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).
LAND DEGRADATION IN THE DEVELOPING WORLD: ISSUES AND POLICY OPTIONS FOR 2020
Land degradation takes a number of forms, including depletion of soil nutrients, salinization, agrochemical pollution, soil erosion, vegetative degradation as a result of overgrazing, and the cutting of forests for farmland. All of these types of degradation cause a decline in the productive capacity of the land, reducing potential yields. Farmers may need to use more inputs such as fertilizer or manure in order to maintain yields, or they may temporarily or permanently abandon some plots. Degradation may also induce farmers to convert land to lower-value uses. For example, farmers may plant cassava, which demands few nutrients, instead of maize, or convert cropland to grazing land.