ELECTROMAGNETIC SIMULATIONS OF DIELECTRIC WALL
ACCELERATOR STRUCTURES FOR ELECTRON BEAM
ACCELERATION*
S. D. Nelson, B. R. Poole, Lawrence Livermore
National Laboratory (LLNL), Livermore, CA 94550, U.S.A.
Abstract
Dielectric Wall Accelerator (DWA) technology
incorporates the energy storage mechanism, the switching
mechanism, and the acceleration mechanism for electron
beams. Electromagnetic simulations of DWA structures
includes these effects and also details of the switch
configuration and how that switch time affects the electric
field pulse which accelerates the particle beam. DWA
structures include both bi-linear and bi-spiral
configurations with field gradients on the order of
20MV/m and the simulations include the effects of the
beampipe, the beampipe walls, and the DWA High
Gradient Insulator (HGI) insulating stack. Design trade-
offs include the transmission line impedance (typically a
few ohms), equilibration ring optimization, driving switch
inductances, and layer-to-layer coupling effects and the
Figure 1: Configuration before and after switch closure
associated affect on the acceleration pulse's peak value.
showing the electric field direction along the wall
GEOMETRY
INTRODUCTION
The impedance of each Blumlein corresponds to the
DWA structures consist of charged Blumlein stacks
parallel-plate transmission line impedance of Z0pp =
which produce an acceleration gradient only during the
Sqrt(µ/ε) d / b, where µ and ε are the permeability and
period of time corresponding to twice the electrical length
permittivity of the dielectric, d is the dielectric thickness,
of the line. Otherwise, there is zero net accelerating
and b is the metallization width. Typically d/b is 1:30 and
gradient and the outer structure of the DWA stack is at
the parallel-plate equation is valid for slow pulses. Note
ground potential. As such, it is similar in concept to a
however, that during the transient switch closure, there is
ferrite-loaded linear induction accelerator cell which is at
coupling between adjacent layers of the Blumleins. The
ground potential except for the period of time
effective impedance of each pair of Blumleins (a single
corresponding to the Volt-second rating of the ferrite core.
stack) is 2Z0pp. Attempts at a single-ended feeds have been
In a DWA design, the collapsing field from the blumlien
unsuccessful to date due to the need to balance the
reaches the beampipe wall, produces an accelerating
magnetic fields in the beampipe. Single-ended feeds
gradient, and accelerates a particle beam (see Figure 1).
produce a strong dipole H field where as double-ended
feeds cancel the H fields in the beampipe. Note that in the
The simulations were performed in 4D (x,y,z,t) using
double-ended configuration, the beampipe loading is
finite difference time domain (FDTD) calculations [1]. In
symmetric.
addition to discrete wires, switches, resistors, capacitors,
2Z0pp
and inductors, LLNL supported the addition of static
initialization via Poisson solutions of capacitive systems;
H
E
to the knowledge of the authors, this is unique in FDTD
E
codes. This increased the throughput of simulations by a
H
factor of 20. Discrete switches allowed for detailed
simulations of systems common to the accelerator
2Z0pp
community and other high power stored energy systems.
Figure 2: View from above showing the beampipe and the
complete double-ended feed configuration
____________________________________________
*This work was performed under the auspices of the U.S. Department of
Energy, the University of California, Lawrence Livermore National
Laboratory under Contract No. W-7405-Eng-48.