Extraction Diode Experiments on PBFA X
On April 24, 1996, SNL scientists concluded the first series of experiments on PBFA II in the extraction ion
mode (PBFA X) before beginning modifications to enable z-pinch implosions (PBFA Z). This ten-month series
showed the operational advantages of an extraction ion diode, which will be required for high yield ICF to
protect diode hardware from intense bursts of x rays and gamma rays. Previously--from December 11, 1985 to
May 22, 1995--PBFA II operated with a barrel-shaped, radially-focusing diode. Features of extraction diode
operation, compared to radial diode operation, included a higher vacuum (by a factor of ten), better optical and
diagnostic access, and a faster shot rate (a maximum of four per day). Using a new magnetic field design
algorithm that accounts for dynamic field modification by electron flow, SNL researchers demonstrated the
capability to improve the radial current distribution in a predictable fashion. The FY96 goals (see SNL Pulsed
Power ICF Program Technical Contract, 11/3/95 draft) for evaluating and improving the electrode surface
cleaning techniques, previously developed on SABRE, were met. Diagnostics showed that cleaning was
enhanced at the higher RF powers (> 600 W) available on PBFA X and that the cleaning significantly
increased the diode impedance throughout the accelerator pulse. Improvements in the advanced
laser-evaporation ion source, based on light laboratory experiments and modeling, were tested on PBFA X and
showed an earlier lithium beam current compared to the passive LiF source. The PBFA-X experiments met the
FY96 goals for ion beam voltage of 8 MV in the low-voltage mode and 15 MV in the high-voltage mode.
However, during this experimental campaign, the beam currents were ~ 0.3 MA rather than the ~ 0.8 MA
needed to study extraction diode physics at current levels relevant to high yield. Future extracted ion beam
experiments will be fielded on SABRE to develop improved active lithium ion sources, demonstrate ion beam
transport, and reduce lithium ion divergence while scaling of the world-record z-pinch results at SNL on the
Saturn accelerator is sought on PBFA Z.
Other Reports on High Energy Density and Inertial Confinement Fusion
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