• K.-C.D. Chan, H. Davis, C. Ekdahl
  LANL, Los Alamos, New Mexico
• T.C. Genoni, T.P. Hughes
  ATK-MR, Albuquerque, New Mexico
• M.E. Schulze
  GA, San Diego, California

The DARHT-II accelerator produces an 18-MeV, 2-kA, 2-μs electron beam pulse. After the accelerator, the pulse is delivered to the final focus on an x-ray producing target via a beam transport section called the Downstream Transport. Ions produced due to beam ionization of residual gases in the Downstream Transport can affect the beam dynamics. Ions generated by the head of the pulse will cause modification of space-charge forces at the tail of the pulse so that the beam head and tail will have different beam envelopes. They may also induce ion-hose instability at the tail of the pulse. If these effects are significant, the focusing requirements of beam head and tail at the final focus will become very different. The focusing of the complete beam pulse will be time dependent and difficult to achieve, leading to less efficient x-ray production. In this paper, we will describe the results of our calculations of these ion effects at different residual-gas pressure levels. Our goal is to determine the maximum residual-gas pressure allowable in DARHT-II Downstream Transport such that the required final beam focus is achievable over the entire beam pulse under these deleterious ion effects.

• T.P. Hughes, T.C. Genoni
  ATK-MR, Albuquerque, New Mexico
• H. Davis, M. Kang, B.A. Prichard
  LANL, Los Alamos, New Mexico

The DARHT-2 facility at Los Alamos National Laboratory accelerates a 2 microsecond electron beam using a series of inductive accelerating cells. The cell inductance is provided by large Metglas cores, which are driven by a pulse-forming network. The original cell design was susceptible to electrical breakdown near the outer radius of the cores. We developed a numerical model for the magnetic properties of Metglas over the range of dB/dt (magnetization rate) relevant to DARHT. The model was implemented in a radially-resolved circuit code, and in the LSP* electromagnetic code. LSP simulations showed that the field stress distribution across the outer radius of the cores was highly nonuniform. This was subsequently confirmed in experiments at LBNL. The calculated temporal evolution of the electric field stress inside the cores approximately matches experimental measurements. The cells have been redesigned to greatly reduce the field stresses along the outer radius.

*LSP is a software product of ATK Mission Research (www.lspsuite.net).

• B.A. Prichard, R.J. Briggs
  SAIC, Alamo, California
• J. Barraza, M. Kang, K. Nielsen
  LANL, Los Alamos, New Mexico
• F.M. Bieniosek, K. Chow, W.M. Fawley, E. Henestroza, L. R. Reginato, W. Waldron
  LBNL, Berkeley, California
• T.E. Genoni, T.P. Hughes
  ATK-MR, Albuquerque, New Mexico

DARHT employs two perpendicular electron Linear Induction Accelerators to produce intense, bremsstrahlung x-ray pulses for flash radiography. The second axis, DARHT II, features an 18 MeV, 2-kA, 2-microsecond accelerator. DARHT II accelerator cells have undergone a series of test and modeling efforts to fully understand their sub par performance. These R&D efforts have led to a better understanding of Linear Induction Accelerator physics for the unique DARHT II design. Specific improvements have been identified and tested. Improvements in the cell oil region, the cell vacuum region, and the PFNs have been implemented in the prototype units that have doubled the cell’s performance. A series of prototype acceptance test are underway on a number of prototype units to demonstrate that the required cell lifetime is met at the improved performance levels. Early acceptance tests indicate that the lifetime requirements are being exceeded. The shortcomings of the previous design are summarized. The improvements to the original design, their resultant improvement in performance, and various test results are included. The final acceptance test results will also be included.

• C. Ekdahl, E.O. Abeyta, R. Bartsch, L. Caudill, K.-C.D. Chan, D. Dalmas, S. Eversole, R.J. Gallegos, J. Harrison, M. Holzscheiter, E. Jacquez, J. Johnson, B.T. McCuistian, N. Montoya, S. Nath, K. Nielsen, D. Oro, L. Rodriguez, P. Rodriguez, L.J. Rowton, M. Sanchez, R. Scarpetti, M. Schauer, D. Simmons, H.V. Smith, J. Studebaker, G. Sullivan, C. Swinney, R. Temple
  LANL, Los Alamos, New Mexico
• H. Bender, W. Broste, C. Carlson, G. Durtschi, D. Frayer, D. Johnson, K. Jones, A. Meidinger, K.J. Moy, R. Sturgess, A. Tipton, C.-Y. Tom
  Bechtel Nevada, Los Alamos, New Mexico
• R.J. Briggs
  SAIC, Alamo, California
• Y.-J. Chen, T.L. Houck
  LLNL, Livermore, California
• S. Eylon, W.M. Fawley, E. Henestroza, S. Yu
  LBNL, Berkeley, California
• T.P. Hughes, C. Mostrom, Y. Tang
  ATK-MR, Albuquerque, New Mexico
• M.E. Schulze
  GA, San Diego, California

When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2-kA, 18-MeV electron beam with more than 1500-ns current/energy "flat-top." In initial tests DARHT-II has already accelerated beams with current pulse lengths from 500-ns to 1200-ns full-width at half maximum (FWHM) with more than1.2-kA, 12.5-MeV peak current and energy. Experiments are now underway with a ~2000-ns pulse length, but reduced current and energy. These pulse lengths are all significantly longer than any other multi-MeV LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated absence of BBU, the pulse lengths were too short to test the predicted protection against ion-hose instability. The present experiments are designed to resolve these and other beam-dynamics issues with a ~2000-ns pulse length beam.