Space-charge transport limits of ion beams in periodic quadrupole focusing channels

It has been empirically observed in both experiments and particle-in-cell simulations that space-charge-dominated beams suffer strong growth in statistical phase-space area (degraded quality) and particle losses in alternating gradient quadrupole transport channels when the undepressed phase advance sigma_0 increases beyond about 85 degrees per lattice period. Although this criterion has been used extensively in practical designs of strong focusing intense beam transport lattices, the origin of the limit has not been understood. We propose a mechanism for the transport limit resulting from classes of halo particle resonances near the core of the beam that allow near-edge particles to rapidly increase in oscillation amplitude when the space-charge intensity and the flutter of the matched beam envelope are both sufficiently large. When coupled with a diffuse beam edge and/or perturbations internal to the beam core that can drive particles outside the edge, this mechanism can result in large and rapid halo-driven increases in the statistical phase-space area of the beam, lost particles, and degraded transport. A core-particle model is applied to parametrically analyze this process. Extensive self-consistent particle in cell simulations are employed to better quantify space-charge limit and verify core-particle model predictions.Comment: 17 pages, 5 figures. Submitted to Nuclear Instruments and Methods A. Includes a long version of a conference talk (trans_limits_talk.pdf) presented on the topic at the "Coulomb'05 -- High Intensity Beam Dynamics" workshop (Senigallia, Italy, 12-16 September 2005). This talk presents further supporting information/plots not included in the abbreviated, draft-format manuscrip

Energy Injection for Fast Ignition

In the fast ignition concept, assembled fuel is ignited through a separate high intensity laser pulse. Fast Ignition targets facilitate this ignition using a reentrant cone. It provides clear access through the overlaying coronal plasma, and controls the laser plasma interaction to optimize hot-electron production and transport into the compressed plasma. Recent results suggest that the cone does not play any role in guiding light or electrons to its tip, and coupling to electrons can be reduced by a small amount of preplasma. This puts stringent requirements on the ignition laser focusing, pointing, and prepulse

Effect of Target Material on Fast Electron Transport

In cone-guided fast ignition (FI) inertial confinement fusion, successful ignition relies on the efficient transport of a relativistic electron beam (REB) through a solid density cone tip to a high-density fuel core. A variety of physics mechanisms affect the quality of beam transport, and these effects vary with tip material. This thesis presents a systematic study of the effect of tip material on REB transport.An experiment was performed using the Titan laser (150 J, 0.7 ps pulse duration, 1 $\mu$m wavelength) at LLNL on multilayered targets with varying transport layers. A more collimated electron beam was consistently observed using high- or mid-Z transport layers as compared to low Z layers, without a significant loss in forward-going electron energy flux. PIC simulations agreed well with experiments, showing the formation of strong resistive magnetic channels ($\sim$80 MG ) enveloped by a global B-field that collimate initially divergent fast electrons (in high-Z targets). These results illustrated the dynamic competition between stopping and collimation that is essential to understand in order to optimize electron flux levels. Hybrid-PIC simulations further investigated transport in various materials at Titan laser conditions. REB energy loss from stopping was similar in low- and mid-Z materials (21 - 27 \%), and much higher in Au (54 \%), dominated by ohmic stopping. Resistive magnetic field growth was shown to depend on the dynamic competition between the resistivity and resistivity gradient source terms in Faraday's Law. Resistivity evolution, in addition, was shown to depend on the Spitzer-like competition between the ionization state and temperature growth rates. Results suggest that, at Titan conditions, mid-atomic number materials like Cu and Ag are optimal for collimation. This work has significant implications for fast ignition. At FI conditions, more energy will be injected into the cone tip very quickly, leading to faster ionization and heating rates. Higher atomic number materials may be favorable at these conditions as ionization can continue for a longer period during a $\sim$20 ps FI pulse. These results motivate further computational and experimental work to investigate how multilayer targets can be exploited to maximize fast electron beam collimation whilst minimizing deposition rates