Short rise- and decay-time Z-pinch currents for soft x-ray laser excitation

The article addresses how to create inductance-free plasma and use it to excite soft x-ray lasers. The method employs a bifilar phenomenon in which one part of the pulsed current flows via the plasma column while the other part runs in the opposite direction via the closely placed external conductor. The electromagnetic fields formed by the plasma and return conductor are adjusted by lowering the distance between them to neutralize the magnetic field of the bifilar. Because the net transient magnetic field is drastically reduced, the plasma-conductor mutual inductance is near zero during current rise and decay. The inductance-free (L ∼ 2 nH) 35 cm-long Z pinches in a 3.1 mm-diameter argon-filled alumina capillary with current rise and decay times of 15 ns, dI/dt > 1012 A/s, and amplitudes up to 17 kA were predicted, realized, and verified to be suitable for pumping soft x-ray Ar+8 lasers. Without the bifilar phenomenon, the 35 cm Z pinches obey a 200 nH inductance that restricts the rise and decay durations of currents to 150 ns. The 35 cm Z pinches with 2 nH inductance generated 46.9 nm laser pulses with up to 4 μJ of energy and a beam divergence of 2 mrad at a low operating voltage of 35–45 kV, compared to 0.1–0.8 MV for similar lasers. The bifilar method could find applications in many research and technological fields, where the rise and decay times of discharge currents play a key role.