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.
Electron‐beam transport for inertial fusion
Two methods for transporting electron beams from the diode to a pellet are analyzed. First, pinched beams can propagate along a Z pinch with current density Jb≲ν/γJp, where Jp is the pinch current density. Heavy high‐density (∼1018cm−3) gas is required in order to (1) reduce plasma turbulence that could prevent total current neutralization and (2) prevent expansion of the pinch by transverse beam pressure. Second, compression in a magnetic mirror with multiple reflections between diode and mirror is considered. Reflex diode current density enhancements ≳100 above Child’s law are required for this approach to succeed.
Observations of nearshore infragravity waves: Seaward and shoreward propagating components
[1] The variation of seaward and shoreward infragravity energy fluxes across the shoaling and surf zones of a gently sloping sandy beach is estimated from field observations and related to forcing by groups of sea and swell, dissipation, and shoreline reflection. Data from collocated pressure and velocity sensors deployed between 1 and 6 m water depth are combined, using the assumption of cross-shore propagation, to decompose the infragravity wave field into shoreward and seaward propagating components. Seaward of the surf zone, shoreward propagating infragravity waves are amplified by nonlinear interactions with groups of sea and swell, and the shoreward infragravity energy flux increases in the onshore direction. In the surf zone, nonlinear phase coupling between infragravity waves and groups of sea and swell decreases, as does the shoreward infragravity energy flux, consistent with the cessation of nonlinear forcing and the increased importance of infragravity wave dissipation. Seaward propagating infragravity waves are not phase coupled to incident wave groups, and their energy levels suggest strong infragravity wave reflection near the shoreline. The cross-shore variation of the seaward energy flux is weaker than that of the shoreward flux, resulting in cross-shore variation of the squared infragravity reflection coefficient (ratio of seaward to shoreward energy flux) between about 0.4 and 1.5.