The design and performance of a portable megagauss magnetic field generator for laser/cluster interactions

Our objective is to create a unique environment consisting of a magnetized high energy density plasma (n ~ 10¹⁹ cm^-3, T ~ 10 keV) produced by laser irradiation of atomic or molecular clusters in a megagauss magnetic field. With deuterium or CD₄ clusters the DD fusion neutron yield can be up to 10⁷ neutrons/shot. Requirements on the magnetic field source are: on-axis field strength of 1-2 megagauss in a cylindrical volume of 1 cm³ (reaching β~ 1 for 10 keV plasma), portability so it can be installed at laser facilities e.g. Texas Petawatt Laser (TPW), operation in vacuum conditions, and constant field for ~100 ns. In a collaboration between the University of Texas at Austin (UT) and Sandia National Laboratories (SNL), we have designed, built, and tested a pulsed power source suitable for cluster fusion experiments at the TPW. Some of the salient design features include use of low inductance circuit elements, a single-turn coil, ten 3.1 μF 100 kV high energy density capacitors (15.5 kJ each), and one high voltage spark gap switch per capacitor. Each capacitor discharges via six high voltage cables into a conical transmission line, which feeds into a vacuum chamber through an insulator stack similar to that of the Z machine at SNL. Current rise time is 1.7 μs with peak current up to 2 MA; system inductance is about 48 nH, including the single-turn coil. A major challenge was to drive the current through the coil in vacuum conditions without a significant inductance penalty or damage to the vacuum system. The single-turn coil, made from 1/8" thick copper, is destroyed every shot. Clearing shot debris, replacing the coil, and achieving the required vacuum pressure of ≤10^-5 Torr reduces the repetition rate to 2 - 3 shots per day.