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

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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 .