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DEVELOPMENT OF 28 GHz-1 MW GYROTRON FOR GAMMA10 ECRH
Mao OTA1 , Tsuyoshi KARIYA1 , Tsuyoshi IMAI1 , Keishi SAKAMOTO2
, Ryutaro MINAMI1 , Yoichi ENDO1 , Hitomi AOKI1 ,
Hideaki IIZUMI1 , and Hideyuki KONDOU1
1Plasma Research Center (PRC), University of Tsukuba, Tsukuba,
Ibaraki, 305-8577, Japan
Email Address : [email protected] 2Japan Atomic Energy
Agency (JAEA), Naka, Ibaraki, 311-0193, Japan
A new 28 GHz - 1 MW gyrotron has been developed for ECRH system
of GAMMA 10 tandem mirror. To achieve the design target of the new
gyrotron, which is 1 MW output for 1 s, the magnetron injection gun
(MIG) and cavity were designed in detail with calculation codes. A
first tube has been fabricated, and performance test is in
progress. The maximum output power of 1.04 MW and the maximum
output efficiency of 40.6% were obtained with short pulse
operation. However, at the same time, it was confirmed that the
output efficiency had decreased with the increase of beam current.
This is explained with α decrease, which is a one of issues to
develop a higher efficiency gyrotron.
I. INTRODUCTION The Electron Cyclotron Resonance Heating (ECRH)
is very effective for the plasma heating, the current drive and the
plasma control in the magnetic confinement fusion devices. Gyrotron
is a high-frequency source for ECRH, the effect of ECRH is
dependent on performance of gyrotron. Therefore, development of
gyrotron has been carried out at several fusion researches. In
GAMMA 10 tandem mirror, ECRH has been applied to electron heating
and potential barriers formation. Three 28 GHz - 0.5 MW gyrotrons
are used as a high power source for GAMMA 10 ECRH system. The rise
of the electron temperature and the expansion of the plug potential
are confirmed by high power operation of these gyrotrons.1 For the
purpose of higher confinement potential and electron temperature,
development of the 28 GHz - 1 MW gyrotron has been started.2 In
this paper, we describe the design and experimental result of a new
28 GHz – 1 MW gyrotron. II. DESIGN OF 28 GHz – 1 MW GYROTRON A set
of design parameters of 28 GHz Gyrotron is shown in TABLE 1. The
one is 0.5 MW gyrotron used in
GAMMA10 at present, the other is a new 1 MW gyrotron. The design
target of new gyrotron is 28 GHz, 1 MW output for 1s. The picture
and cross-section structure of a new gyrotron is shown in Fig.1.
The triode gun is adopted to control the electron beam by anode
voltage. The cavity mode is TE8,3 mode, to reduce current density
at the cathode. The TE8,3 mode RF wave is converted to a
Gaussian-like beam by a built-in Quasi-Optical mode converter. The
RF beam is transmitted with four-mirrors system, and taken out
through the single sapphire window. In this gyrotron, the collector
doesn’t adopt electric potential depression (W/O CPD). Figure.2
shows the anode voltage Vak dependence of the pitch factor α (=
perpendicular velocity to the magnetic field / parallel velocity)
and the α spread calculated by MIG simulation code. In this
calculation result, electron beam parameter of α=1.1~1.3 with α
spread ≦ 8% was obtained at Vak=38~40kV. Beam current Ic
dependences of the output power Po and output efficiency η
calculated by cavity simulation code are shown in Fig.3. At beam
current Ic=40A, target value of
TABLE 1. Design parameters of 28 GHz gyrotrons
Present Gyrotron New Gyrotron
Frequency 28 GHz
Output Power 0.5 MW 1 MW
Pulse Length 0.1 s 1 s
Beam Voltage 75 kV 80 kV
Beam Current 20 A 40 A
MIG Diode Triode
Cavity Mode TE4,2 TE8,3
Output Mode Gaussian Like
Output Window Alumina Sapphire
Collector W/O CPD
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Fig.1 The picture and cross-section structure of 28 GHz – 1 MW
Gyrotron.
Fig.2 Anode voltage dependence of the pitch factor (solid line)
and the α spread (dashed line) calculated by MIG simulation
code.
Fig.3 Beam current dependence of the output power (solid line)
and efficiency (dashed line) calculated by cavity simulation
code.
(a) (b) Fig.4 (a)The calculated RF power profile and (b)the
measured RF burn pattern at the output window. the output power
Po=1MW was achieved. Moreover, the output efficiency of over 35 %
was obtained. III. PERFORMANCE TEST The performance test of a new
28 GHz – 1 MW gyrotron, which had been fabricated based on the
calculation results, was executed with short pulse operation on RF
test stand at the Plasma Research Center. Figure.4(b) is the
experimentally measured RF burn pattern at output window. From this
measurement, it was confirmed that RF beam had Gaussian-like and
centrally peaked profile at the window. This result corresponds
well to the calculated RF power profile shown in Fig.4(a). The
oscillation frequency of 28.035 ~ 28.05 GHz was obtained, and this
frequency was stable in all beam current. The output power Po and
the output efficiency η were measured calorimetrically by SiC dummy
load. Cooling water circulates through the SiC dummy load, and
absorbs the RF beam power injected from gyrotron. The output power
was calculated from the temperature rise of cooling water as
follows:
where Po is the output power of RF beam, C is the specific heat,
Q is the water flow rate, d is the density, ΔT is the temperature
rise of cooling water, D is the duty factor of gyrotron. A cavity
magnetic field strength dependence of the output power was measured
at each beam current. A serious mode competition was not observed.
Beam current dependence of the output power and the output
efficiency is shown in Fig.5. Po increase with increasing Ic. The
maximum Po of 1.04 MW at Ic=40.4A and the maximum η of 40.6 % at
Ic=16A were obtained. In this experiment, the design target of 1 MW
output was achieved successfully. However, at the same time, the
output efficiency decrease with increasing Ic was observed at
Ic>16A. The output efficiency at Po=1.04MW was 32.3 %, so that
the value of η decrease was 8.3 %.
€
Po = 4.18 J cal[ ] ×C cal g⋅°C[ ] ×Q ml s[ ] × d g cm 3[ ] ×ΔT
°C[ ] ×1D
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Fig.5 Beam current dependence of the output power (closed
circle) and efficiency (open circle) measured by SiC dummy load.
IV. DISCUSSION
A comparison of the output efficiency measured by short pulse
test with calculated by cavity simulation code is shown in Fig.6.
The pitch factor α was estimated to be 1.4 at Ic≦16A. However, it
was presumed that the α decreased with increasing Ic at Ic>16A.
There is a possibility that the α decreased to about 1.0 at
Ic=40.4A. To investigate the decrease of α with increasing Ic, the
recalculation by the MIG simulation code was executed by using
actual values of the external magnetic field and the each electrode
voltage at the performance test. Figure.7 is a result of this
calculation. It was confirmed similarly that the pitch factor α had
decreased with increasing Ic. The maximum α is 1.28 at Ic=6.3A and
the minimum α is 0.88 at Ic=40.4A. At the same time, α spread is
increasing from 3.3% to 5.8%.
The decrease in the pitch factor α could explain as the reason
why the output efficiency decreases. The improvement of the
electron beam deterioration is needed to achieve higher output
power and frequency. V. SUMMARY The new 28 GHz – 1 MW gyrotron was
developed for GAMMA10 ECRH. A new tube has been fabricated based on
the design study, and the performance test with short pulse
operation was executed. In the test, the target output power of 1
MW and the maximum output efficiency of 40.6 % have been achieved.
However, deterioration of the electron beam parameter with
increasing beam current was confirmed, and it is the issue for the
development of the higher efficient gyrotron. Long pulse test
aiming for 1 MW – 1 s operation is in progress.
Fig.6 A comparison of the output efficiency measured by
experiment with calculated by cavity code.
Fig.7 Beam current dependence of the pitch factor α (closed
circle) and α spread (open circle) calculated by MIG simulation
code with experimental conditions.
ACKNOWLEDGMENT The author thanks the member of the GAMMA10 group
of the University of Tsukuba for their collaboration and valuable
discussion during this study. This work is partially supported by
NIFS collaboration research program (NIFS09KUGM032 and
NIFS04KUGM009).
REFERENCES 1. T.IMAI et al., “Upgrade Program of ECRH
System for GAMMA 10”, Trans of Fusion Science and Technology,
51, 2T(2006)208-212
2. T.KARIYA et al., “Development of 28 GHz and 77 GHz Gyrotron
for ECRH of Magnetically Confined Plasma”, Trans of Fusion Science
and Technology, 55, 2T(2009)91-94
