Selective Properties of Planar Bragg Reflectors and Their Application for Multichannel Masers
A.V. Arzhannikov, aN.S. Ginzburg, P.V. Kalinin[*], A.S. Kuznetsov,
aN.Yu. Peskov, bP.V. Petrov, aA.S. Sergeev, S.L. Sinitsky, cM. Thumm
Budker Institute of Nuclear Physics, Russian Academy of Sciences (RAS), Novosibirsk, 630090, Russia;
aInstitute of Applied Physics RAS, N-Novgorod, 603600, Russia;
bRFNC-VNIITF, Snezhinsk, 456770, Russia;
cForschungszentrum Karlsruhe, IHM, Germany.
In this paper we discuss selective properties of planar reflectors composed by a pair of 1-D or 2-D Bragg gratings with various types of their surfaces corrugation, which can realize one- or two-dimensional distributed feedback in a maser. The properties of reflectors are investigated by using theory, computer simulations and “cold” measurements. Obtained results allow one to create a superpower generator by combining several identical planar FEM-oscillators in common multichannel device.
PACS codes: 41.60.Cr, 42.25.Fx, 84.40.Ik
Keywords: FEL; Spatial coherence; Distributed feedback; Bragg resonator; High-power microwaves
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  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 . 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 .
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 . 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 . According to this, the project of 4-channel FEM experiments with combined Bragg resonators and Bragg deflectors for radiation output is developed .
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.