Progress in lithium beam focusing and beam-target interaction experiments at Sandia National Laboratories

Significant progress in the generation and focusing of ion beams generated by PBFA-II has enabled us to begin experiments in ion beam coupling and target physics. Data from these experiments indicates that we can reproducibly deliver ∼50 KJ of 5 MeV protons at an average power intensity of 3.5 TW/cm2 to a 6 mm diameter by 6 mm tall cylindrical target. The implosion of spherical exploding pusher targets and the radiation production from foam-filled cylindrical thermal targets were studied in these experiments. They demonstrated that high quality target data can be obtained on PBFA-II. Specific deposition rates of about 100 TW/g were achieved in these experiments. This deposition rate marks the boundary between the regime where enhanced ion deposition and equation-of-state (EOS) physics are studied (10–100 TW/g) and the regime where radiation-conversion and radiation-transport physics are studied (100–1000 TW/g). Experiments in the radiation-conversion regime are now of primary importance in our program because these experiments will test the target physics basis for ion-driven ICE Experiments using a thin film LiF source have produced an intensity of 1 TW/cm2 of lithium ions. This beam has a potential specific deposition rate of 300–400 TW/g in hydrocarbon foams. However, radiation conversion experiments will require an increased total energy content of this beam in order to overcome the specific internal energy of the foam. Further increases in ion beam intensity and energy content are being pursued in a multi-pronged attack. Understanding and controlling ion beam divergence is the highest program priority. Present understanding indicates that instabilities in the electron sheath cause significant ion beam divergence. Our understanding suggests that this contribution to the ion divergence can be decreased by operating the diode at a low enhancement through the use of high applied magnetic fields or by modifying the electron distribution near the anode via electron limiters. The new 9 cm radius “Compact Diode” has the capability of generating ≥8 T applied magnetic fields which will enable divergence experiments in the low-enhancement, high-B regime. Experiments with the LEVIS (Laser Evaporation Ion Source) lithium source have demonstrated the existence of a preformed plasma, as determined by visible-emission spectroscopy of the anode plasma. Work on improving lithium purity with this source is in progress. This active anode plasma will be used in experiments testing the effectiveness of electron limiters in controlling ion beam divergence. We are also working to understand the interrelation between accelerator coupling, diode physics, and ion beam focusing in order to optimize the diode configuration to maximize the power intensity on target. Success in these experiments will provide an adequate lithium beam for performing target experiments exploring radiation conversion and radiation transport physics in ion-driven ICF.