DEVELOPMENT OF A COMPACT ROTATING-WAVE
ELECTRON BEAM ACCELERATOR
Jose E. Velazco
Microwave Technologies Incorporated, Fairfax, Virginia 22030
Peter H. Ceperley
Departments of Physics and Electrical Engineering, George Mason University, Fairfax, Virginia 22030
Abstract. We present the successful prototype development results of a novel compact rotating-wave electron beam
accelerator (RWA). The RWA uses a single cylindrical cavity holding a transverse-magnetic resonant mode in
combination with an axial static magnetic field to accelerate electrons to higher energies. With approximately 80
kilowatts of microwave power fed into a C-band cavity, we have been able to successfully accelerate a 3 keV electron
beam to ~760 keV. The compact RWA accelerator could be the basis for a new class of compact and affordable 1-10
MeV microwave accelerators for military, medical and industrial applications.
1. INTRODUCTION
Rf in
Waveguide
The purpose of this project was the development of
a compact rotating-wave electron accelerator (RWA)
[1-3]. The RWA uses a single cylindrical cavity in
conjunction with an external magnetic field to
gradually accelerate a low energy electron beam to
high energies. A schematic of the rotating-wave
accelerator is shown in Fig. 1. The RWA prototype
consists of an electron gun, accelerator cavity, and a
set of electromagnets. The design parameters for the
RWA prototype development are listed in Table I.
Electron
gun
Electron beam
envelope
Cylindrical
cavity
Magnetic
coils
15 cm
2. SYSTEM COMPONENTS
FIGURE 1: Schematic of rotating-wave accelerator.
RWA cavity
resonator with two rectangular waveguides to feed
high power microwaves into the cavity. Two WR-137
rectangular waveguides arranged 90o apart azimuthally
on the front face of the cavity are used to generate the
TM110 mode inside the cavity. The cavity is made out
of 304 stainless steel and is silverplated inside in order
to minimize wall losses. A picture of the RWA cavity
is shown in Fig. 2. The coupling waveguides are
The RWA uses a short cylindrical smooth-wall
resonator operating in the TM110 mode to produce
beam acceleration.
We designed the prototype
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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terminated in two pressure microwave windows that
are used to maintain vacuum integrity inside the
cavity.
TABLE I: RWA Design Parameters
Parameter
Units
Cavity Mode
Cavity Radius
cm
Cavity Length
cm
Initial Beam Energy
keV
Beam Current
mA
Pulse Width
microsecond
Pulse Rate
Hz
Peak focusing field
kG
Final Beam Energy
MeV
Value
TM110
3.2
15
3-5
10-50
4
1-10
3
0.5-0.8
FIGURE 3: Picture of high-current pulse modulators
assembled inside a rack. The pulse modulators are designed
to deliver up to 250 V, 50 A pulses to the electro-magnets.
Each magnet is driven independently by one pulse
modulator.
FIGURE 2: Picture of high-power RWA cavity.
High-Current Pulsers and electromagnets
In this project we also designed, built, and tested a
set of seven high-current pulse modulators for driving
the electromagnets of the RWA. The pulsers were
designed to provide output voltages up to 250 V and
currents as high as 50 amps. After each modulator
was fully tested, we assembled them inside a rack as
shown in Fig. 3. In order to provide electrical
isolation between the switching devices (IGBTs) from
each modulator, we also designed a controller box for
these modulators.
This box provides electrical
isolation between each IGBT by using a photocoupler
driver in the gate of each IGBT switch. The controller
electronics also include means for delaying the pulse
signal of each modulator with respect to the others so
that they can be properly aligned in time. This box is
designed to control 7 modulators and has 2 additional
5 V outputs for driving the electron beam and rf
pulsers. Figure 4 shows pictures of the finalized
controller box.
FIGURE 4: Picture of controller box for high-current pulse
modulators.
The current RWA prototype uses 7 magnets to
focus the electron beam and to provide
synchronization of the electron orbits with the field of
the TM110 mode. Six electromagnets are identical and
have the following dimensions: 3” ID, 7” OD, and
1.5” length. A seventh magnet with dimensions: 7”
ID, 11” OD, and 1.5” length is inserted between the
electron gun and the RWA cavity to improve transport
of the electron from the gun to the beginning of the
accelerator cavity. Figure 5 shows a picture of the 7
electromagnets assembled together. We used hightemperature plexiglass components plates and hard
nylon rods to fasten the magnets together (all materials
used in the holder system are non-magnetic).
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upon impinging on a copper target located 2”
downstream the end of the cavity. Note that a lead
shield is placed around the accelerator output which
provides for radiation protection as well as shields the
detector from natural background radiation.
3. SYSTEM TESTING AND
ACCELERATION RESULTS
Figure 6 shows the layout of the RWA
experimental apparatus. The magnetron source is
connected to the driving waveguides of the RWA
cavity via straight pieces of waveguides. Directional
couplers are inserted between the magnetron and
cavity in order to measure the amount of forward and
reflected power.
Two lightly-coupled capacitive
sampling probes are also inserted in the RWA cavity
to monitor the microwaves inside the cavity. Highfrequency crystal detectors are attached to the output
of each sampling probe for monitoring the sampled
signals in a 4-channel Tektronix oscilloscope. A
scintillator for imaging the beam is attached to the end
of the RWA chamber via a glass insulator which
allows the scintillator to also act as a collector for
beam current measurement at that point. A CCD
camera is used to monitor the spot produced by the
electron beam on the scintillator. The scintillator is
baked with a copper mesh which acts as the main
target for the generation of x-ray radiation. The
optically-isolated controller box provided electrical
pulses for driving the pulse modulators and other
pulsers in the system.
FIGURE 5: Picture of 7 electromagnet system used to
produce the RWA focusing magnetic field.
We routinely performed field mapping of the magnetic
field produced by the seven magnets located along the
axis of the system using an F.W. Bell 5000 series
Gaussmeter. We devised a field-mapping system that
allowed us to move a gaussmeter probe axially along
the axis of the device. The probe can be moved along
the axis of the coil system and discrete field
measurements can be taken at any axial point. The
current in each high-current pulser was adjusted
properly so as to produce the desired profile and
amplitude of the magnetic field.
Magnetron rf system
A 0-200 kilowatt magnetron rf system was used for
driving the RWA cavity. This package uses a Varian
SFD-3410 coaxial magnetron oscillator capable of
delivering up to 200 kW of microwave power. This
system operates at 5.85 GHz in pulse mode and
consists of one rack that houses a pulse modulator
system and a coaxial magnetron. Triggering the rf
system with a 15 volt pulse, we are able to obtain up to
200 kW of microwave power from the magnetron.
The high-power output is pulsed with a pulse width of
~4 microseconds. The pulse repetition frequency can
be varied from 1 Hz to 100 Hz. RWA experiments are
typically run at 1 Hz.
In a typical run, we turned on the magnetron system
and powered the cavity via the driving waveguides.
The sampling probes were monitored in the scope to
assure that the proper mode is excited at the design
frequency. Next, we turned on the electron beam rack
and injected the electron beam along the axis. We
powered the electromagnet system and adjusted the
current delivered by each modulator in order to
achieve the desired focusing magnetic field profile.
X-ray Diagnostics for Beam Acceleration
With all the sub-systems optimized for beam
acceleration, we adjusted the amount of rf power fed
into the RWA cavity as we monitored the MCA for
energy readings of the x-ray radiation detected by the
NaI detector. (The image of the beam’s cross-section
was also monitored on the scintillator.)
One of the main tasks performed during this project
was the implementation of a very sensitive
spectroscopy system to measure beam acceleration.
The system consists of an NaI detector, a preamplifier
system and a multi-channel analyzer Canberrra Series
35 Plus. The NaI detector measures energy of x-ray
radiation produced by the accelerated electron beam
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Magnetron
200 kW
Adapter
Isolator
RF
CCD camera
P
v
Waveguide
RF feed
a
i
n
d
a
e
s
o
Vac Ion
pump
o
n
c
i
a
c
m
e
High-power
cavity
r
NaI detector
a
Electron
gun
To MCA
X-ray
radiation
Radiation
shield
FIGURE 6: Schematic of rotating-wave accelerator prototype experimental
apparatus.
Electron Beam Energy (keV)
A data point from the MCA was obtained for each
case of input rf power. Figure 7 shows a typical plot
of the measured radiation as a function of rf power.
The injection energy of the electron beam was 3 keV
and the beam current was 10 mA. As can be observed
in the plot, a maximum energy of ~760 keV has been
obtained so far during the RWA testing. Figure 8
shows a picture of the beam cross-section produced on
the scintillator. The beam depicts a ring on the
scintillator as a result of the transverse energy gained
by the electron beam during acceleration. It should be
noted that with the RF off a small solid spot is
observed in the center of the scintillator.
900
800
700
600
500
400
300
200
100
0
20
30
40
50
60
70
Input RF power (kW)
80
90
FIGURE 7: Plot of measured radiation produced with the
new high-power RWA cavity. The experimental data is
obtained with the spectroscopy system described above.
System parameters are listed in Table I. The initial beam
energy is 3 keV and the beam current is 10mA.
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Based on the successful testing of the RWA, we are
now ready to move to the next stage of commercial
development. The compact accelerator developed here
could be the basis for a new class of compact and
affordable 1-10 MeV microwave accelerators for
military, medical and industrial applications.
5. ACKNOWLEDGMENTS
This work was funded by the Missile Defense
Organization under the Small Business Innovation
Reseach Program.
FIGURE 8: Picture of beam cross-section on scintillator.
Note that due to the acceleration electrons undergo inside the
cavity, the beam is deflected off axis describing a ring on the
scintillator.
6. REFERENCES
4. CONCLUSIONS
1. Jose E. Velazco and P. Ceperley, The Study of Compact
Rotating-Wave Accelerators for Medical and Industrial
Applications, Proceedings of the Fourteen International
Conference on the Applications of Accelerators In
Research and Industry, DE5, November 6-9, 1996,
University of North Texas, Denton, Texas.
The acceleration results obtained thus far are
a milestone in the RWA development. These results
clearly show the acceleration capabilities of the novel
RWA scheme. We plan to continue further testing of
the RWA in order to produce acceleration of the
electron beam to 1 MeV and beyond.
2. J. Velazco and P. Ceperley, A Discussion of Rotating
Wave Fields for Microwave Applications, IEEE Trans.
Microwave Theory Tech. MTT-41 (1993).
For commercial applications of the RWA we
plan to replace the electromagnets with a permanent
magnet. This will eliminate the need for pulse
modulators and will considerably reduce the size and
weight of the RWA. Figure 9 shows sketches of a
commercial RWA machine.
3. Peter H. Ceperley and Jose E. Velazco, Tuning of
Rotating Mode Resonators, Rev. Sci. Inst., 66 (1)
256-260, Jan 1995.
Figure 9: Sketch of commercial RWA using a permanent magnet.
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