ALBERT

Notes: The concept of converting a multi-kA relativistic e beam into an ultrahigh-power rf pulse with a magnetically swept e-beam switch is studied. Two short VHF pulses are created by rapidly switching a 60-ns, 900-keV, 15-kA e beam between a pair of inverse e-beam diodes. The switching is done by a transverse 500-G, sinusoidal B field, oscillating at 50 MHz. The e-beam pulses are transported 1.6–4.6 cm in vacuum and ballistically or magnetically focused onto the collectors of the inverse diodes. The inverse diodes extract electrical energy from the deflected e-beam pulses and transport the energy to resistive loads that varied between 30 and 130 Ω. Numerical simulations of the e-beam transport with self-electric and magnetic fields are performed that show the sources of electrical inefficiency. These occur because the e beam is near the space-charge limit during transport. Peak unipolar powers of 2.0 and 0.5 GW are coupled to the resistive loads for the ballistically and magnetically transported e-beam geometries, respectively.

Notes: The post-acceleration of a 400-keV, 10-kA proton beam by a 200-kV magnetically insulated gap is investigated. The defections from self- and applied E and B fields are measured and compared to calculated values. Several important principles of post-acceleration gap operation are observed for the first time that are important for multigap operation. First, the beam is inadequately space-charge neutralized without gas puffs or preformed plasma in regions of transverse applied-B field to allow efficient transport. The beam is also noncurrent neutralized in these regions. Second, the applied-B field defines equipotential surfaces in the gap allowing the voltage on the gap to steer and focus the beam, and it has an axial field component that acts like a pair of solenoidal lenses to focus the beam. It is also pointed out how azimuthal asymmetries in the beam current density and cathode plasma cause beam self-field asymmetries that lead to growth of beam emittance. Finally, we discuss a model which can be used to extrapolate the experimental results to multigap operation.

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

Notes: An improved understanding of the factors that control the axial focus of applied-B ion diodes was obtained from time-resolved diagnostics of ion-beam trajectories. This resulted in a new selection of anode shape that produced a proton focus of 1.3-mm diameter from a 4.5-cm-radius diode, which is a factor of 2 improvement over previous results. We have achieved a peak proton power density of 1.5±0.2 TW/cm2 on the 1-TW Proto I accelerator. The radial convergence of this proton beam, defined as the ratio of the anode diameter to focused beam FWHM, is 70. Time-resolved information about virtual cathode evolution, the self- and applied-magnetic-field bending, and the horizontal focus of the beam was also obtained. In addition, the diffusion of the magnetic field into the anode plasma is estimated by measuring the horizontal focal position as a function of time. Finally, we discuss the effects of gas cell scattering on the beam focus.

Notes: Light ions deposit their energy in target materials by interaction with bound and free electrons. As the target heats toward inertial confinement fusion temperatures a progression of ionization states will be encountered. The stopping power of each ion created in this process will depend upon details of the respective bound electron states. In general, the net ion stopping power will increase compared to cold matter due to the free electron contribution. We report an experimental and theoretical study of enhanced ion stopping powers in targets heated by 0.5–1.4 TW/cm2 proton beams. The experiments were performed on the Proto-I accelerator with aluminum and nickel foil targets. The theoretical effort incorporated free and bound electron stopping terms in hydrocode simulations of the target response. At these intensities we observe and calculate stopping power enhancements of 100% for aluminum and 50% for nickel.

Notes: The radiation induced conductivity (RIC) of polyethylene terephthalate (Mylar) produced by the electron beam from a pinched electron beam diode has been measured experimentally. Data were obtained for 4–10 ns duration electron beam pulses at radiation dose rates up to 4×1016 rad/s (100 kA/cm2 current density). The conductivity is roughly proportional to radiation dose rate up to about 100 Mrad and at higher doses scales approximately with radiation dose to the (3)/(2) power. The experimental data are compared to the predictions of a model of RIC using dispersive electronic transport and bimolecular recombination. The parameters used in the model were obtained from published low dose (〈100 rad) time-resolved photoconductivity measurements on Mylar. The data agreed well with the calculations before the onset of significant sample heating (10 Mrad). The highest reported RIC results are compared to the expected results for a weakly ionized plasma experiencing electron neutral collisions. In this case the experimental conductivity is significantly higher than the calculated value.

Notes: Two magnetically insulated ion diodes that utilize a radial applied-B field are described. Both diodes generate an annular beam that is extracted along the diode axis. The first diode operated at 1.2 MV and 600 kA for 25 ns and generated a 300-kA ion beam. The second operated at 300 kV, 100 kA and generated 15 kA of ion current. The first diode was used to study diode performance as a function of inner and outer anode-cathode gaps, the applied-B field, and transmission line current ratios. The second diode was used to study anode plasma formation. The diodes were operated below Bcrit, resulting in electron leakage to the anode, especially near the outer cathode. A definition of Bcrit applicable to extraction diodes is given and methods of improving ion production efficiency in these diodes are suggested. The strong correlation of ion production with visible light emission suggests, however, that the electron loss played an important role in anode turn-on. The breakdown of neutral gas desorbed by electron impact is thought to be the anode plasma production mechanism. The grazing incidence leakage electrons affect the breakdown by significantly enhancing space-charge-induced electric fields in the dielectric-filled anode grooves.

Notes: We present the first experimental results of a uniformly insulated extraction applied-B ion diode. The diode typically attained beam generating efficiencies of 70% at the time of peak beam power. This is a significant improvement over previous extraction applied-B ion diodes, demonstrating the importance of uniform insulation as suggested by theory [J. Appl. Phys. 59, 2685 (1986)]. Furthermore, we found that the beam current is nearly independent of the ion-emitting area down to a minimum area. At the minimum area, the beam current density was enhanced by 150 times the monopolar Child–Langmuir value. This data supports a recently proposed theory [Phys. Rev. Lett. 59, 2295 (1987)] that predicts large enhancements due to the electron diamagnetic effect on the virtual cathode.

Notes: Particle Beam Fusion Accelerator II is a light-ion fusion accelerator that is presently capable of irradiating a 6-mm-diam sphere with ∼50 kJ of 5.5-MeV protons in ∼15 ns. An array of particle and x-ray diagnostics fielded on proton Inertial Confinement Fusion target experiments quantifies the incident particle beam and the subsequent target response. An overview of the ion and target diagnostic setup and capabilities will be given in the context of recent proton beam experiments aimed at studying soft x-ray emission from foam-filled targets and the hydrodynamic response of exploding-pusher targets. Ion beam diagnostics indicate ∼100 kJ of proton beam energy incident within a 1.2-cm radius of the center of the diode with an azimuthal uniformity which varied between 6% and 29%. Foam-filled target temperatures of 35 eV and closure velocities of 4 cm/μs were measured.