| In flash x-ray radiography, one of the key goals is to focus an electron beam onto a high-atomic number target to generate intense x-rays. One of several goals is to create a small radiographic spot. The paraxial diode, which utilizes a gas-filled transport cell to focus the beam into a small spot, has been investigated for several decades. It has been shown in simulations using L SP, a particle-in-cell code, that the primary limitation to achieving a small spot size is due to time-dependent net currents in the transport cell which cause the beam's focal plane to shift axially during the timescale of the pulse. This movement, often referred to as "beam sweep", smears out the radiographic spot, leading to a larger size than is desired. Further calculations with LSP suggest that replacing the gas with a preionized plasma with density on the order of 1016 cm-3 and temperature 3 eV significantly slows down the net current growth in the transport cell. For a stable voltage, the spot remains stable for the duration of the pulse (50-70 ns), and this results in a decreased time-integrated radiographic spot.; Recent experiments utilizing gas and plasma-filled transport cells have been performed on the RITS-3 accelerator (4.5 MV, 20-30 kA beam current, 50-70 ns FWHM) at Sandia National Laboratories to study beam sweep. The preionized plasma was produced by a z-discharge in 500 mTorr hydrogen incorporated inside the transport cell. The beam was injected into the plasma with densities ranging from 2.7 +/- 0.40 x 1015 cm-3 to 1.2 +/- 0.20 x 1016 cm-3, based on laser interferometry and spectroscopic measurements.; The basic physics principle of stabilizing the beam spot by incorporating a high density, preionized plasma in the transport cell was demonstrated. The spot was maintained at 7.75 +/- 0.78 mm for 45 ns while the voltage was stable. The plasma density was 9.0 +/- 3.0 x 1015 cm-3 at the time of beam injection.; Beam sweep was observed for several variations of the paraxial diodes in 1-2 Torr air and the plasma-filled paraxial diode with lower densities, ∼ 1015 cm-3. |
