Experiments and non-linear MHD simulations of hot vertical displacement events in ASDEX-Upgrade

Abstract

Hot Vertical Displacement Events (VDEs) are one of the worst case scenarios for high current tokamaks as they are associated with large heat loads and electro-magnetic forces. 
Non-linear magneto-hydrodynamic (MHD) simulations of the thermal and current quench can help to understand their dynamics and consequences. 
In order to make predictions for future devices, the validation of codes against present machines is crucial.
Dedicated experiments were performed in ASDEX Upgrade to provide a basis for simulations with the non-linear extended MHD code JOREK. 
2D as well as non-axisymmetric simulations at realistic parameters can reproduce quantities like the edge safety factor $q_{95}$ at the thermal quench (TQ) onset, the halo current magnitude and the level of vertical forces, while the exact width of the halo current area requires more sophisticated boundary conditions and is left for future work. Small horizontal forces are observed during the hot VDEs in the experiment as well as in the simulations.

The mechanism of the global vertical force reduction in disruptions mitigated by massive material injection

“…Similar as in JET, the toroidal resistance is increased by bellows in between the sectors [53], so that the induced eddy current by the VDEs close inside the sector and thus have a larger effective path. It has been shown [38], that the movement of the hot VDE can be reproduced with the simulation and the current in the PSL evolves consistently with the experimental measurements.…”

“…The electron density n e rises up to a value of around 5 × 10 20 m −3 as measured by the CO2-interferometry [28,40]. The radiated energy during the TQ and CQ is 0.6 MJ-0.7 MJ as measured by the bolometry system [41] makes up around 50% of the total energy [38].…”

“…The results presented here consist of three SPI mitigated VDEs that were performed with the SPI system to study the mitigation of the loads on the machine for displaced plasmas. The discharges are based on the hot VDE discharge # 39655, that has been described in detail in [38]. In order to get a reproducible motion, the VDE was triggered by a downward displacement of 2 cm within 20 ms by the plasma by the control coils.…”

“…Also, two temperature effects are not included, so that in the following T, designates the total temperature (T = T e + T i with T i = T e ). The code has been used and benchmarked before for studies of VDEs in 2D [46] as well as 3D [37,38,47]. Note that the convention of the sign of I p is machine dependent and can differ from the JOREK coordinate system.…”

The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas

“…In reference [168], the growth rate of resistive wall modes was compared to analytical values and a comparison of VDEs in a simplified ITER-like geometry was compared to the CEDRES++ code including realistic wall resistances. Basic verification tests (not shown here) have been performed including convergence checks, the induction of currents by the plasma in a conducting wall or passive coil, the mutual interaction between conductive structures and a comparison of an axisymmetric and non-axiymmetric coil field on the JOREK boundary employing the Biot-Savart law [72,92,169]. Further validation work is included in reference [92] and some more recent and more advanced tests are shown in the following.…”