HEM-MURI
High Energy Microwave Research at the University of California at Berkeley
Sponsored by the
U.S. Air Force Office of Scientific Research
The High Energy Microwave MURI program is a national three consortium
effort to study high energy and high power microwave sources. The
University of California at Berkeley is a member of the Western
Consortium, led by the University of
California at Davis. The Western Consortium also includes
Stanford University/SLAC, as well as the University of California at
Los Angeles. A summary of the MURI projects at Berkeley follow.
Parallel XOOPIC:
This is an effort to extend the XOOPIC code to run on parallel
computing platforms, including symmetric multiprocessor (SMP)
computers, distributed workstations (either via standard or fast
ethernet, or in a network of workstations (NOW) configuration), and
massively parallel computers. Currently the electromagnetic and particle
solvers are operational and show 95% of linear scaling to 8 nodes.
Simple automatic domain decomposition has been implemented as well.
XOOPIC
3d XOOPIC:
This is an effort to extend the XOOPIC code from two dimensions in x-y
and r-z coordinates to three dimensions (x-y-z and r-theta-z
respectively). The 3d version is presently in the design stages, with an
initial implementation anticipated by November 1998. The 3d version will
require parallel processing capability (see above) for larger problems.
XOOPIC
Multipactor Discharge
This project is a collaboration with University of Michigan, Texas
Tech, and University of Maryland. The study incorporates simulation,
theory and experimental elements to study initiation and growth of
multipactor discharges. Two types of discharges can occur.
The two-surface discharge occurs between metal electrodes with
an rf bias between them. This is relevant to HEM since the two-surface
multipactor can occur with moderate fields in cavities and gaps. It
saturates via either detuning of the cavity/gap resonance or by space
charge defocusing.
The single-surface discharge occurs on dielectric surfaces with
an rf electric field tangential to the surface. This is relevant to
HEM microwaves since a wave with a transverse electric component
propagating through a window is a an important example of a single
surface multipactor discharge. The discharge can grow and damage the
window, reducing transmission or compomising its physical integrity.
The single-surface discharge is saturated by space charge effects,
and recent investigations indicate new mechanisms for saturation as well.
RF Breakdown
This is a collaboration with G. Scheitrum (Stanford Univeristy) and
L. Laurent (University of California at Davis). Their experiments
indicate that the field emission current in an RF cavity exceeds the
Fowler-Nordheim current by a factor of about 200, far more than can be
explained by geomoetric enhancement. Plasma formation in the gap is
postulated as a mechanism for such a large enhancment of the surface
fields.
At University of California - Berkeley, E. Kawamura is simulating the
formation of the plasma and the resulting fields and field emission
current using the XPDP1 code in one dimension. XPDP1 is self
consistent, and includes space charge, kinetic effects, and
time-dependent effects such as virtual cathode formation and
oscillation. Atomic hydrogen is assumed to desorb from the copper
surface, and is then self consistently ionized by an initial field
emission current due to RF fields in the gap. A plasma is formed in
the gap of sufficient density to neutralize the the fields, leaving a
sheath region in which the fields are significantly enhanced. The
simulation presently includes no mechanism for current limit, which
occurs in the experiment when the current causes damage to the copper
surface.
High Power Microwave Applications:
Relativistic Klystron Oscillator (AFRL)
Radial Acceletron (AFRL)
Klystrino (Stanford/SLAC)
Multipactor: two surface (metal) and single surface (dielectric)
Links:
UC Berkeley Plasma Theory and
Simulation Group Home Page