Overlap of intense charged particle beams for intertial confinement fusion
One approach to controlled thermonuclear fusion for power production requires the irradiation of centimetre-size targets containing heavy hydrogen (D–T) fuel by intense charged particle beams (CPB). Beam energy and pulse length requirements are ∼1 MJ and 10ns. In certain studies1 the CPBs are beams of light ions, such as helium, with kinetic energies per nucleon in the range 1–5 MeV. These beams can be transported by preformed, current-carrying, plasma discharge channels. The fields from the channel currents (supplied by capacitor banks) confine the CPBs. These channels can be laser-initiated in a background gas that has a pressure between 10 and 100 torr. This gas blanket enables first wall loading, by X rays, shocks and thermal radiation to be mitigated. Although intense relativistic electron beams (REBs), which were developed before intense light ion beams, have achieved higher intensities2, ion beams have more favourable deposition characteristics in targets3. Proton beams have driven exploding pusher targets4 to implosion velocities of 20 cm µs−1 and have attained power of ∼1 TW cm−2, which is a factor of 40 below that estimated (G. Allshouse, personal communication) to be necessary for target ignition and for net system energy gain. To close the gap between present capabilities and future requirements, beam bunching and beam overlap are being studied. The current density gain G is important to the CPB overlap process and is the ratio of beam current density at the target surface to that of the individual beams in the transport channels; it depends on particle species, transverse energy, geometry of the channel-target interface, and so on. Detailed calculations5–7 indicate that for electron beams, G<3, while for practical cases of light ions, G can exceed 10. This is due to the difference in masses and effective temperatures of the two types of beams. Sources providing multiple ion beams are not available to test this overlap theory experimentally, hence it has been tested using a multiple electron beam system. Here we outline the experiment and describe the apparatus, and then summarize the theoretical treatment of beam overlap, describe diagnostic procedures, and compare the experimental results with calculations.
Experimental Verification of Non-Equilibrated Bremsstrahlung Dosimetry Predictions for 0.75 MeV Electrons
Absolute Monte Carlo predictions of the combined x-ray production and energy deposition in low-Z (Al) and high-Z (Ta) encapsulated thermoluminescent dosimeters for a 0.75-MeV electron/bremsstrahlung source have been experimentally verified over a large range of encapsulation thicknesses and conditions. The effect of the Al is primarily manifested by an alteration in the x-ray flux reaching the active CaF2 material in the dosimeter. For Ta encapsulation, however, there is clear evidence of high-Z/low-Z interface phenomena, where both fluorescence and photoelectrons from the Ta make significant contributions to enhanced energy deposition in the CaF2. During the experiment, this interface enhancement-factor for the encapsulation relative to Al encapsulation was measured directly. The good agreement between the absolute predictions and measurements reported here represents an important verification of the Monte Carlo model for predicting integral interface phenomena.
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%