This study was motivated by some experiments in which it was found that during the voltage rise, instead of expected excitation of a high-frequency parasitic mode, the excitation of a lower-frequency parasitic mode takes place in a certain range of voltages. For explaining this fact, the dependence of start currents of possible competing modes on the beam voltage was carried out in the cold-cavity approximation and by using the self-consistent approach. It was found that in the case of cavities, which consist of the combination of a section of constant radius waveguide and a slightly uptapered waveguide, these two approaches yield completely different results. Thus, experimentally observed excitation of the low-frequency parasitic mode can be explained by the self-consistent modification of the axial profile of the excited field, which has strong influence on the diffractive quality factor of competing modes. This modification is especially pronounced in the case of excitation of modes with many axial variations which can be excited in the region of beam interaction with the backward-wave component of such modes.

1.
G. G.
Denisov
,
V. E.
Zapevalov
,
A. G.
Litvak
, and
V. E.
Myasnikov
, “
Megawatt gyrotrons for ECR heating and current-drive systems in controlled-fusion facilities
,”
Radiophys. Quantum Electron.
46
,
757
768
(
2003
).
2.
H.
Jory
,
M.
Blank
,
P.
Borchard
,
P.
Cahalan
,
S.
Cauffman
,
T. S.
Chu
, and
K.
Felch
, “
CPI Gyrotrons for fusion ECH heating
,”
AIP Conf. Proc.
807
,
180
((
2006
).
3.
K.
Sakamoto
, “
Progress of high-power-gyrotron development for fusion research
,”
Fusion Sci. Technol.
52
,
145
153
(
2007
).
4.
G. S.
Nusinovich
,
O. V.
Sinitsyn
,
L.
Velikovich
,
M.
Yeddulla
,
T. M.
Antonsen
, Jr.
,
A. N.
Vlasov
,
S. R.
Cauffman
, and
K.
Felch
, “
Starup scenarios in high-power gyrotrons
,”
IEEE Trans. Plasma Sci.
32
,
841
852
(
2004
).
5.
S. N.
Vlasov
,
G. M.
Zhislin
,
I. M.
Orlova
,
M. I.
Petelin
, and
G. G.
Rogacheva
, “
Irregular waveguides as open resonators
,”
Radiophys. Quantum Electron.
12
,
972
978
(
1969
).
6.
D. S.
Tax
,
O. V.
Sinitsyn
,
W. C.
Guss
,
G. S.
Nusinovich
,
M. A.
Shapiro
, and
R. J.
Temkin
, “
Experimental study of the start-up scenario of a 1.5-MW, 110-GHz gyrotron
,”
IEEE Trans. Plasma Sci.
41
,
862
871
(
2013
).
7.
K.
Felch
, personal communication (
2010
).
8.
M.
Botton
,
T. M.
Antonsen
, Jr.
,
B.
Levush
,
K. T.
Nguyen
, and
A. N.
Vlasov
, “
MAGY: A time-dependent code for simulation of slow and fast microwave sources
,”
IEEE Trans. Plasma Sci.
26
,
882
892
(
1998
).
9.
Yu. V.
Bykov
and
A. L.
Goldenberg
, “
Influence of resonator profile on the maximum power of a cyclotron resonance maser
,”
Radiophys. Quantum Electron.
18
,
791
792
(
1975
).
10.
I. G.
Zarnitsyna
and
G. S.
Nusinovich
, “
Stability of self-excited single-mode oscillations in a gyromonotron
,”
Radiophys. Quantum Electron.
17
,
1418
1424
(
1974
).
11.
V. L.
Bratman
,
M. A.
Moiseev
,
M. I.
Petelin
, and
R. E.
Erm
, “
Theory of gyrotrons with a non-fixed structure of the high-frequency field
,”
Radiophys. Quantum Electron.
16
,
474
480
(
1973
).
12.
G. S.
Nusinovich
,
M.
Yedulla
,
T. M.
Antonsen
, Jr.
, and
A. N.
Vlasov
, “
Start-up scenario in gyrotrons with a nonstationary microwave-field structure
,”
Phys. Rev. Lett.
96
,
125101
(
2006
).
13.
E. M.
Choi
,
A. J.
Serfon
,
I.
Mastovsky
,
M. A.
Shapiro
,
J. R.
Sirigiri
, and
R. J.
Temkin
, “
Experimental results for a 1.5 MW, 110 GHz gyrotron oscillator with reduced mode competition
,”
Phys. Plasmas
13
,
023103
(
2006
).
14.
O. V.
Sinitsyn
,
G. S.
Nusinovich
, and
T. M.
Antonsen
, Jr.
, “
UMd program update
,” in
Annual ECH Technology Meeting, General Atomics, San Diego, CA
(
2011
).
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