Millimeter-wavelength free-electron lasers in PPL
JINR
Microwave system of the FEL-oscillator
In the experiment a two-mirror Bragg resonator with the parameters close
to calculated ones was used. It was consisted of 22 mm diameter circular
waveguide and two Bragg reflectors in the form of waveguide sections with
the same diameter and weak corrugation of side walls. As an operating wave
the lowest H11 wave of the circular waveguide was chosen. In correspondence
with calculations, two effective reflections into backward waves were observed
in "cold" microwave measurements of the Bragg reflectors: in E11 wave in
the vicinity of the frequency 31.5 GHz and in E12 wave in the vicinity
of 37.5 GHz. The width of these reflection zones was about 2% and 3% correspondingly.
The maximum reflection was 95% for te cathode-side reflector and 85% for
the collector-side reflector for the both reflection zones. The resonator
with such reflectors and 40 cm length regular part had four high-Q-factor
longitudinal modes (with different numbers of variation along longitudinal
coordinate) in the both zones. Maximum Q-factor of these modes was about
1000.
Photograph of the Bragg mirors of different length.
Reflectivity of Bragg mirrors
| Type of backward wave |
H1,1
|
E1,1
|
E1,2
|
| Bragg reflection band, GHz |
28,2...29,9
|
30,6...32,1
|
37,9...39,0
|
|
Length of reflector, cm
|
Reflection coefficients, %
|
|
|
|
16,2
|
66
|
82
|
92
|
|
10,8
|
51
|
63
|
73
|
|
5,4
|
38
|
57
|
65
|
Note, that there were modes in this resonator which were caused by the
reflections of operating wave to the backward wave of the same type (H11)
at the frequency near 29 GHz. However, the maximum reflection coefficient
in this case did not exceed 50%. This value was rather small for the threshold
of generation of the FEL with such resonator.
The new RF-input (Denisov's splitter) for FEL-amplifier
was designed in cooperation with IAP (Nizhny Novgorod). It uses the effect
of RF-beam multiplication in nonregular rectangular waveguide. The
main features of the splitter are high efficiency of the RF-power input
(about 85%) and absence of any elements in the beam aperture.
The input scheme is also shown in the figure (PDF
format - 12.5 kB).
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Last updated 26.02.98.
Sergey N. Sedykh Contact E-mail: snsed@sunse.jinr.ru