Planar reflectors composed by various Bragg gratings give good possibility to create a high selective resonator for masers. For example, in recent experiments at the ELMI-device [1] its resonator consisted of two different reflectors. The first one is up-stream 2-D Bragg reflector, which provides synchronization of radiation across a sheet electron beam. The down-stream 1-D Bragg reflector provides a sufficient reflectivity for the FEM self-excitation.
Spectral properties of 1-D reflectors (Fig.1) with rectangular thread of gratings are investigated with different theoretical models, in computer simulations and in “cold” measurements. At frequencies 72-76 GHz the incident wave H10 scattered preferentially in backward H10 wave, and at 76-79 GHz – in backward E12 wave. In “cold” measurements only H10 wave could be measured and that is reason for the difference between experiment and theoretical results.
Fig. 1. Spectral properties of 1-D Bragg reflector (corrugation length - 6 cm, width - 19 cm, depth - 0.15 mm, period –
2 mm).
2-D reflectors composed by Bragg gratings with several types of corrugation were investigated [2]. It was established that spectral properties of chessboard corrugation are very close to the “ideal” sine corrugation (see Fig.3). Results of “cold” measurements for reflectors with these two types of corrugation are presented in two cases: reflectors are closed in transverse direction with metal waveguide walls (a) and reflectors with transverse extraction of energy (b). Side reflections in case (a) causes the additional areas in spectrum at 73.5 and 77 GHz. In case of the opened reflectors (b) only reflection at 75 GHz is present. These results coincide well with theoretical investigations [3].
To
separate the mm-wave radiation and the sheet electron beam by wave scattering
in transverse direction a special deflector composed by a pair of 1-D Bragg
gratings with corrugation at the angle 450 in relative to direction
of the incident wave was proposed [4]. Such deflector was made and tested
(Fig.4). It is seen that the bandwidth of this device 
Fig. 4. Properties
of the Bragg deflector (corrugation size – 10x10 cm, depth - 0.3 mm, period –
2.82 mm). P^
- deflected power, P0 – incident power.
(~2-2.5 GHz) is enough
for radiation output from resonator composed by 2-D Bragg reflectors with
bandwidth ~1 GHz and this deflector is used in resent experiment at the
ELMI-device. According to the computer simulations, geometry of the
corrugated area on the deflector plates can be optimised for maximum output or
for the required distribution of deflected radiation.
Theoretical investigations show, that 2-D Bragg reflectors can produce mutual exchange of transverse radiation flows between adjacent FEM-oscillators in a multichannel device and synchronize generation in the FEMs [5]. According to this, the project of 4-channel FEM experiments with combined Bragg resonators and Bragg deflectors for radiation output is developed [6].
References
1. N.V. Agarin et al.
Abstracts of 22nd Int. FEL Conf., Durham, USA, 2000, p.33.
2. N.S. Ginzburg et
al. Nucl. Instr. and Meth. A 475 (2001) 287.
3. N.S. Ginzburg et
al. Phys. Rev. E 60 (1999) 935.
4. A.V. Arzhannikov
et al. Abstracts of 5th Int. Workshop “Strong Microwaves in
Plasmas”, N.Novgorod, Russia, 2002, S30.
5. A.V. Arzhannikov
et al. Digest of Tech. Papers of Pulse Power Plasma Science, Las Vegas, Nevada,
USA, 2001, p.561.
6. A.V. Arzhannikov
et al. Abstracts of 14th Int. Conf. on High-Power Particle Beams,
Albuquerque, New Mexico, USA, 2002, p. 203.