23P175-8
Frequency Selectivity of Hybrid Bragg Resonator for Free Electron Maser
大強度FEMに用いるハイブリッド・ブラッグ共鳴器の優位性について
YAMADA Naohisa, KITAMURA Taro, IWATA Kazuma, YASUDA Kiyotaka,
YOSHIDA Mitsuhiro1), SOGA Yukihiro, KAMADA Keiichi, N.S.Ginzburg2)
山田尚久, 北村太郎，岩田和馬，安田清貴，
吉田光宏1)，曽我之泰，鎌田啓一，N.S.ギンツブルグ2)
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
金沢大学大学院自然科学研究科 〒920-1192 金沢市角間町
1) High Energy Accelerator Research Organization, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan高エネルギー加速器研究機構〒305-0801 つくば市大穂町 1-12) Institute of Applied Physics, Russian Academy of Science, Nizhny Novgorod, 603600, Russia
ロシア科学アカデミー応用物理学研究所 ニージニー

ノブゴロド，603600，ロシアA normal Bragg resonator utilizing two traditional Bragg reflectors and a hybrid Bragg resonator with anadvanced and a traditional Bragg reflector were designed to develop the frequency selectivity of the freeelectron maser with frequency of 40 GHz using a helical wiggler coil and an intense relativistic electron
beam. In the cold test, the frequency resolution of the hybrid Bragg resonator was appeared to be 0.05 GHz,
while that of the normal one was 0.5 GHz.

1. Introduction
There are still few light sources in the range of
0.1-10 THz. As candidates of the THz source, laser,solid state components, an electron tube, etc. arementioned. THz sources using laser and solid statecomponents have developed in advance withaverage output power of less than mW and haveexplored every new possibility of application.
Electron tubes are expected to realize a steady state
THz source with output power of > 1 W or anintense pulsed THz source with power of >1 MW.
The intense THz source will apply for thermonuclear
fusion, particle accelerators, radars, etc. Amongelectron tubes, gyrotrons[1] have realized forthermonuclear fusion with output power of 1 MWand frequency of ~170 GHz.
One of the other candidates of electron tubes for
an intense THz source is a free electron maser(FEM) using an intense relativistic electron beam(REB). However, one of the problems for the FEM
using REB is the wide frequency spectrum becauseof its strong self electric field. Usually, a normal
Bragg resonator is utilized to develop the frequency
selectivity. Traditional Bragg reflectors work as
mirrors in the optical distributed feedback laser[2-3]. However, as the frequency increases, thetraditional Bragg reflector loses the selectivefeatures over transverse indexes. An advanced
Bragg reflector proposed by N. S. Ginzburg[4]utilizes the interaction between a propagating wave
and a quasi-cutoff mode to provide a higher

selectivity over the transverse index than a
traditional Bragg reflector. Structural difference
between the traditional and the advanced reflectors
is mainly on the periodic length of the corrugation.
The period of corrugation of an advanced Braggreflector is longer nearly twice than that of a
traditional Bragg reflector. In the hybrid Bragg
resonator [5], an advanced Bragg reflector is used at
the entrance side and at the exit side the traditional
Bragg one is utilized. The hybrid Bragg resonator is
expected to realize a selective frequency band
narrower than the traditional Bragg resonator.
Normal Bragg resonator
Traditional Bragg reflector

Traditional Bragg reflectror

Hybrid Bragg resonator
Advanced Bragg reflector

Traditional Bragg reflector

Fig. 1. Schematics of the normal Bragg resonator(upper) and the hybrid Bragg resonator(lower). A
REB is injected from left side.

L=500mmFig. 2. Transparent characteristics of normal andadvanced Bragg reflectors (cold test). And simulated
reflective indexes of Bragg reflectors are alsoindicated.

Figure 1 shows the difference between the normal
and the hybrid Bragg resonators.
2. Cold Tests of Bragg Reflectors
Cold tests of the Bragg reflectors were carriedout using network analyzer E8354C (AgilentTechnology). The microwave passed through theBragg reflector was detected. The advanced Braggreflector reflected the frequency of around 39.85
GHz. The width of the reflected frequency of theadvanced Bragg reflector is about 10 times
narrower than that of the traditional one as shown inFig.2. The simulated reflective indexes of theBragg reflectors are also shown in Fig.2 with dottedlines. The experimental results of the Braggreflectors show good agreements with the simulatedresults.
3. Cold Tests of Bragg Resonators
Cold tests for normal and hybrid Bragg
resonators were carried out with the same network
analyzer used for Bragg reflectors. Microwave wasradiated at the center of the Bragg resonator. Andthe microwave passed through the traditional Braggreflector located at the downstream side of bothresonators was detected. The distance between two
reflectors was changed from 100 to 500 mm atinterval of 100 mm.
The experimental result of the normal Bragg
resonator is shown in Fig. 3. Several peaks wereobserved within the frequency range 39.8  0.5
GHz in which the microwave was reflected by thetraditional Bragg reflector. The frequencies of eachpeak are the function of the distance between twoBragg reflectors. As the distance increased, thefrequency width of each peak decreased but thenumber of frequency peak increased.

Fig. 3. Transmitted frequency spectra of thenormal and the hybrid Bragg resonators.

The frequency spectrum passed through thehybrid Bragg resonator is also shown in Fig. 3. A
single narrow peak is observed at the same
frequency 39.85  0.05 GHz that was observed in
the cold test of the advanced Bragg reflector. As thedistance of the two reflectors increased, thefrequency width of the peak decreased a little butthe numberof the peak kept one with the same
frequency. The output power is a little lower than
the normal one. It might indicate more careful
adjustment of the distance of two reflectors.
An intense FEM using a REB radiates
microwave with wide frequency spectrum. Thoughmany peaks should be observed with the normal
Bragg resonator, with the hybrid Bragg resonator asingle peak should be detected. Therefore, we canexpect the higher frequency selectivity of the hybrid
Bragg resonator.
4. Conclusion
From the results of the cold test of both Bragg
resonators, the frequency selectivity of the hybrid
Bragg resonator is about 10 times narrower than the
traditional Bragg resonator. The hybrid Bragg
resonator can be used not only in the intense FEM
using a REB but the other steady state THz source.
References[1] A. Kasugai et. al.: Nuclear Fusion, Vol. 48, Issue 5,pp. 054009 (2008).
[2] I. Boscolo: Appl. Phys. B57, pp. 217-225, (1993)[3] T. S. Chu et. al.: Phys. Rev. Lett., 72, 2391-2394(1994)[4] N. S. Ginzburg et. al.: Phys. Rev. ST-AB, vol. 8,040705, (2005).
[5] K.Kamada et. al.: Proc. of 7th International
Workshop “Strong Microwaves: Sources andApplications” (Nizhny Novgorod, Russia July. 2008)
pp215-223.