Search Results (10 total)

We describe capabilities of an integrated software suite to simulate pinched-electron-beam diodes for pulsed radiography. In contrast to other reported work using particle-in-cell methods, we employ a ray-tracing code (Trak) with advanced capabilities for modeling beam-generated magnetic fields. Ray tracing is a direct approach to a steady-state solution and involves less work than a particle-in-cell calculation. The second software component, GamBet, is a new Monte Carlo code for radiation transport that incorporates effects of the complex electric and magnetic fields at the radiation target. The ray-tracing approach exhibits good convergence in calculations for the diode geometry of the compact radiography (CRAD) program at Lawrence Livermore National Laboratory. With a 1.5 MV, 30 ns driver, we predict that the diode can produce a beam with axial length ∼1  mm that generates isotropic bremsstrahlung radiation exceeding 1 rad at 1 m. The ray-tracing procedure encounters convergence problems when applied to the rod-pinch geometry, a configuration used in several pulsed radiographic machines. We observe a fundamental difference in the nature of electron orbits in the two diodes. There is an increased chance for particle-orbit feedback in the rod pinch, so that equilibrium solutions are sensitive to small changes in emission characteristics.

We present the first measurements of radiation temperatures and stimulated scattering losses in laser-driven, gas-filled hohlraums. They show efficient coupling when applying laser beam smoothing techniques. Scattering losses are reduced to the 3% level while the radiation temperatures increased by ∼15 eV for smoothed laser beams. We observe peak radiation temperatures in excess of 230 eV in gas-filled hohlraums consistent with detailed hydrodynamic LASNEX modeling.

Radiation transport through high-opacity materials can be described using the Rosseland mean opacity of the medium, which is dominated by low-opacity regions in the frequency-dependent opacity. By mixing gold and gadolinium, we can fill in low-opacity regions of one material with high-opacity regions of another material, resulting in a material with a Rosseland mean opacity 1.5× higher than either of the constituents. For a given laser energy, this can raise the temperature of the laser heated hohlraums, or for a given desired temperature, require less laser energy.

The phase change of an electromagnetic wave propagating through a free-electron laser (FEL) amplifier has been measured. The FEL operated at 34.6 GHz, with both a uniform and a tapered wiggler. The results of the experiment show a nearly constant phase derivative with increasing wiggler length through the exponential gain region and are in good agreement with analytical theory. For the tapered-wiggler amplifier, the phase derivative decreases at a point approximately one third of a synchrotron period past saturation; the decrease is in good agreement with numerical simulation. For the untapered-wiggler amplifier, however, numerical simulations predict an increase in phase derivative past saturation, whereas in the experiment the phase derivative abruptly vanishes. Several explanations for the discrepancy are discussed.

We have substantially increased the output power and extraction efficiency of a free-electron laser operating at 34.6 GHz by tapering the wiggler magnetic field. In the exponential-gain regime, the laser exhibited a measured gain of 34 dB/m. With a 50-kW input signal, the amplifier saturated in 1.3 m with a 180-MW output signal. By using a taper that brought the magnetic field at the end of the wiggler down to 45% of its initial (peak) value, we increased the output signal to 1.0 GW. This corresponds to an extraction efficiency of 34%.

A high-gain, high–extraction-efficiency, linearly polarized free-electron laser amplifier has been operated at 34.6 GHz. At low signal levels, exponential gain of 13.4 dB/m has been measured. With a 3-=kW input signal, saturation was observed with an 80-MW output and a 5% extraction efficiency. The results are in good agreement with linear models at small signal levels and nonlinear models at large signal levels.

Tandem-mirror plasma with sloshing-ion end plugs has been produced. Ion-velocity distribution functions are peaked at 47° to the magnetic field lines. Low-level ion-cyclotron fluctuations were detected at 1.8 times the plug-midplane frequency and with properties predicted by theory; they did not degrade plasma confinement in the plugs or central cell.

A magnetron operating at a wavelength of 10 cm has been constructed with six resonant cavities cut in a cylindrical anode block. A graphite cylinder acting as a field-emission cathode delivers ∼12 kA in an accelerating radial potential of ∼360 kV. The magnetic field directed along the diode axis is ∼8 kG. Linearly polarized microwaves of 30 nsec duration at powers of ∼1.7 GW are achieved. The conversion efficiency of electron energy into microwave is ∼35%.