Rf guns: bright injectors for FEL - Triangle Universities Nuclear ...

Nuclear Instruments and Methods in Physics Research A304 (1991) 285-296 285
North-Holland
Section V .
Accelerator technology
Rf guns :
bright injectors for FEL
C . Travier
Laboratoire de !'Accélérateur Linéaire, IN2P3-CNRS and Université de Paris-sud, 91405 Orsay Cédex, France
Invited paper
Free electron lasers (FELs) require very bright electron beams . During the past few years, rf guns have demonstrated their
potentiality to produce such bright beams . A review of the on-going worldwide effort to develop this new technology is given . Present
limitations and possible improvements are also presented . General design considerations are introduced .
1 . Introduction
FELs require very good quality electron beams . A
low emittance is necessary to obtain short wavelengths .
A high peak current provides a good gain . These requirements
are already met by the best conventional
injectors (dc gun + buncher) for the infrared spectrum .
However, a need for brighter beams is real to improve
IR-FEL performances and to obtain shorter wavelength
lasers . Therefore, new ideas are necessary since conventional
injectors have nearly reached their limits [1] .
In 1983, Madey and Westenskow [2] proposed to put
a thermionic cathode in an rf cavity. This new gun
named "rf gun" or "microwave gun" was used as a
bright electron source for the Mark III FEL . Meanwhile,
Lee et al . [3] reported that very high current densities
could be obtained from semiconductor photocathodes .
Then, Fraser and Sheffield [4,5] experimented the use of
such cathodes in an rf gun . The "first demonstration of
a FEL driven by electrons from a laser irradiated photocathode"
was made at Stanford in 1988 [6] .
Since that time, many laboratories have undertaken
the necessary effort to design and build an rf gun, with
or without a photocathode . Almost 30 projects are
identified in this review [7] . Fig . 1 shows the exponential
evolution of the number of projects over the past eight
years . It also shows the increasing preponderance of
photoinjectors . Fig . 2 shows that half of these rf gun
projects are devoted for FEL injectors . The other half is
developed for different purposes such as TeV colliders,
storage ring injectors, accelerator test facilities, etc.
A review of these projects is presented . The most
significant are described .
2. Rf gun principle
The rf gun principle is rather simple . A cathode is
placed in an rf cavity . When the electric field is accel-
cil
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N THERMIONIC RF GUNS
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83 84 85 86 87 88 89 90
YEAR
Fig . 1 . Current number of rf gun projects .
erating, electrons can be extracted . Fields obtained in
an rf cavity are much higher than do fields . This property
is favourable since at low energy, electron beams
are subject to space-charge forces that lead to emittance
growth .
There exist three types of rf guns according to the
cathode that is used .
83 84 85 86 87 88 89 90
YEAR
Fig . 2 Rf gun projects dedicated to FEL .
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
V . ACCELERATOR TECHNOLOGY

28 6 C Travier / Rf guns bright injectors for FEL
2.1 . Thermionic rf gun
Electrons are continuously emitted by a thermionic
cathode but can only be extracted and accelerated during
half an rf cycle. The electrons, emitted during the
accelerating half cycle, but with a too large phase (typically
larger than 100') do not have enough energy to
reach the cavity output and are accelerated backward to
the cathode .
The pulse that actually comes out the cavity is very
long (about one fourth of the rf period) and has a very
large energy spread . It is thus necessary to place behind
the gun a magnetic bunching system (e .g ., an "a-magnet")
with energy slits to reduce the pulse length, increase,the
peak current and select a given spectrum .
2 .2 . Laser-driven rf guns
A photocathode placed in an rf cavity is illuminated
by a laser which delivers short pulses . Therefore as they
leave the cathode, electrons are already bunched .
Besides the obvious advantages (high field, bunched
beam), laser-driven guns have other attractive features .
The pulse format is more flexible than that of conventional
injectors and that of thermionic rf guns. It depends
essentially on the laser pulse format which can be
varied over a wide range. Photocathodes can deliver
much higher current densities than thermionic cathodes :
more than 400 A/cm2 has been reported [3] .
2 .3 Field emission rf guns
When placed in a high field, sharp needles can
produce electrons by the so-called field emission process
. This principle can be used in an rf gun . The
difficulty is to find stable operating conditions . When
high current is emitted, the needle is heated and easily
destroyed .
3 . Worldwide review
Since 1983, many laboratories have launched R&D
programs on rf guns . Some are already producing experimental
results while others are still at the preliminary
design stage. Reviewing all these projects is a
difficult task, due to the fast changing situation and the
lack of literature for the most recent projects . Thus, the
review presented here is probably incomplete and for
some cases not up-to-date . May those who will not find
their work reported here or who will find it reported
inaccurately, be comprehensive and believe that they
are not the subject of a special "censure" . Another
difficulty met during this review was to produce consistent
lists of parametersmorder to fairly compare the
different projects . Each author uses his own units and
definitions of the parameters and often all the parameters
are not given for the same operating conditions .
The following conventions are assumed . Design data
for emittance are always given as four times the rms
normalized emittance . When an author uses a different
definition, necessary adjustments are made . Experimental
data are reported as given in the publications . Since
in some cases real beams have much longer tails than
ideal beams and have not necessarily Gaussian or uniform
profile, the "4 rms" emittance may not be appropriate
to describe them .
For laser-driven guns, the rmcropulse length is taken
as 4a b , where ab is the rms bunch length . Rms definition
(lab) is adopted for thermionic guns because of the
pulse shape which has a long tail and a very sharp front
end . In this case, la b includes most of the particles .
When only the charge Q and the bunch length T are
given, the peak current I is calculated in a conservative
way assuring a uniform distribution (I = Q/T ) .
Table 1 lists the different projects for each type of rf
gun . Status of the projects and their main purpose are
given . In table 2, the main parameters of each project
are given . According to the project status, these parameters
correspond to experimental or design values . A
more detailed description of each project can be found
in ref. [1] .
3.1 . Stanford (HEPL) - Duke (DFELL)
The first thermionic rf gun was built at Stanford
High Energy Physics Laboratory (HEPL) by Westenskow
and Madey [2] to serve as an injector for the Mark
III accelerator dedicated to FEL studies . The design
work started in 1983 . The whole system (rf gun + lmac
+ wiggler) was operated as a laser oscillator in September
1985 . The system was then disassembled and moved
to the Stanford Photon Research Laboratory where it
was lasing again in September 1986 . In August 1987, a
new momentum filter which increased the peak current
was installed . In September 1988, a new gun cavity
allowed a more stable operation with longer rf macropulses
(8 Ws) repeated at a higher frequency (15 Hz) .
The thermionic gun consists of a LaB6 cathode placed
in an rf cavity operated at 2857 MHz . The cavity is
followed by a transport system including an "a-magnet"
with momentum filter and several quadrupoles, as shown
in fig. 3 . In order to control the backbombardment, a
transverse magnetic field at the cathode was added [8] .
A summary of the gun's main parameters and performances
is given in table 2 [9] . After the "ci-magnet", 2
to 3 ps electron pulses corresponding to 20 to 40 A peak
current are obtained with emittances as good as 41T
mm mrad in the vertical plane .
In collaboration with Rocketdyne division of
Rockwell Corporation, the same gun was operated as a

C. Trainer / Rf guns - bright injectors for FEL 28 7
photoinjector and led in November 1988 to the first
successful operation of a photoinjector driven FEL [6] .
The LaB6 cathode was illuminated by a tripled
Nd : YAG laser. Because the laser pulse was quite long
(100 ps), the "a-magnet" was still necessary. Although
the experiment was run for only a very short time, this
photoinjector gave better results (in terms of current
and emittance) than the thermionic gun [10] . The FEL
performances suggest rms emittances as good as 2m
mm mrad in the vertical plane . The parameters and
performances are given in table 2 [9] .
The whole system was then disassembled again in
December 1988 to be moved to Duke University Free
Electron Laser Laboratory (DFELL) where it was
scheduled to be operating again in 1990 .
3.2. Los Alamos National Laboratory
Since at least 1984, Los Alamos National Laboratory
(LANL) is involved in R&D programs about laserdriven
rf guns and related topics like lasertron .
In 1985, Lee et al . [3] demonstrated the possibility of
extracting high current density (over 200 A/cm2) from
CS 3 Sb photocathode illuminated by a frequency doubled
Nd : glass laser. The first rf gun design consisted of
one cavity operating at 1 .3 GHz and incorporating a
Table 1
List of rf gun projects
Laboratory Country Status Beginning Beginning Purpose
September 90 of design of tests
Therauonic rf guns
Stanford - Duke [2,10] USA tested 1983 2/85 FEL
IHEP Bering [451 China tested 6/86 2/88 FEL
Mitsubishi [461 Japan tested
IPT Kharkov [47] USSR tested ? storage ring injector
AET/Varian/SSRL [431 USA under tests 1988? 1989 storage ring injector
KEK [48] Japan construction 1989 1989 FEL/other
North Carolina Central USA construction ? 1991 microwave
University (NCCU) [49]
generator
Stanford University [50] USA design 1990 ? FEL
Laser-driven rf guns
Los Alamos [12] USA tested/stopped 1984? 1985 rf gun
Los Alamos (HIBAF) [24] USA operating 9/88 5/89 FEL
Stanford -> Duke [6] USA tested 1987? 9/88 FEL
CEA [17,19] France under tests 3/86 1989 FEL
Brookhaven [20,21] USA under tests 1987? 1989 new accelerator
methods/FEL
CERN [511 Switzerland rf tests 1988? 1990 collider
Argonne [52] USA construction 1988 ? wakefield accelerator
Wuppertal [53] Germany construction 1987? ? bright injector
Twente [36] Netherlands construction ? ? FEL
Duke [9] USA construction 9/89 6/91 storage ring injector
Boeing [54] USA construction ? ? FEL
Rockwell [55] USA construction ? ? FEL
Los Alamos (AFEL) [151 USA design 1988? ? FEL
INFN Milan [37] Italy design 1988? ? FEL
LAL Orsay [56] France design 9/89 1991 rf gun
INFN Frascati [57] Italy design ? 9 FEL
MIT [58] USA design 1990 ? collider
KEK [591 Japan design ? ? collider
UCLA [601 USA ? ? ° FEL
Vanderbilt University [9] USA ? ? ? FEL
LLNL/SLAC/LBL [611 USA stopped 1987 - high gradient
accelerator
Field emission rf guns
IHEP Beijing [62] China preliminary design 1988? ? FEL
LAL Orsay [63] France stopped 1988 1/89 rf gun
V ACCELERATOR TECHNOLOGY

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V. ACCELERATOR TECHNOLOGY

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C. Tracter / Rf guns- bright injectorsfor FEL
T
QUADRUPOLE
VERTICAL STEERING
-i `- HORIZONTAL STEERING
Fig. 3 . Stanford/Duk e rf gun layout (from ref. [44]) .
Cs 3Sb photocathode illuminated by a frequency doubled
Nd : YAG laser [4] .
The cavity shape was designed to minimize the nonlinear
components of the radial electric field, assuming
a do approximation . This gun produced a 1 MeV beam
of 70 ps pulses with a peak current of 200 A and a
normalized enuttance of 40m mmmrad [I1].
A second cavity independently powered and phased
was then added to the original one . Meanwhile, the
adjunction of a pulse compressor to the laser allowed to
generate 16 ps optical pulses . This new gun tested in
1987 and 1988, provided a 2.7 MeV beam . The maximum
extracted charge was 13 .2 nC corresponding to a
peak current of 600 A. No enuttance measurements
were possible due to the drift space between the second
cavity and the "pepper-pot" device that led to excessive
space-charge emittance growth [12] .
During this period, the photocathode lifetime was
significantly improved by replacing Cs 3Sb with CsK 2 Sb .
This lifetime remained very dependent though on the
operating conditions . Table 2 summarizes the performances
of the two experiments named for convenience
phase I and phase 11 .
Based on the gained experience, a new photoinlector
was built to serve as an electron source for the HIBAF
infrared FEL [13] . This new gun which replaced the
previous conventional injector provides a 350 A, 16 ps
and 351T mm mrad beam. It consists of six cells where
the first cell contains the CsK 2Sb photocathode (fig . 4) .
This photoinlector has been designed for maximum
flexibility in order to do parametrization studies for
LOS ALAMOS NATIONAL LABORATORY
ACCELERATOR TECHNOLOGY DIVISION
GROI.P AT-1
1300 Mi; PHOTOINJECTOR LINAC
Fig . 4 . Los Alamos HIBAF rf gun (from ref . [14]) .

C. Travier / Rf guns : bright injectors for FEL
291
Fig. 5 . CEA photoinjector (from ref . [19]).
simulation codes verification . Nominal performances
are given in table 2 [14].
Because of the possible industrial use of FEL, it is
interesting to develop a very compact device. This work
has been undertaken by LANL [15] . A compact FEL
named AFEL is now under design . The photoinjector
will consist of ten cells powered by one rf source. All
flexibility will be removed and the parameters will be
chosen for an optimum design according to simulations .
This injector should be able to produce a beam brightness
of the order of 10 12 A/(m2 rad 2 ) (table 2) .
Carlsten proposed a very powerful way to reduce the
emittance of a photoinjector [161 . After identifying the
four different emittance growth mechanisms : (a) linear
space charge, (b) nonlinear space charge, (c) nonlinear
time-independent rf effect, (d) linear time-dependent rf
effect, he shows that for a pulse produced by a laser, it
is possible to remove most of the correlated emittance
(variations in the transverse phase space correlated with
longitudinal position) by a proper choice of position
and strength of magnetic lenses . Such a theory is not
applicable for conventional injectors since bunching
~F TUNER
Fig . 6 . BNL rf gun cavity (from ref. [21]).
V ACCELERATOR TECHNOLOGY

292
C Travier / Rf guns: bright injectors for FEL
introduces mixing between longitudinal and transverse
phase-space planes . AFEL design is based on this theory
.
3 3. CEA Bruyères-le-Châtel
The Commissariat à l'Energie Atomique (CEA) in
Bruy6res-le-Châtel (France) has built a laser-driven rf
gun as an injector for a high-power infrared FEL [17] .
In order to improve the beam characteristics, a frequency
of 433 MHz was chosen for the linac and a subharmonic
frequency of 144 MHz for the gun . At first,
one cavity is used providing a 1 MeV beam (fig . 5) .
Later, a second cavity will be added . Cs 3Sb and CsK,Sb
photocathodes are used . Other types are also tested . The
goal is to produce a 50-100 ps, 10-20 nC beam pulse .
Design parameters [18] are summarized in table 2 . This
gun is now under testing . An accelerating field of 25
MV/m on the cathode has been obtained . 20 nC, 100
ps pulses have been accelerated to 1 MeV . An emittance
of 901T mmmrad was measured for a 10 nC pulse [191 .
3 .4 . Brookhaven National Laboratory
.
In order to study several ideas related to future
particle acceleration methods, Brookhaven National
Laboratory (BNL) is building an accelerator test facility
(ATF) [20] which will also be used for FEL physics . The
injector for the accelerator is a laser-driven rf gun
operating at 2856 MHz. It consists of a one and a half
cell cavity incorporating a photocathode in the end wall
of the first cell . The cavity shape was designed so that rf
fields cause minimal nonlinear distortion of the phase
spaces (fig . 6) . This was obtained by using a Fourier
decomposition of the rf fields and by setting the radial
field to be linear [21] .
The gun is designed to operate with a 100 MV/m
electric field at cathode. The laser will provide a 4 to 6
ps pulse . 1 nC is expected from the yttrium photocathode
The main design parameters are shown in table
2 . The gun has already been tested with a 20 ns pulse
from an excimer laser . A 90 MV/m accelerating field
was obtained at the cathode . A peak current of 0.6 A
was accelerated to 3.6 MeV [20] . The picosecond
Nd : YAG laser is now available and experiments will
proceed soon .
Meanwhile, extensive studies of metallic photocathodes
are done [22] .
10 4 "
~ M LASER-DRIVEN GUNS EXPERIMEN-AL RESU_TS
-ASER-DRIVEN GUNS DESIGN
PARAhETEPS
HERMIONIC GJNS EX~EPir'ENTAL PARAMEIERS
the brightness as a function of the peak current and the
bunch length respectively, for the different rf gun projects.
These figures are only indicative and should be considered
with some care . Fig . 7 also shows the brightness
corresponding to different wavelength FEL . From these
figures a few general comments can be made. Both
thermionic and laser driven rf guns have demonstrated
their capability to produce very bright beams (roughly
one order of magnitude higher than conventional injectors)
. Thermionic rf guns provide smaller peak currents
than photoinlectors . The best laser-driven rf gun should
be sufficient to reach wavelengths m the XUV domain
provided that suitable wigglers can be built for such
short wavelengths . X-ray domain is out of reach for the
current state of the art .
E
Q
cn
cn
w
z
10' 4
10 12
10 100 1000 10000
CURRENT (A)
Fig. 7 Rf gun brightness vs peak current .
LASER-DRIVEN GUNS EXPERIMENTAL RESU-IS
O LASER-D-2IVEN GUNS DESIGN PARAMETERS
'HER - 1ONIC GUNS EXPERIMENTAL PARA - E - ERS
4 . State of the art
A figure of merit of the beam quality can be the
peak brightness defined here as two times the peak
current divided by the square of the normalized emittance
. It is expressed in A/(m 2 rad2 ) . Figs . 7 and 8 show
ro
10
1 10 100 1000
BUNCH LENGTH (PS)
Fig . 8 . Rf gun brightness vs bunch length

C Tracter / Rf guns bright injectors for FEL
293
What are the present limitations in rf gun technology?
As mentioned earlier, thermionic rf guns suffer from
the backbombardment problem. The extra heat due to
the backbombarded electrons increases the peak current
during the rf macropulse which is not good for FEL
operation. It also limits the duty cycle . Adding a transverse
magnetic field improves the situation since the
most energetic electrons come back on the cavity wall
beside the cathode . Using this technique, Stanford could
obtain a 10 ps macropulse repeated at 15 Hz without
current increase during the macropulse [10) . If more
current and higher duty cycles are needed, the backbombardment
problem will still be a limitation .
Conventional thertmonic cathodes have current densities
of a few tens of A/cm2 . Recently, however, more
than 100 A/cm 2 has been reported [23] . These values
are nevertheless much smaller than that reported for
photocathodes.
Since in a thermionic rf gun, every rf bucket is filled,
the beam loading is very important . Reflected electrons
and secondary electrons also participate to beam loading
. At equivalent peak current, thermionic rf guns
require more rf power than laser-driven rf guns . This
fact could be a limitation for high currents .
Photoinjectors rely on photocathodes and laser technology .
The optimum photocathode (good quantum efficiency,
long lifetime, good ability to withstand high
fields) has not yet been found . The most successful
photocathodes mainly studied at Los Alamos (Cs3Sb,
CsK2Sb) have a very good quantum efficiency (2 to
8%) . Their lifetime depends on the operating conditions .
For their typical operating conditions (200 times 5 nC
micropulses repeated at 1 Hz, 2 x 10-9 Torr pressure),
Los Alamos HIBAF photocathodes have a 15 h lifetime
[24), the production time for one cathode being 3 h . In
order to improve this lifetime, Sheffield suggested to use
a glow discharge to clean the cavity prior to its operation
[251 .
The main alternative to these multi-alkaline photocathodes
are the thermionic cathodes (dispenser, LaB6 )
and the metallic cathodes . They both have a lower
quantum efficiency and are more robust . Only LaB6 has
been seriously tested in an rf gun [6). Dispenser cathodes
are now studied in a do gun at Orsay [261.
Metallic cathodes have been extensively studied at
BNL both in picosecond and femtosecond regimes
[27,28] in a do configuration . Quantum efficiency as
high as 7.25 x 10-4 has been measured for samarium
photocathodes . A current of 21 kA/cm2 has been extracted
from a 7 mm2 yttrium photocathode . This result
corresponds to the "highest current density reported to
date for a macroscopic photoemitter" . These photocathodes
have now to be tested in an rf gun configuration
.
Several other photocathode studies are currently un-
.
.
der way in many different laboratories . A good overview
can be found in ref. [29) .
An important issue for any type of photocathode is
the dark current (current due to field emission) . This
current depends on the photocathode material and the
field level in the cavity At Los Alamos, for the operating
conditions mentioned above, a current of 100 fC per
rf cycle was measured [301 . In case of cw operation, this
dark current could carry a significant power .
The laser used to illuminate the photocathode should
provide very short pulses, at the wavelength corresponding
to the photocathode spectral response . Given the
photocathode quantum yield, the laser macropulse
should carry enough energy to extract the desired charge .
The laser should be synchronized to the rf, i .e . the laser
micropulse should correspond to a determined phase of
the rf signal . This phase should be repeated from pulse
to pulse with a timing jitter much less than the pulse
length The laser micropulse should be as uniform as
possible in both transverse and longitudinal directions .
Uniform trains of micropulses are required at a repetition
rate that varies from a few tens of MHz for FEL
to a few GHz for linear colliders' driving beams .
Over the last few years, lasers suitable for rf gun
operation have been developed . A lot more work is
required to improve some of the performances and
bring them to a high level of reliability . The most
critical issues are the uniform pulse shape, the phase
and amplitude stability and the very high micropulse
repetition rate especially when very short energetic
pulses are required. The typical present performances
are :
micropulse length 10-20 ps,
micropulse energy 10-100 pJ,
repetition rate 20-100 MHz,
amplitude jitter 1%,
timing jitter 1-2 ps .
Table 3
Comparison of rf gun vs conventional injectors
Conventional
injector
Rf gun
thermionic laser-driven
High accelerating field - + +
Current density
Short pulse
-
-
-
-
+
+
Pulse format flexibility - - +
Cathode lifetime + + -
Rf system simplicity - + +
Compactness - - +
Timmgjitter + +
Energy fitter + +
Development state + -
Cost
V. ACCELERATOR TECHNOLOGY

294
C Traoier / Rf guns bright injectors for FEL
5 . Rf guns vs conventional injectors
Conventional injectors including a do gun and one or
several bunchers have already produced very bright
beams (e .g., SLC [31], Boeing [32]) . Therefore several
new FEL designs make use of a conventional injector
(e .g ., CLIO [33], FELIX [34], CDRL Berkeley [35]) .
Meanwhile several other recent designs rely on rf guns
(e.g ., Duke [10], Los Alamos [15], Twente University
[361, Milan [37], etc .) . Both technologies have advantages
and drawbacks that are summarized in table 3 .
The two technologies are in a very different state of
development . The statements presented in table 3 are
thus subject to evolution in the next few years .
Infrared FEL users' facilities to be built now are
very likely to use a conventional injector since its performances
are sufficient and its reliability much greater .
For operation in the visible regime, rf guns are advisable
owing to the fact that the considerable efforts made
to improve their performances (stability, reliability)
should succeed soon . For even shorter wavelengths, they
are probably the only choice to date . Pbotoinjectors are
also very interesting for compact infrared FELs that
might be developed for commercial use . Compactness
allows to reduce the amount of concrete necessary for
shielding thus making the FEL cheaper.
6 . Design philosophy
From the Kim equations [38], it can be shown that
the minimum emittance expressed in iT mm mrad for an
rf gun beam is equal to
e
mm
- 1 .57 X 10-_
~faxa6~2
where I is the peak current to A, f the rf frequency in
MHz, a, the rms transverse beam size in mm, a b the
rms bunch length in ps . This equation is valid for short
pulses and Gaussian distributions, This minimum emittance
is obtained for an optimum accelerating field
expressed in MV/m :
0
_J
w
w
0
I = 100 A - sigma . . = 3 mm
10
100 1000
FREQUENCY (MHz)
Fig . 9 . Optimum electric field
emittances are shown in fig. 10 . When the optimum
field is higher than the practical limit, the emittance is
calculated for this practical field limit .
Figs . 9 and 10 provide a quick estimate of rf gun
performances . For higher current, the results are easily
scaled since both E.Pt and f r , are proportional to the
square root of the current. For the rms bunch length of
interest (less than 10 ps), fig . 10 shows that the best
frequency to choose is around 1 GHz which corresponds
to L-band.
If very short pulses can be produced from the laser,
higher frequencies are possible . If only long pulses are
possible or if very high charges are needed, lower frequencies
are to be chosen . In this case the rf gun has to
be followed by a magnetic compressor . This scheme is
probably the best way to obtain currents larger than 500
A.
In order to minimize the emittance growth occurring
I = 100 A - sigma x = 3 mm
Eopt - 7.2 x 10 5 f7Y a b Sax_+ l .506
Fig . 9 shows the optimum field as a function of the
frequency for a current of 100 A, a bunch size ax of 3
mm and different pulse lengths . Since the above equations
are only valid for short pulses, fractions of the rf
period are shown to indicate what are the reasonable
bunch lengths to consider . The practical limit of two
times the Kilpatrick value is also represented showing
that it is not always possible to reach the optimum field.
From this picture it appears that low frequencies require
a very high field. The corresponding minimum
E
E
C
w
U
Z
Q
E-
ui
100
10
100 1000
FREQUENCY (MHz)
Fig . 10 . Minimum enuttance .

C. Traurer / Rf guns . bright injectorsfor FEL
295
during drifts at low energy, it is better to use several
cells thus producing a beam with an energy larger than
10 MeV at the end of the gun .
The above equations did not include the use of a
magnetic lens to reduce the emittance as suggested by
Carlsten . However, Kim [39] showed that this method
can be applied only if
In yq < 1, arid 30 .7Ezr
2
where yq is the electron energy (m units of MC Z ) at the
location of the lens, I is the peak current in A, E the
gradient in MV/m and r the beam radius in mm .
For
typical values of current, beam radius and electric field,
the above conditions show that emittance reduction due
to the magnetic lens is only possible for low frequencies .
Several other methods to reduce the emittance are
being investigated such as laser pulse shaping [40]
and
time-dependent rf focusing [41,42] . Computer simulations
show significant reduction of the beam emittance
but the practicality of such methods is to be demonstrated
. With these schemes, the residual emittance is
mainly due to the thermal emittance of the cathode .
7 . Conclusion
Between 1983 and 1988, it was proven that thermionic
and laser-driven rf guns could produce bright
electron beams. Starting in 1989, rf guns began to be
widely studied and used in different laboratories .
Efforts are made to push the technology to the limit
while trying to improve its reliability . The first routine
operation of an rf gun is starting at SSRL [43] .
Starting in the fall of 1990, several laboratories will
be ready to experiment their rf guns (CEA, BNL, CERN,
DUKE, etc.) . It will then be possible to test the different
types of cathodes in an accelerator environment .
Long hours of operation experience will
give birth to
new ideas to bring the rf gun technology to a mature
state .
Acknowledgements
I would like to thank very much J. Gao, H. Liu and
B . Carlsten for helpful discussions . S.V . Benson, S.C .
Chen, A. Enomo, G.J . Ernst, W. Gai, S. Joly, K.J . Kim,
A. Michalke, R.L . Sheffield, Y. Tur and S . Zhong are
acknowledged for providing me with a parameter list
for their projects.
Finally, I am indebted to J. Le Duff who gave me
the opportunity to work on this subject .
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