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We demonstrate record high energy of 2 mJ, with four nanosecond pulses a peak power of ⪆420 kW and average power of 660 W, in a fiber amplifier using a novel 26 μm mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1kW. This is achieved at a diffraction limited beam quality of M2=1.14.
Glick et al. (Tue,) studied this question.