov,et al.  Runaway generation during disruptions in JET and TEXTOR , 2009 .

[42]  Stephen C. Jardin,et al.  A high-order implicit finite element method for integrating the two-fluid magnetohydrodynamic equations in two dimensions , 2007, J. Comput. Phys..

[43]  J. Manickam,et al.  Chapter 3: MHD stability, operational limits and disruptions , 2007 .

[44]  J. Kinsey,et al.  Access to sustained high-beta with internal transport barrier and negative central magnetic shear in DIII-D , 2006, Physics of Plasmas.

[45]  L. Lao,et al.  MHD Equilibrium Reconstruction in the DIII-D Tokamak , 2005 .

[46]  Bengt Lennartson,et al.  Feedback stabilization of nonaxisymmetric resistive wall modes in tokamaks. I. Electromagnetic model , 2000 .

[47]  M. Rosenbluth,et al.  Theory for avalanche of runaway electrons in tokamaks , 1997 .

[48]  R. R. Khayrutdinov,et al.  Studies of plasma equilibrium and transport in a Tokamak fusion device with the inverse-variable technique , 1993 .

[49]  L. L. Lao,et al.  Separation of β̄p and ℓi in tokamaks of non-circular cross-section , 1985 .

[50]  D. Spong,et al.  Macroscopic kink instabilities in toroidal relativistic beams , 1977 .

[51]  R. R. Khayrutdinov,et al.  Assessment of the Runaway Electron Energy Dissipation in ITER , 2017 .

[52]  H. Furth.  HHD STABLE REGIME OF THE TOKAMAK , 2009 .

[53]  J. Wesson.  Hydromagnetic stability of tokamaks , 1978 .