Page 117 - High Power Laser Handbook

86 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 87

going normal. It is also impossible to impose desired solenoid com-
pensatory fields at the cathode because of the superconductor’s
shielding. Finally, the compatibility of the cathode itself with the
superconducting environment is a potential issue. Ongoing research
is aimed at answering these questions.
4.3.3 Accelerators
RF accelerators work by injecting short bunches of electrons in proper
phase with an oscillating microwave field inside a cavity. The longi-
tudinal electric field of the microwaves accelerates the electrons as
energy is extracted from the microwaves. Electrons are such light
particles that they travel at nearly the speed of light once they are
greater than 1 MeV in energy; therefore, proper phasing of the micro-
wave fields is straightforward. High-acceleration gradients are estab-
lished by the fields: 60 MV/m or more in pulsed copper accelerators
and 20 MV/m in modern CW SRF accelerators. High ohmic losses in
copper cavities lead to severe heat loads in high-duty-factor copper
accelerators, even with gradients reduced to 6 MV/m. As a conse-
–3
quence, most copper accelerators operate at duty factor of 10 ,
which is sufficient for scientific research applications but useless for
high-average-power applications. An exception is the low-frequency
180-MHz recuperator system, developed at the Budker Institute; this
system produces a continuous 30-mA 18-MeV electron beam for FEL
lasing. Upgrades to higher energy are underway.
The difficulty with copper ohmic losses led to the development of
superconducting accelerator cavities made of niobium (Fig. 4.4). The

Figure 4.4 Niobium cavities inside a cryomodule with the RF waveguide
feeds in red. The electron beam enters from the pipe in the right foreground.