Page 115 - High Power Laser Handbook

84 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s High-Power Fr ee-Electr on Lasers 85

limited by field emission to 6 to 10 MV/m. Other strategies are
employed in this case to help maintain brightness in continuous wave
(CW) beam production.
A proponent of high-voltage dc guns with thermionic cathodes is
the Budker Institute of Nuclear Physics in Russia. Researchers there
have successfully produced up to 22 milliamperes (mA) average cur-
13
rent with a normalized emittance of 30 mm·mrad. Producing high
average current in thermionic cathodes is straightforward. The main
difficulty in applying this technology to short-wavelength FELs is
that the emittance of such a system tends to be marginal for operating
in the shorter infrared regions due to the degrading effect of the mod-
ulating grid. It is also technically difficult to produce the very short
bunches needed for subsequent acceleration; therefore, RF buncher
cavities are required in addition to accelerating cavities. The trans-
port of the electrons through these cavities at low energies gives space
charge forces an opportunity to degrade brightness.
To eliminate the need for grids and to accelerate the beam quickly
so that space charge forces do not cause the electron beam quality to
degrade while the electrons are at low energy, a group at Boeing pro-
duced a high-average-current RF photoinjector. The copper cavity
14
operated at 433 MHz. The injector used a mode-locked green laser on
a CsKSn cathode to produce 25 percent duty factor pulses of 135 mA
average current. The normalized emittance was 12 mm·mrad, which
is suitable for short infrared (IR) lasing. The limitation of such a system
was twofold—first, the cathode degraded due to the relatively poor
vacuum environment in the RF cavity (roughly 3 hours in this case),
and second, the RF power dissipation on the walls of the copper cav-
ity was quite significant and led to difficulty in cooling the cavity, in
addition to representing a significant overall power drain. Because of
the power dissipation, average accelerating gradients are limited in
such systems to around 6 MV/m. Nonetheless this effort, which was
performed in 1986, remains a benchmark for this technology.
High-voltage direct current guns with photocathodes were used
by the Thomas Jefferson National Accelerator Facility (Jefferson Lab)
to produce a high-quality short-pulse beam of greater than 9 mA with
long life. This long life was available because the geometry of dc guns
is better for vacuum pumping. The electron beam quality was suit-
15
able for lasing into the visible region, despite limitations in the volt-
age gradient to less than 4.5 MV/m due to high-voltage breakdown.
This gradient limitation may be a factor in determining whether such
a system can be scaled to yet higher currents, but efforts are under-
way at the Jefferson Lab to scale up the performance.
A technical challenge in the design of all photoinjectors is the
need for an ultrahigh vacuum to avoid poisoning the cathode mate-
–10
–9
rial. Vacuums of 10 to 10 torr are required for most cathode mate-
rials, with water vapor being a key poisoning element. Typically
–11
partial pressures of 10 torr of water are desired to maintain high