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.
Tuning the DARHT Long-Pulse Linear Induction Accelerator
Flash radiography of large hydrodynamic experiments driven by high explosives is a well-known diagnostic technique in use at many laboratories. The Dual-Axis Radiography for Hydrodynamic Testing (DARHT) facility at Los Alamos produces flash radiographs of large hydrodynamic experiments. Two linear induction accelerators (LIAs) make the bremsstrahlung radiographic source spots for orthogonal views of each test. The 2-kA, 20-MeV Axis-I LIA creates a single 60-ns radiography pulse. The 1.7-kA, 16.5-MeV Axis-II LIA creates up to four radiography pulses by kicking them out of a longer pulse that has a 1.6- μs flat top. The DARHT LIAs use solenoidal focusing for transport of the beam through the accelerators. The long-pulse Axis-II LIA has 74 accelerating cells, and uses 91 solenoids and 80 pairs of dipoles for focusing, transporting, and steering the beam. The setting of the currents of these 251 magnets is called tuning the accelerator. Tuning is done in two stages. First, the solenoidal focusing magnets are set to values designed to provide a beam with minimal envelope oscillations, and little or no instability growth. Then, steering dipoles are adjusted to minimize the low-frequency motion of the beam centroid and center it at the LIA exit. The design of the focusing tune is computationally intensive. Focusing tune design methods, simulations, and validation are the main topics of this article.
Compact Circularly Polarized Composite Cavity-Backed Crossed Dipole for GPS Applications
In this paper, we present a circularly polarized (CP) composite cavity-backed crossed dipole antenna for global positioning system (GPS) applications. We produce the CP radiation by crossing two dipoles through a 90° phase delay line of a vacant-quarter printed ring, which also has a broadband impedance matching characteristic. Two techniques, insertion of meander lines in the dipole arm and arrowhead-shaped trace at its end, are employed to reduce the sizes of the primary radiation element. The compact radiator is backed by a cavity reflector to achieve a wide CP radiation beamwidth. The proposed antenna exhibits a measured bandwidth of 1.450～1.656 GHz for a voltage standing wave ratio (VSWR) < 2 and 1.555～1.605 GHz for AR < 3-dB. At 1.575 GHz, the antenna has a gain of 7 dBic, a frontto-back ratio of 27 dB, AR of 1.18 dB, and 3-dB AR beamwidths of 130° and 132° in the x-z and y-z planes, respectively.