3D PIC simulation of ion debris mitigation by B-field for LPP-EUV source

In laser produced plasmas (LPP) for EUV light source, the maximum ion kinetic energy sometime exceeds 10 keV. Such fast ion debris from LPP may cause damage on a collecting mirror of the light source. Therefore the mitigation of the fast ions is one of the critical issues that should be overcome for the practical use of LPP as EUV source for lithography. We have investigated both free plasma expansion and plasma expansion in an external magnetic field by using 3D particle simulation. The maximum ion energy in free expansion is essentially determined from the ratio of initial target radius to initial Debye length. Based on this relation we discuss the possibility to reduce the maximum ion energy. We show that the maximum ion energy perpendicular to the magnetic field is also reduced and that the ion radial flight distance becomes less than the gyro diameter estimated from the maximum energy. We also show that generated electric field causes the interchange instability of the plasma, but the instability is limited within a certain radial region. Simulation results indicate that the fast ions can be efficiently exhausted along the magnetic field without causing crucial damage on an EUV collecting mirror with the use of a single magnetic coil.