ALBERT

Notes: A series of experiments was performed with an Applied-B ion diode on the Particle Beam Fusion Accelerator-I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied-B diode operation from the sub-TW level on single module accelerators to the multi-TW level on a low impedance, self-magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100-TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of the diode impedance behavior and beam quality with electrical drive power is obtained from comparison with lower-power experiments.The diode impedance lifetime was about 10 ns, several times shorter than for lower-power experiments. Azimuthal and top-to-bottom variations of the diode and ion currents were found to be approximately ±10%, compared with an estimated requirement of 5%–7% uniformity to avoid focal blurring by self-magnetic field effects. The ion production efficiency was 80%–90%. However, only 50%±10% of the ion current was carried by protons; the balance was carried by multiply charged carbon and oxygen ions. Activation measurements showed a proton beam energy of approximately 50 kJ. A gas cell filled with 5 Torr of argon was used for beam transport. The macroscopic divergence was 15±10 mrad and the microscopic divergence was 20±15 mrad, values that are similar to those from lower-power experiments. A model of beam focusing is formulated that predicts the proton charge focused onto 0.47-cm radius lithium targets, taking into account beam purity, magnetic bending, small-angle multiple scattering, and intrinsic divergence. The model results and activation measurements of the number of protons focused onto targets agree, and indicate that the spatially averaged (over about 3 cm2) peak focal power was about 0.5 TW/cm.2 The most important limitations on power concentration were found to be the low proton content of the beam, the short impedance lifetime of the diode, and the asymmetric current feed of the accelerator. The short impedance lifetime limited the power coupled to the diode, and caused the voltage at peak ion power to be low, which exacerbates the small-angle scattering problem. The asymmetric feed caused focal blurring through nonuniform self-magnetic bending. At least partly because of the experience gained with low impedance beams during these experiments, the next generation accelerator, the 100-TW Particle Beam Fusion Accelerator-II, has been configured to produce a 25–30-MV Li+ beam rather than a 5-MV proton beam. off