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.
Transport studies of cusp and multichannel REB fusion concepts
Single‐particle trajectory calculations have been used to study the effectiveness of beam concentration near a target pellet in two relativistic electron‐beam‐driven inertial fusion concepts: magnetic cusps and multiple current‐carrying channels in a wagon‐wheel configuration. Angular momentum constraints favor the multichannel transport scheme. The role of angular momentum nonconserving reflections from plugged‐cusp loss cones is discussed. Minimum radii are calculated for various injection conditions in a 40‐channel configuration. The resulting superposition of beams may provide an order‐of‐magnitude improvement in beam concentration over single‐beam results.
The Earth's Angular Momentum Budget on Subseasonal Time Scales
Irregular length of day (LOD) fluctuations on time scales of less than a few years are largely produced by atmospheric torques on the underlying planet. Significant coherence is found between the respective time series of LOD and atmospheric angular momentum (AAM) determinations at periods down to 8 days, with lack of coherence at shorter periods caused by the declining signal-to-measurement noise ratios of both data types. Refinements to the currently accepted model of tidal Earth rotation variations are required, incorporating in particular the nonequilibrium effect of the oceans. The remaining discrepancies between LOD and AAM in the 100- to 10-day period range may be due to either a common error in the AAM data sets from different meteorological centers, or another component of the angular momentum budget.