Formation and propagation of cooling front in open-field-line-plasma*

When the magnetic field lines suddenly intercept solid surfaces that recycle the plasma particle and energy, a magnetized plasma can undergo a thermal collapse that is dominated by parallel transport of extreme kinetic character. This can happen, for example, in the thermal quench during a tokamak disruption when large-scale MHD activities can turn nested flux surfaces into globally stochastic field lines that connect fusion grade core plasma directly to the divertor/first wall. In fully kinetic VPIC simulations of thermal collapse in open field line plasmas intercepting a recycling wall, we find that the thermal quench comes in the form of a cooling front that propagates at ion thermal speed from the wall into the core plasma. Such a cooling front is driven by the cold plasmas from the wall that significantly reduces the perpendicular electron and ion temperature, especially in weakly collisional case. Inside the cooling front, the ion dynamics are not affected by the cold recycling particles, where a steady recession of ion parameters is found. Whereas outside the cooling front, the plasma changes abruptly in a narrow region. One direct result is that a whistler instability is driven locally by trapped electrons in the cooling front affecting the thermal quench process.