Very-High-Brightness Picosecond Electron Source
H. Bluem
Advanced Energy Systems, PO Box 7455, Princeton, NJ 08543
Abstract. Bright, RF photocathode electron guns are the source of choice for most high-performance research
accelerator applications. Some of these applications are pushing the performance boundaries of the present state-of-theart guns. Advanced Energy Systems is developing a novel photocathode RF gun that shows excellent promise for
extending gun performance. Initial gun simulations with only a short booster accelerator easily break the benchmark
emittance of one micron for 1 nC of bunch charge. The pulse length in these simulations is less than 2 ps. It is expected
that with more detailed optimization studies, the performance can be further improved. The performance details of the
gun will be presented. In addition, we will discuss the present design concept along with the status of the project.
INTRODUCTION
DESCRIPTION
Continued progress in many evolving and proposed
advanced accelerator applications requires continuing
advancement in the capabilities of electron beam
sources. Such applications include next generation
linear colliders, advanced light sources, and linacs for
basic research. Although a higher quality electron
beam is the underlying driver in most applications, the
approach to achieving the required electron beam can
vary widely. A common factor in these advanced
sources, however, is that they are based on
photocathode electron guns that can produce short
pulse, high-brightness electron bunches.
The basic concept of the design is shown in Figure
1. This figure shows the RF structure of the gun. The
device is perfectly cylindrically symmetric with no
radial protrusions on the gun body. Thus, it can be
exactly simulated in SUPERFISH[2]. The input RF is
brought in coaxially to the first fractional cell. The
end of the coaxial inner conductor comprises the
cathode. There are no radial tuning inserts. The
tuning will also be accomplished coaxially. Because
there are no radial protrusions from the gun body, the
solenoid can be placed over the gun cells with the
bucking solenoid being placed behind the cathode.
Advanced Energy Systems (AES) has been active
in the development and application of advanced, highbrightness electron sources for a variety of
applications.
The present paper discusses the
investigation of an axisymmetric RF gun design that,
according to simulations, can produce extremely
bright, 1 nC level electron pulses.
The benefits of this design are several. Firstly, as
already noted, the solenoid can be placed in an optimal
location for emittance compensation. Additionally,
emittance growth from higher order multipole RF
fields is expected to be eliminated due to the rotational
symmetry. These two features should result in
increased brightness over existing designs. To further
enhance the beam brightness, the gun will be designed
to operate in X-band. Coupling at the cathode end, as
opposed to the downstream end of the gun[3], allows
maximum flexibility in setting the optimal gun to linac
spacing for emittance compensation, while still
allowing sufficient space for the photocathode drive
laser entry port and all necessary diagnostics. It also
This gun represents the next step in emittance
reduction efforts. It seeks to eliminate contributions to
emittance from non-axisymmetric modes. In addition,
it allows for the optimal placement of the emittance
compensation solenoid over a short BNL-type gun[1].
The design presented here is in X-band at 11.424 GHz,
but the overall concept can be scaled to any frequency.
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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allows for flexibility in the length of the solenoid. The
ability to place the solenoid over the first cell aids in
the transport of higher charge per bunch that would
otherwise lead to excessive radial bunch expansion by
the time the beam reaches the end of the gun. Hence,
we would expect to achieve higher peak current out of
this system as compared to other configurations.
Finally, one or two additional accelerating cells can be
added to the gun without pushing the solenoid farther
from the cathode.
FIGURE 1. Gun cavity shape and field lines from SUPERFISH.
radial electric field near the cathode is slightly larger
in the coaxial design, the variation is still
approximately linear. By the middle of the first cell,
these small differences are gone.
RF DESIGN
The primary question to be answered in the RF
design is whether the transition between the coaxial
line and the first accelerating cavity, i.e. the location of
the cathode, has a reasonable field distribution. The
goal of the initial RF design was to achieve minimal
differences in the field distribution between this design
and a design with a flat end wall.
FIGURE 3. Comparison of the radial electric field strength
as a function of radius at a position 0.002 inches from the
cathode surface.
One would not expect these small field differences
to have a substantial effect on the beam dynamics and
this was verified in the subsequent beam simulations.
FIGURE 2. Comparison of the axial electric field strength
as a function of radius at a position 0.002 inches from the
cathode surface.
BEAM DYNAMICS
The beam dynamics in this gun design were studied
using PARMELA[4]. The initial radial beam profile
was assumed to be flat topped. The initial axial beam
shape was also assumed to be a top hat shape. For this
study the solenoid was placed over the entire gun, and
the RF field profile as shown in Figure 1 was used for
Figures 2 and 3 show a comparison between the
coaxial feed and a flat backplate. Other than the
details in the cathode region, the two geometries were
identical. As can be seen in the figures, there are small
differences in the field dependencies. Although the
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the accelerating fields. There was no effort to
optimize the solenoid position or the shape of the
solenoidal field.
TABLE 1. Beam Output Comparison Between Cases
with the Solenoid Over the Gun and After the Gun.
Parameter
Solenoid
Solenoid
Units
Over Gun After Gun
Three different configurations were simulated. The
standard configuration consisted of a 1.5 cell coaxial
gun with 0.1 nC of charge. This is the configuration
that is compared to a conventional gun with the
solenoid placed after the last cell and to an equivalent
gun with a flat backplate. The other configurations
studied were a 2.5 cell coaxial gun with 0.1 nC of
charge, where the additional cell was just added to the
end of the standard configuration to investigate the
benefits of slightly higher energy, and a 1.5 cell
coaxial gun followed by a four cell booster accelerator
with 1.0 nC of charge. The higher bunch charge in
this case necessitated the use of a booster accelerator
to aid in the emittance compensation just as it does for
all short RF guns.
Charge
Position of Min.
Emittance
Radius
εxn
0.10
61.5
0.10
70.2
nC
cm
0.26
0.165
0.25
0.197
Bunch Length
Energy Spread
Energy
1.02
1.3
3.3
1.04
1.3
3.3
mm rms
mmmrad
ps rms
%
MeV
The next question to be answered concerning the
new design is: Does the coaxial feed in the backplate
cause any degradation in beam performance due to RF
field effects? To address this issue, a case was also
run for a gun with a flat, continuous cathode backplate.
The coaxial portion was removed and the radius of the
first cell was adjusted to compensate for the resulting
frequency change. Otherwise, the gun and solenoid
geometry was identical to the base configuration. The
flat backplate case results were also relatively well
optimized. The result of this set of simulations
revealed that the coaxial line geometry results in an
emittance penalty of less than 2% when compared to a
gun with a uniform backplate. This penalty is
insignificant when compared to the potential emittance
gains that the proposed geometry offers through
solenoid positioning (shown above) and elimination of
higher order mode asymmetries. Additionally, it
should be noted that no effort was expended to
optimally adjust the coaxial geometry in order to
achieve the absolute best beam performance. This
would be a worthwhile endeavor in the future.
The first step in the beam dynamics simulations
was searching the parameter space for something close
to the best performance. The peak cathode field and
the bunch charge were kept fixed, but the bunch radius
and length, the peak magnetic field, and the bunch
phase were varied. Due to time constraints, the search
can not be considered exhaustive. However, it is
believed that for the base setup, the results are quite
close to optimum.
When the initial case was
completed, the solenoid was moved to the rear of the
gun, and the parameter search was conducted once
more. Again, it is believed that the results are quite
close to optimum for this particular case. The output
beam parameters for the gun with the solenoid over the
first cell and the gun with the solenoid at the
downstream end of the gun are shown and compared
in Table 1. For the numbers presented in the table, the
bunch was allowed to drift out to the minimum
emittance point. All values in the table correspond to
this point, which is different in each configuration.
Lastly, to extend the demonstration of the gun
performance, two additional cases were examined.
First, an additional cell was added to the gun to extend
it to 2.5 cells. In the final case, the charge was
increased to 1.0 nC per bunch and a short booster
accelerator was included in the simulation. These two
cases were not optimized due to time considerations,
however their results point to the potential of this gun
concept for various applications.
As can be seen from the table, most of the final
beam parameters are virtually identical, as would be
expected. The two differences are the axial position at
which the minimum emittance occurs and the value of
the minimum emittance. This data indicates that a gun
design that forces the solenoid to the rear of the gun
can lead to an emittance increase of approximately
20%. This emittance penalty is due to solenoid
positioning alone and does not take into account the
RF asymmetries introduced by other designs.
Accounting for the RF asymmetries is beyond the
scope of this project, but they would certainly be
expected to cause a further increase in the transverse
emittance.
The 2.5 cell gun’s purpose is solely to provide an
easy method of increasing the gun output energy. The
additional cell increased the energy to over 5 MeV. Its
performance in other areas was slightly improved over
that of the 1.5 cell gun. For instance, the emittance of
the best run is down to 0.15 µm, and the pulse length
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is below 1 ps rms. From a performance standpoint, the
2.5 cell gun would appear to be the preferred choice,
especially if there is no additional acceleration taking
place after the gun. Any additional acceleration
through a booster could most likely be optimized such
that, in the end, both 1.5 cell and 2.5 cell guns would
provide the same ultimate emittance.
From a
fabrication standpoint, the 2.5 cell gun is certainly
more difficult, mostly due to tuning issues. However,
the 2.5 cell version of the gun seems to be a feasible
alternative for those applications requiring only
slightly more energy than the 1.5 cell gun can provide
or that would benefit from the improved performance
without needing an additional accelerator.
SUMMARY
Advanced Energy Systems is developing a
promising new RF gun geometry. This new gun is
expected to be able to exceed the performance of the
present standard gun designs, providing extremely
bright bunches of electrons at meaningful charge
levels. Simulations indicate that the benchmark of 1
micron transverse emittance at 1 nC of charge can be
easily surpassed.
ACKNOWLEDGMENTS
The 1.0 nC case utilized the base gun
configuration, however, due to the increased charge, a
short 4-cell booster accelerator was added to the
simulation. The booster was placed, according to the
emittance compensation prescription, at the waist of
the focussed beam that exits the gun. Without much
effort at optimization, a transverse emittance of 0.76
µm was achieved at an energy of 8.7 MeV. The
emittance and beam envelope evolution are shown in
Figure 4. The emittance spike due to the vector
potential of the solenoid is intentionally cut off to
provide a better view of the downstream emittance
evolution. The smaller emittance spikes, visible at a
little over 60 cm, are due to the focusing and
defocusing effects in the booster accelerator.
The author would like to gratefully acknowledge
the assistance of his coworkers in this endeavor,
especially the many useful discussions with Alan
Todd. The support of this work under DOE SBIR
contract DE-FG02-01ER83135 is also gratefully
acknowledged.
REFERENCES
1. http://nslsweb.nsls.bnl.gov/AccTest/Menu.html
2. J.H. Billen and L.M. Young, “POISSON/SUPERFISH
on PC Compatibles,” Proc. 1993 IEEE Particle
Accelerator Conference, IEEE 93CH3279-7, 2 (1993)
790.
6
Emittance (mm-mr)
3. F.B. Kiewiet, O.J. Luiten, G.J.H Brussaard, J.I.M.
Botman, and M.J. Van der Wiel, “A DC/RF Gun for
Generating Ultra-Short High-Brightness Electron
Bunches,” in Proceedings of EPAC 2000, Vienna,
Austria, pp. 1660-1662.
Radius (mm)
5
4
3
4. http://laacg1.lanl.gov/laacg/services/parmela.html
2
1
0
0
20
40
60
80
100
Axial Position (cm)
FIGURE 4. Transverse beam envelope evolution for 1 nC
charge bunch through gun and 4-cell booster.
It is fully expected that even better performance
could be achieved with a concerted effort at
optimization. A longer booster accelerator should also
provide better ultimate emittance.
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