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MULTI-MODE GENERATOR FOR THE COLD TEST OFBROADBAND QUASI-OPTICAL GYROTRON MODE CONVERTERS
D. Wagner1, M. Thumm2, G. Gantenbein2, J. Flamm2, J. Neilson3, R. Vernon4
1MPI für Plasmaphysik, Association EURATOM-IPP , D-85748 Garching, Germany2Forschungszentrum Karlsruhe, Association EURATOM-FZK, IHM, D-76021 Karlsruhe, Germany
3Calabazas Creek Research, 690 Port Drive, San Mateo, CA 94404-1010, USA4University of Wisconsin, Madison, Wisconsin, 53706-1691, USA
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
Outline: - Introduction - Broadband beam excitation
- Cavity design- Measurements
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
INTRODUCTION (1)
• Testing of quasi-optical mode converters requires the excitation of high-order volume modes at low power levels.
• Mode converters for advanced multi-frequency gyrotrons need to be efficient for different modes over a wide frequency range
• Mode generator required for cold tests to excite several modes with high mode purity in this frequency range
• In our case main modes of interest are:
TE22,6 @ 110.0 GHz
TE24,7 @ 124.7 GHz
INTRODUCTION (2)
Principle:(Alexandrov et al., Int. J. Infrared and Millimeter Waves, 13 (1992), pp.1369)
caustic
waveguide
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
INTRODUCTION (3)
-1.0
-0.5
0
0.5
1.0
-1.0 -0.5 0 0.5 1.0
rc
B
k0
kz
kc
– Field distribution of high-order modes can be decomposed into a spectrum of plane waves– All rays ( ) are tangential to the caustic with radius:
– The reflection angles of the rays at the waveguide wall are given by:
*HE
Smn
c x
Rmr
R
z
mnx
m)cos(
fR
cx
k
k mncB
2)sin( 0
0
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
BROADBAND BEAM EXCITATION (1)
• Gaussian beam excitation using a smooth Gauss horn (output mode mixture app. 86% TE11 + 1% TE11(180°) + 12.6% TM11(180°) + 0.4% TM12 (180°) )
• Linear horn (2 phasing sections)
-2
-1
0
1
2
0 5 10 15
length /
radi
us /
• calculated bandwidth 7%
• center frequency: 122.5 GHz
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
BROADBAND BEAM EXCITATION (2)
• Ex.: horn pattern at center frequency (122.5 GHz, at z = 59 mm)
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
calculation measurement
BROADBAND BEAM EXCITATION (3)
• Lens horn contains 2 cylindrical teflon lenses
• Designed to generate astigmatic beam at f = 122.5 GHz with w01 = 10 mm, w02 = 33 mm at d2 = 353 mm
• Measured lens horn beam at the position of the quasi-parabolic mirror:
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
CAVITY DESIGN (1)
• Cavity profile optimized for - high output mode purity - high Q
mode f (GHz) QD (%) TE22,6 110.00 1658 99.91 TE24,7 124.25 2095 99.77
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
-20
-10
0
10
20
0 25 50 75 100length (mm)
rad
ius
(mm
)
coaxial mode generator cavity
19
20
21
0 25 50 75 100length (mm)
rad
ius
(mm
)
outer cavity wall
0
0.2
0.4
0.6
0.8
1.0
0 25 50 75 100
TE22,6
length (mm)
|V| (
no
rma
lize
d)
Modal Amplitude Distribution
(Ri/Ro=0.44)
CAVITY DESIGN (2)
• Coupling holes over whole circumference reduces counter rotation minimizes re-radiation
• Calculated frequency dependence of the coupling factor:
-30
-25
-20
-15
110 120 130 140
frequency (GHz)
dB
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
CAVITY DESIGN (3)
• Optimum mode separation by appropriate choice of inner conductor radii
• Calculated eigenvalue spectra (Ro=19.65mm)
TE22,6 resonance TE24,7 resonance
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
40
42
44
46
48
50
0.2 0.3 0.4 0.5 0.6 0.7
29,4
17,8
37,2
5,13
43,1
7,12 14,9
36,228,4
9,11 19,7
32,3 25,5
21,7
21,6
22,5
22,7
22,6
Ri/Ra
eig
en
valu
e
Ra = 19.651 mm
o
47
49
51
53
55
57
0.2 0.3 0.4 0.5 0.6 0.7
19,9
38,3
7,14
34,4 49,1
9,13 16,10
27,6
37,321,8
11,12
30,642,2
24,7
Ri/Ra
eig
en
valu
e
Coaxial Waveguide Eigenvalue
o
CAVITY DESIGN (4)
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
45.0
45.2
45.4
45.6
45.8
46.0
0.40 0.42 0.44 0.46 0.48 0.50
TE22,6
Ri/Ra
eig
en
valu
eR
a = 19.651 mm
o
MEASUREMENTS (1)
• Measured frequency response (Ri/Ro=0.44)
• Calculated and measured mode pattern (vertical polarization) @ 110.0 GHz)
TE22,6
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
-80
-70
-60
-50
-40
-30
-20
109.0 109.5 110.0 110.5 111.0 111.5 112.0f / GHz
dB
mode f (GHz) QD (%) TE22,6 110.00 1658 99.91
51.0
51.2
51.4
51.6
51.8
52.0
0.40 0.42 0.44 0.46 0.48 0.50
TE24,7
Ri/Ra
eig
en
valu
eCoaxial Waveguide Eigenvalue
CAVITY DESIGN (5)
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
o
-80
-70
-60
-50
-40
-30
-20
124.0 124.5 125.0 125.5f / GHz
dB
MEASUREMENTS (2)
• Measured frequency response (Ri/Ro=0.44)
• Calculated and measured mode pattern (vertical polarization) @ 124.2 GHz
TE24,7
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
mode f (GHz) QD (%) TE24,7 124.25 2095 99.77
-80
-70
-60
-50
-40
-30
-20
124.0 124.5 125.0 125.5f / GHz
dB
MEASUREMENTS (3)
• Measured frequency response (Ri/Ro=0.435)
• Calculated and measured mode pattern (vertical polarization) @ 124.33 GHz
TE24,7
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
mode f (GHz) QD (%) TE24,7 124.38 2093 99.77
51.0
51.2
51.4
51.6
51.8
52.0
0.40 0.42 0.44 0.46 0.48 0.50
TE24,7
resonance TE24,7
Ri/Ra
eig
en
valu
eR
a=19.651 mm
CAVITY DESIGN (6)
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
o
-80
-70
-60
-50
-40
-30
-20
124.5 124.6 124.7 124.8f / GHz
dB
MEASUREMENTS (4)
• Measured frequency response (Ri/Ro=0.42)
• Calculated and measured mode pattern (vertical polarization) @ 124.65 GHz
TE24,7
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
mode f (GHz) QD (%) TE24,7 124.67 2079 99.73
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
MODE GENERATOR SETUP
lens horn
quasi-parabolicmirror
cavity
MEASUREMENTS (5)
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
CONCLUSIONS
US-EU-JPN RF Heating Technology Workshop, September 16-18, 2009, Fukuoka, Japan
• Mode generator for TE22,6 @ 110 GHz and TE24,7 @ 124.7 GHz built and tested.
• First results show clear mode patterns with low counter rotation. Frequency matches design values within 20 MHz.
• Mode generator shipped to University of Wisconsin.
