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Abstract —Improving frequency stability and enhance of the
frequency tunability by using self-injection locking by delayed
reflection is studied. Theoretical analysis, numerical calculations
and experimental observations of the frequency stabilization
effect for second-harmonic 0.394 THz gyrotron is considered.
I. INTRODUCTION
EDIUM-POWER (10-100 W) THz gyrotrons operating
in continuous-wave (CW) mode are extremely important
for many applications such as DNP-NMR spectroscopy,
plasma diagnostics, non-destructive inspection, stand-off
detection of radioactive materials, medical applications, etc.
[1]. For these applications, high frequency stability and
tunability within 1-2 GHz frequency range is typically
required. The gyrotron operation frequency may fluctuate
owing to voltage power-supply ripple, residual gas ionization,
thermal expansion of the cavity, and other reasons. Apart from
different existing techniques for frequency stabilization [2],
self-injection locking of a gyrotron by a small part of its output
signal reflected back into the cavity has been suggested [3].
Basic theory of the frequency stabilization effect has been
developed and confirmed by numerical simulations, but it has
not been yet demonstrated experimentally.
Note that the influence of the signal reflected from a remote
load may cause change of the oscillator frequency and power
(autodyne effect). Thus, controlled variation of the position of
the reflecting element can lead to frequency tuning or
frequency/power modulation [4]. This method has an
advantage over the variation of beam voltage or magnetic field
since it has less impact on the gyrotron output power and
efficiency that decreases undesired power modulation.
The delayed reflections may strongly affect mode
competition phenomena in gyrotron [5]. Sub-THz and THz
gyrotrons usually operate at the second cyclotron harmonic.
For second-harmonic (SH) gyrotrons, competition with
fundamental harmonic (FH) modes is most dangerous.
Different dynamic interaction regimes of FH and SH modes
have been observed [6]. The situation becomes even more
complicated in case of competition between modes with soft
and hard self-excitation. Using the delayed reflection seems a
promising way to control competition between FH and SH
modes.
In this paper, the capability of delayed reflections for
improving performance of the THz gyrotron is studied
numerically and experimentally. FU CW IIB 0.394 THz SH
gyrotron with axial radiation output developed at FIR Fukui
University [7] for DNP-NMR is considered. The study
addresses improving frequency stability by self-injection
locking and combining with the existing PID feedback
techniques by beam parameters for improving the stabilization
quality [2], as well as frequency tunability and modulation by
controllable shift of the reflecting element.
II. NUMERICAL SIMULATIONS
For the numerical simulation, the well-known self-consistent
formalism of multimode gyrotron oscillator theory [8],[9] is
used. FU CW IIB gyrotron is designed to operate at SH of
counter-rotating TE-2,6 mode at 394 GHz. At dc beam voltage of
19kV, the range of magnetic field is about 7.25-7.3 T. The main
danger for this mode is competition with FH TE2,3.
(a)
(b)
Fig. 1. Power (a) and frequency (b) vs. magnetic field for TE-2,6 mode with and
without the reflection. The arrows show direction of the magnetic field
variation.
In Fig. 1, output power and oscillation frequency for the SH
TE-2,6 mode are presented for different reflection magnitudes R
and different distances to the reflector L. One can see that, in
agreement with theoretical predictions [3], increase of R and L
provides expanding of the oscillation zones as well as
Eduard M. Khutoryan1,Toshitaka Idehara1
Maria M. Melnikova2, Andrey G. Rozhnev2, and Nikita M. Ryskin2
1Research Center for Development of Far-Infrared Region, University of Fukui, Fukui, 910-8507 Japan
2Saratov State University, 83 Astrakhanskaya st., Saratov, 410012 Russia
Influence of Reflections on Frequency Stability, Tunability and Mode
Competition in the Second-Harmonic THz Gyrotron
M
frequency stabilization. Variation of the magnetic field
provides stepwise frequency tunability caused by excitation of
different axial modes. The transitions between the axial modes
occur with hysteresis.
III. EXPERIMENTAL RESULTS
In the experiment, influence of the reflection on competition
between FH and SH modes was studied. An example of
reflections influence on the competition between SH and FH is
presented in Fig. 2(a) where the waveforms of the FH and SH
signals measured by two detectors are shown. In the
experiment, modulation of reflection was provided by a
rotating chopper with two wings. The chopper was installed
just after the waveguide output. We observe a great difference
in gyrotron operation at different moments of chopper wing
position with respect to the waveguide output. When the
waveguide output is not cut off by chopper wings ( 0R
) SH
mode dominates. At the moments when a chopper wing cuts off
the waveguide (the two wings have slightly different reflection
factors 1,2
R), excitation of the FH mode is observed. Thus,
modulation of the gyrotron output power takes place. Similar
effect were observed in [4] for a FH gyrotron.
(a)
(b)
Fig. 2. (a) FH and SH detected signals when modulating of reflection
coefficient. B=7.305 T, Uc=19 kV. (b) Simulation of transients for the case of
competition between FH (TE2,3) and SH (TE-2,6).
Fig. 2(b) demonstrates simulation of transients for B=7.27 T
for various reflection coefficients R. One can see that the
reflections provide excitation of the TE2,3 FH-mode which, in
turn, provides small increase of the output power (cooperative
mode interaction). The results of simulations are qualitatively
in a very good agreement with the experimental ones shown in
Fig. 2(a), especially taking into account that we do not know
exact values of the reflection coefficient for both modes.
IV. CONCLUSION AND DISCUSSION
The presented results of numerical simulation and
experimental measurements demonstrate that using partial
reflection of gyrotron output radiation is very promising for
increase of frequency tuning range, mode competition control,
etc.
Strong reflection decreases output power for the fundamental
axial mode but can increase it for high-order axial modes,
especially at lower magnetic field when electron beam mainly
interacts with forward wave that seems to be promising way to
increase frequency tunability. In addition, numerical simulation
shows that the reflector formed by a stop-band filter (i.e., acting
as a reflector only at second harmonic) may increase Q-factor
for the SH mode and provide its wider single-mode operation
range.
V. ACKNOWLEDGEMENTS
This work is partly supported by the Russian Foundation for
Basic Research grant # 15-02-01798a.
REFERENCES
[1] T. Idehara, T. Saito, I. Ogawa, S. Mitsudo, Y. Tatematsu, and S. Sab-
chevski, “The potential of the gyrotrons for development of the sub-terahertz
and the terahertz frequency range — A review of novel and prospective
applications,” Thin Solid Films, vol. 517, 1503–1506, 2008.
[2] E.M. Khutoryan, T. Idehara, A.N. Kuleshov, Y. Tatematsu, Y. Yama-
guchi, Y. Matsuki, and T. Fujiwara, “Stabilization of gyrotron frequency by
PID feedback control on the acceleration voltage,” J. Infrared Millim.
Terahertz Waves, vol. 36, 1157–1163, 2015.
[3] M.M. Melnikova, A.G. Rozhnev, N.M. Ryskin, A.V. Tyshkun,
M.Y. Glyavin, and Y.V. Novozhilova, “Frequency stabilization of a 0.67-THz
gyrotron by self-injection locking,” IEEE Trans. Electron Dev., vol. 63,
1288-1293, 2016.
[4] N. Kharchev, A. Cappa, D. Malakhov, J. Martinez, E. Konchekov,
A. Tolkachev, V. Borzosekov, K. Sarksyan, and M. Petelin, “Influence of
controlled reflected power on gyrotron performance,” J. Infrared Millim.
Terahertz Waves, vol. 36, 1145–1156, 2015.
[5] M.M. Chumakova, S.A. Usacheva, M.Y. Glyavin, Y.V. Novozhilova, and
N.M. Ryskin, “Mode competition in a two-mode gyrotron with delayed
reflections,” IEEE Trans. Plasma Sci. vol. 42, 2030-2036, 2014.
[6] T. Saito, Y. Tatematsu, Y. Yamaguchi, S. Ikeuchi, S. Ogasawara, N. Ya-
mada, R. Ikeda, I. Ogawa, and T. Idehara, “Observation of dynamic interaction
between fundamental and second-harmonic modes in a high-power
sub-terahertz gyrotron operating in regimes of soft and hard self-excitation,”
Phys. Rev. Lett. vol. 109, 155001, 2012.
[7] T. Idehara, Y. Tatematsu, Y. Yamaguchi, E. M. Khutoryan, A.N. Ku-
leshov, K. Ueda, Y. Matsuki, and T. Fujiwara, “The development of 460 GHz
gyrotrons for 700 MHz DNP-NMR spectroscopy,” J. Infrared Millim.
Terahertz Waves, vol. 36, 613–627, 2015.
[8] M. I. Airilla, O. Dumdrajs, P. Kall, and B. Piosczyk, “Influence of
reflections on the operation of the 2 MW, CW 170 GHz coaxial cavity gyrotron
for ITER,” Nucl. Fusion, vol. 43, pp. 1454-1457, 2003.
[9] N.A. Zavolsky, G.S. Nusinovich, and A.B. Pavelyev, In: “Gyrotrons”
(Gorky: Academy of Sciences of USSR) p 84, 1989.
