The Charged Particle Physics Branch performs basic and applied research on topics relevant to Navy and DoD missions with potential spin-offs to the private sector. The Branch is comprised of 10 PhD physicists, two engineers, two technicians, and several part time undergraduate research assistants. The physicists have an average of nearly 15 years of experience in plasma physics, diagnostics, beam physics, high power microwaves, and related areas. Experimental research is performed in labs at the Naval Research Laboratory in Washington, DC as well as occasionally at other facilities around the US. Research is centered in two primary areas: applications oriented plasma programs, and studies of the near earth space environment. Current applications programs include the investigation of a new technique to make large area plasmas for materials processing, the study of electrodeless discharge lamps for efficient white light production, and the investigation of the effects of plasma discharges on airplane drag. Space research includes investigations of the coupling of solar energy into the ionosphere and magnetosphere, the generation of dusty plasmas and their effects on the ionosphere, and the propagation of electromagnetic waves in the ionosphere. Additional work in broad instantaneous-bandwidth radar beam steering, the origin and characterization of lightning discharges, spacecraft and aircraft electrostatic charging effects, beam propagation in the atmosphere, applications of hyperspectral imagery to surface analysis, and the development of high velocity electric launchers is performed. The common thread of plasma generation and detailed diagnostics couples the different fields studied within the Branch. Members of the Branch work closely with other groups within the Plasma Physics Division, other Divisions with the NRL and with many other laboratories and universities.
The Charged Particle Physics Branch has been investigating a new techique to make a plasma suitable for materials processing. This technique involves injecting a low current (10's milliamp/sq. cm.), moderate energy (several kilovolts) beam of electrons into a gas filled chamber. The beam electrons are confined with a magnetic field produced by a series of coils surrounding the chamber. The technique is unique in that it can produce a very large area (square meters), thin (centimeters), cold (sub-electron volt electron temperature) plasma layer which can be located close to a surface. The plasma generation technique is almost completely decoupled from the surface allowing one to adjust the parameters (e.g., plasma temperature, density) or the composition (e.g., relative concentration of different atomic or molecular species) of the plasma in order to optimize the desired materials effect.
The figure to the left shows a schematic representation of the beam-generated
plasma technique. A sheet beam of electrons is injected into a gas chamber
and is reabsorbed at the opposite side. The electrons pass through the
gas, ionizing the background to form a sheet of plasma. Ions, neutrals,
free radicals, or other products generated in the plasma then strike the
surface located near the plasma. The energy of ions extracted from the
plasma can be regulated using a RF bias network on the material processing
The picture at the far left shows a the e-beam source chamber. The rectangular
coils are capable of generating up to 300 gauss magnetic fields in the
source region. A 1 meter x 1 cm beam source is located inside of the chamber.
The second picture shows the main processing chamber. The source chamber
replaces the end flange. The field coils can produce up to 300 Gauss field
with a few percent uniformity over the entire 1 meter x 1 meter processing
area. The chamber contains diagnostic ports on all surfaces to allow access
to the plasma.
The photo at left shows an early example of etching of a photoresist
using a beam produced plasma. The aluminum mask was placed over a 0.5 micron
thick layer of photoresist on top of a piece of silicon. The chemical action
of oxygen free radicals produced by the beam etched the photoresist wherever
it was not covered by the aluminum mask. No bias voltage was used in this
case making the etch isotropic and producing the hollowed out region under
the edge of the mask.
The schematic at left shows the concept behind the new electron beam
source. A rectangular cross section volume (hollow cathode) with a slot
on one surface is filled with a low pressure gas (50-150 mTorr). An electrode
with a similar slot is placed approximately 1 cm downstream from the hollow
cathode. This slot is covered by a fine stainless steel mesh. A 50-300
Gauss magnetic field is superimposed on the source. 300-400 V placed across
the two electrodes drives a cw discharge between the hollow cathode and
the screen. Depending on the gas pressure, voltage, and geometry the discharge
can produce10's of milliamperes/sq. cm current densities at the screen.
Another electrode located approximately 1 cm downstream from the screen
can extract electrons from the holes in the wire mesh. These electrons
gain the voltage imposed on the gap. The extraction gap can pull up to
90% of the discharge current out of the plasma and can be run cw or in
a pulsed mode.
The pictures at left show an edge-on view of a beam and a top view with
a probe stuck into the beam. The light comes from a nitrogen background
gas fill. The beam can be run continuously producing a well defined plasma