Beam clean-up zone calculations for 2.5MV, 1.4kA experiments on DARHT-2
The injector for the second axis of the Dual-Axis Radiographic Hydrotest Facility (DARHT) produces 2 us, 2kA, 3.2 MeV electron beam with a nominal rise time of 600 ns. The injector diode is powered through a ceramic column by a Marx generator. The off-energy electrons (beam head and tail) during the rise and fall time are mismatched to the magnetic transport channel. These electrons may hit the accelerating gaps, resulting in insulator flashover or a vacuum arc. The beam cleanup zone (BCUZ) has been designed to remove low energy beam electrons and to keep mismatched charge from reaching the downstream accelerator. Eliminating the beam head requires a strongly dispersive magnetic field tune in the BCUZ. In this paper, we will present simulation results on developing a beam clean-up tune and on the beam transport through BCUZ for 2.5MeV, 1.4kA experiments on DARHT-2 using the particle-in-cell code Lsp and the envelope code Lamda. The energy loss and wall temperature rise, electron stimulated desorption due to beam scrape-off in the BCUZ and the effects of slow approach to the flat top on transport of the beam body are also investigated. In addition, the experimentally measured voltage profiles for the injector diode and the accelerating cells were used in these simulations. The current transport from Lamda calculations is compared with Lsp results.
Design of beam cleanup zone for DARHT-2
The DARHT-2 linear induction accelerator is designed to produce a 2 kA, 20 MV, 2 /spl mu/s flat-top electron beam. The injector is driven directly by a Marx bank, and may have a risetime as large as one microsecond. As a result, there will be a considerable amount of beam charge which is mismatched to the magnetic transport channel. Depositing some of this charge near accelerating gaps could potentially cause insulator flashover. Deposition near the thermionic cathode could lead to cathode poisoning by desorbed neutrals and ions. The approach that has been adopted is to transport the beam-head without loss through the first eight cells, and then eliminate most of the beam-head in a "beam cleanup zone" (BCUZ). This consists of a 3 m section of beam pipe, two apertures, and three static solenoids. We describe two different beam tunes through this region: an aggressive "head cleanup" tune which removes the low-energy beam-head at the expense of a large local envelope modulation in the beam-body, and a "smooth" tune, which transports the beam-body with minimal envelope modulation. Calculations of emittance growth, electron energy deposition, secondary electron emission, and surface neutral and ion desorption in the BCUZ for each tune will be presented. We will also show the effect on transport through the rest of the accelerator. The calculations make use of an envelope code (LAMDA), beam-slice code (SPROP), and particle-in-cell code (LSP).
Using Multispectral Imaging to Measure Temperature Profiles and Emissivity of Large Thermionic Dispenser, Cathodes
Thermionic dispenser cathodes are widely used in modern high-power microwave tubes. Use of these cathodes has led to significant improvement in performance. In recent years these cathodes have been used in electron linear accelerators (LINACs), particularly in induction LINACs, such as the Experimental Test Accelerator at Lawrence Livermore National Laboratory and the Relativistic Test Accelerator at Lawrence Berkeley National Laboratory. For induction LINACs, the thermionic dispenser cathode provides greater reproducibility, longer pulse lengths, and lower emittance beams than does a field emission cathode. Los Alamos National Laboratory is fabricating a dual-axis X-ray radiography machine called dual-axis radiograph hydrodynamic test (DARHT). The second axis of DARHT consists of a 2-kA, 20-MeV induction LINAC that uses a 3.2-MeV electron gun with a tungsten thermionic-dispenser cathode. Typically the DARHT cathode current density is 10 A/cm{sup 2} at 1050 C. Under these conditions current density is space-charge limited, which is desirable since current density is independent of temperature. At lower temperature (the temperature-limited regime) there are variations in the local current density due to a nonuniform temperature profile. To obtain the desired uniform current density associated with space-charge limited operation, the coolest area on the cathode must be at a sufficiently high temperature so that the emission is space-charge limited. Consequently, the rest of the cathode is emitting at the same space-charge-limited current density but is at a higher temperature than necessary. Because cathode lifetime is such a strong function of cathode temperature, there is a severe penalty for nonuniformity in the cathode temperature. For example, a temperature increase of 50 C means cathode lifetime will decrease by a factor of at least four. Therefore, we are motivated to measure the temperature profiles of our large-area cathodes.