J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Development of the long-pulse ECRF system for JT-60SA
Takayuki KOBAYASHI1, Akihiko ISAYAMA1, Damien FASEL2, Kenji YOKOKURA1,
Mitsugu SHIMONO1, Koichi HASEGAWA1, Masayuki SAWAHATA1, Sadaaki SUZUKI1,
Masayuki TERAKADO1, Shinichi HIRANAI, Koichi IGARASHI1, Fumiaki SATO1,
Kenji WADA1, Minoru SHIBAYAMA1, Jun HINATA1, and Shinichi MORIYAMA1
1

Japan Atomic Energy Agency, Naka, Ibaraki, Japan

2

Centre de Recherches en Physique des Plasmas, Lausanne, Switzerland(Received: 12 November 2009 / Accepted: 22 April 2010)

Improvements of the Electron Cyclotron Range of Frequency (ECRF) system in JT-60U and design works areunderway towards steady-state operation in JT-60SA. Since the ECRF system in JT-60U was designed as a 5 s
system, improvements in almost all the system are required to achieve the target pulse length of 100 s with agyrotron output power of 1 MW for JT-60SA. The issues and present status of the design and development aresummarized from a view point of requirements for obtaining long pulse ECRF system.
Keywords: long-pulse ECRF system, JT-60SA
others) for obtaining the long pulse of 100 s in JT-60SA,
and to show the progress of the design and developmentsof the system mainly about the gyrotron, transmission line
and launchers those need some R&D, and plans for near
future toward JT-60SA using the JT-60 ECRF system. We
do not discuss the system for the integrated research phasein this paper.

1. Introduction

Design and development of an Electron Cyclotron
Range of Frequency (ECRF) system for JT-60SA isunderway. A phased construction concept consisting of
initial, integrated and extended research phases is applied
for JT-60SA [1, 2]. In the initial research phase, the ECRF
system will provide an injection power of 1.5 MW for
100 s using two gyrotrons and additional 1.5 MW for 5 s
using the other two gyrotrons. In the integrated researchphase, an injection power of 7 MW for 100 s will be
provided using nine gyrotrons. Nominal output power of
each gyrotron is 1 MW. Each gyrotron has an evacuatedwaveguide transmission line. Four launchers will be
installed on the upper oblique ports of the JT-60SA
tokamak for the nine transmission lines. The
millimeter-wave frequency is 110 GHz which is the sameas that used in JT-60U. Since the ECRF system developed
for the JT-60U [3] was designed for obtaining pulse lengthof 5 s at a gyrotron output power of 1 MW, improvements
are required in many components of the system forachieving pulse length of 100 s. In the initial researchphase, two units of power supplies are fabricated andinstalled by EU. The two 100 s gyrotrons (110 GHz) areprocured by JA. Two units of the power supplies andgyrotrons used in JT-60U will be used for 5 s units as theyare. Since the frequency of the output millimeter-wave in
JT-60SA is the same as that in JT-60U, four
superconducting magnets (B < 4.5 T) can be reused. Fourtransmission lines and two launchers are installed by JA.
The requirements of the improvements on the JT-60
ECRF system had been discussed between JAEA and EU
in recent two or three years. This paper aims at to clarify
such requirements on each ECRF system component(gyrotron, power supply, transmission line, launcher and2. Power Supply
DC Generator primary AC voltage

18 kV

Secondary DC voltage-65 kVDC current

65 A

Voltage ripple< 1%

Duty cycle

1/18

Fast DC switch shutoff< 10 micro s

Modulator frequency> 5 kHz

Acceleration power supplies
DC voltage

50 or 90 kV

DC current

0.3A CW

Voltage ripple< 0.5%

Table 1 Design specification of the ECRF power supply in theinitial research phase.

The power supplies used on the JT-60U ECRF system
limited the pulse duration to 30 s even with the reduced
output power of 0.4 MW [4]. Newly developed power
supplies will be fabricated and installed by EU enabling
pulse length of 100 s at 1 MW in the initial research phase.
Since the JT-60 gyrotron equips a triode type magnetron
ignition gun (MIG) with a collector potential depression
technology, the power supply will consist of a main highvoltage power supply (MHVPS), body power supply(BPS) and anode power supply (APS) or anode voltage

author’s e-mail: kobayashi.takayuki@jaea.go.jp363©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

T. Kobayashi et al., Development of the Long-Pulse ECRF System for JT-60SA

divider (AVD). The design specification of the power
supplies is shown in Table 1 [5]. The choice of the
gyrotron supply topology is one of the major issues, since
the BPS and the APS (AVD) can be either referred to thecathode, or independently referred to the collector(ground) for the BPS and to the cathode for the APS(AVD). A selection between these two topologies will
occur in a near future. However, a 100 s operation at anominal operation parameter (the voltage and the currentof each power supply) for 1 MW will not have a significant
technical difficulty by each topology. A detailed discussion
between EU home team and Japanese home team will be
continued in order to define the detailed specifications.

Figure 1 Progress of the collector conditioning of the improved
gyrotron.

3. Gyrotron

problems come up for the 110 GHz gyrotron in long pulse
operation, additional counter measures are required for
JT-60SA. Therefore, an experimental verification of theeffectiveness of the improvement was essential for the
110 GHz gyrotron. For testing the newly fabricatedgyrotron at long pulse, a MHVPS previously used for the
Lower Hybrid (LH)-A system in JT-60U was connected to
the ECRF system in 2009. Since the current capacity of the
LH-A system (60 kV/130 A) is higher than that of theprevious ECRF system (60 kV/65 A) by a factor of two,
the gyrotron can be tested at long pulse of at least 30 s with
an operation current of less than 65 A. The conditioning
operation of the gyrotron has already been started. Figure 1
shows a progress of conditioning of the gyrotron. As ionpump current of the gyrotron, which was a measure of thegas pressure in the gyrotron, decreased, the pulse length
increased without a hitch. The pulse length at an output
power of 1 MW has been reached to 12 s within fivemonths of conditioning operation (July, 2009). The pulse
length was significantly longer than the pulse lengthachieved in ten years operations in JT-60U by the previous
gyrotrons (5 s at 1 MW), and no significant increase of
the gas pressure in the gyrotron was observed. The
extension of the pulse length is expected in near future
toward 100 s after confirming the feasibility of thetransmission line that was modified as described in thefollowing section.
Although the highest oscillation efficiency achievedby the JT-60 gyrotron was close to 50% at short pulse [6],the efficiencies at the operations in JT-60U experiments
were 40% or lower. This fact was mainly caused by a large
amount of the ripple voltage of the MHVPS used in
JT-60U which was originally designed for the Lower
Hybrid (LH)-system, and therefore, reduction of theefficiency was needed for obtaining stable oscillation using
such a power supply. It is, however, expected that the
MHVPS newly designed by EU will provide lower ripple
voltage than that of the previous one and it may result in
obtaining high efficiency of around 50% in an ideal
situation. The gyrotron may be sometimes operated with

A 110 GHz gyrotron used in JT-60U was originallydesigned for obtaining 1 MW output power with a pulse
length of up to 5 s. After achieving the target output powerof 1 MW for 5 s [6], improvements of the gyrotron andperipheral equipments for obtaining longer pulse lengthand higher power had been continued. For example, a large
sized DC-break made of an alumina ceramic between bodyelectrode and collector electrode was replaced with thatmade of a Si3N4 ceramic due to its property of high thermal
strength and low rf absorption rate. Flow rates of coolingwaters at some internal components were increased. The
longest pulse length of 30 s was successfully achieved in
an experimental campaign of JT-60U in 2007 with areduced output power of 0.4 MW. A reduction of diffracted
rf power loss in the gyrotron and an increase in theoscillation efficiency are mainly required for achieving apulse length of 100 s at 1 MW for the gyrotron.
For reducing the diffracted power in the gyrotron, an
improved 110 GHz gyrotron was fabricated in 2008. The
gyrotron equips a newly designed built-in mode convertor
in order to reduce the stray radiation power in the gyrotron.
The previous mode convertor consisted of a single helix
type launcher and quasi-optical mirrors including a pair ofphase correction mirrors. The new launcher was designedby means of the Launcher Optimization Tool (LOT) [7, 8].
The launcher has a complicated wall shape optimized by
the LOT. Moreover, the new mode convertor does not havephase correction mirrors in order to reduce risk of
deformation of phase pattern at the window due tomisalignment of those mirrors in fabrication process. The
technique used for designing the mode convertor was thesame as that used in designing a 170 GHz gyrotron for
ITER in JAEA, which achieved an oscillation of 1 MW for
800 s [9]. Gyrotrons designed with the same technology
have been fabricated for the large helical device and the
GAMMA 10 tandem mirror at frequencies of 77 GHz and
28 GHz, respectively [10-12]. As studied in Ref. 10, thelower frequency can cause problems due to large amountof diffraction loss even with the same technology. If such

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T. Kobayashi et al., Development of the Long-Pulse ECRF System for JT-60SA

Figure 2. Schematic view of the transmission line of the ECRF system in the JT-60 gyrotron room in 2009. A waveguide type dummy load system
is shared by EC-#2 and EC-#3 units by the use of a waveguide switch (SW-C1).

Since the diameter of 60.3 mm is very close to that ofwaveguides in ITER (63.5 mm) and the similar design of
ITER transmission line components is applicable also for60.3 mm components, a design work of such a large sizeddiameter components may not be a major issue.
On the other hand, the use of 31.75 mm waveguides
can reduce construction cost of the ECRF system in
JT-60SA especially in the gyrotron room where themaintenance is relatively easy compared with maintenancein the torus hall. In this case, the waveguides close to
miter-bends must be cooled externally. The theoretical lossof the HE11 mode for 31.75 mm waveguide is 0.69 kW for
1 m at the transmitted power of 1 MW. The heat capacity
per unit length of the 31.75 mm waveguide made ofaluminum is 1.7 kJ m-1 K-1, thus, the expected temperaturerise for 100 s is about 40 oC assuming adiabatic condition;
it is acceptable level. Consequently the external cooling for
the waveguides away from the miter-bends may not be
required if the heat deposition due to higher order mode iswell localized around the miter-bends. It is, however,recognized that the practical loss on the waveguide
sometimes differs from the theoretical one. A quantitativeevaluation of the distribution of the temperature rise on the
waveguide transmission line has not been done sufficiently.
Therefore an experimental evaluation of the deposition
length of the higher order mode excited at the miter-bend
and a measurement of the practical loss on the waveguideare essential for confirming the availability of the
31.75 mm waveguides and for designing the coolingsystem.
Since the JT-60U experiments was finished in 2008
and disassemble will be started soon (in 2010), thetransmission line in the torus hall was separated from thetransmission line in the gyrotron room. This modification
enables us to test transmission line components at highpower using a dummy load system (1 MW for continuous
wave (CW) operation) in the gyrotron room independentlyof disassembling of the JT-60U tokamak. Figure 2 shows aschematic view of the transmission line in the gyrotronroom at present. A waveguide type dummy load system,
which consists of a pre-dummy-load, pumping waveguide,

reduced oscillation efficiency when the ideal situation is
not obtained due to some experimental reasons (forexample, gyrotron arcing, electrical noises due to otherdevices, and insufficient time for optimization). Therefore
the gyrotron may be operated with an efficiency range of
30% (worst case) to 50% (ideal case). Such an
improvement of the maximum oscillation efficiency in an
ideal condition is favorable from the viewpoint of the
failure cycle of the collector. The improvement of themode convertor described above may also give higher
efficiency, and further improvements of gyrotron itself may
be carried out.

4. Transmission line

In JT-60SA, waveguide transmission lines connect the
gyrotrons in the gyrotron room and the launchers at thetokamak. Since we have many 31.75 mm/60.3 mm
waveguide components (waveguide, miter-bend, window,
DC-break, gate-valve, dummy load, and so on) used in
JT-60U, the candidates of the waveguide diameter for
JT-60SA are also 31.75 mm and/or 60.3 mm for reducing
construction cost. The theoretical loss on the waveguide for
HE11 mode of 31.75 mm and 60.3 mm are 6.7%/100 m and
0.71%/100 m, respectively [13, 14]. However, the practicaltransmission loss on the waveguides strongly depends on amode conversion at miter-bends or that caused by amisalignment of the transmission line. The number of themiter-bends is expected to be seven to nine per line in
JT-60SA. The theoretical mode conversion loss for one
miter-bend of 31.75 mm and 60.3 mm are 1.4% and 0.5%,
respectively, and one-half of them are expected to behigher order mode that heats the waveguides near themiter-bend locally. The mode conversion loss due tomisalignment of the waveguide of the large diametercomponent is relatively large compared with that of thesmall diameter component while the excited mode is
expected to be lower order mode and it can propagate long
distance with low attenuation. Therefore, the large
diameter of 60.3 mm is desired for eliminating local
heating of the waveguide components and for reducing
total loss of the HE11 mode.

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T. Kobayashi et al., Development of the Long-Pulse ECRF System for JT-60SA

dummy-load and termination-load is shared by the EC-#2
and EC-#3 using a waveguide switch (SW-C1). The flowrate of the cooling water of the termination load, which
was originally used as a short pulse load (1 MW/0.2 s at
10 L/min) was increased to ~ 40 L/min which gives acapability of 0.12 MW for CW operation. In addition to themain transmission line (direct connection from the miter
bend (MB-C1) to the dummy load system, a roundabout
route is installed between the MB-C1 to the pre-dummyload using two waveguide switches (SW-C2 and SW-C3)
and it will be used for testing high power capability ofsome components such as a window and polarizer. At
present, 31.75 mm diameter components are installed andtested. They will be replaced with 60.3 mm diametercomponents after sufficient measurements are carried out
with the present transmission line.
Since the waveguide is made of aluminum and its
emissivity is too low, the temperature was measured bymeans of an Infra-Red (IR) camera with a tape of whichemissivity was known (~ 0.95). Thermocouples (TCs in
Fig. 2) were also installed at the center of the outer surfaceof the waveguides and the backward surface of the miter
bend mirrors.
An experiment was performed with a gyrotron output
power of 0.45 MW for 10 s using EC-#2 unit. The
temperature rise of the miter-bend mirror, which was
actively cooled by water, was acceptable level for 100 s
operation at 1 MW as discussed in the previous work [4].
The temperature of the waveguide, which was not cooled
actively, was increased almost linearly and kept the peaktemperature for several tens of seconds or longer as shownin Fig. 3. Figure 4 shows the distribution of thetemperature rise of the waveguides (not including
Matching-Optics-Unit (MOU), miter-bends, switches andwaveguide couplings) measured by the IR-camera. The
highest temperature rise of about 15 oC was observed justafter the SW-2, and the temperature rise of other points
close to the miter-bends and switches were also relatively
high ~ 10 oC as expected. Although the temperature riseestimated by the theoretical loss of the HE11 mode was
1.8 oC for transmitted energy of 4.5 MJ with adiabatic
condition, the observed temperature rise exceeded 1.8 oC
elsewhere; it was typically 5 oC which corresponds to
100 oC for the case of 100 s operation at 1 MW. It seems
like too high for repetitive operations. The longest lengthof the straight lines was about 9 m (between the MB-C1
and dummy load) in this experiment (neglecting the
SW-C2 and SW-C3 those were used as short gaps). This
result suggested two possibilities. One is that the length ofover 5 m was required for absorbing the higher order mode
excited at the MB-C1. The other is that there was a large
amount of lower order mode. It was difficult to identify theorigin of such a mode conversion loss. Therefore, we will
try to observe the temperature rise with longer transmission
line and also try to observe temperature rise by changing

Figure 3 Temperature rises of the waveguides measured bythermocouples installed at the center of the waveguides asshown in Fig. 5. The gyrotron output power and pulse length
are about 450 kW and 10 s, respectively.

Figure 4 Distribution of temperature rise of the waveguides
measured by an IR-camera. The theoretical temperature risefor pure HE11 mode is 1.8 oC for the 31.75 mm waveguides
made of aluminum.

the coupling of the rf beam into waveguide at the MOU
actively in near future in order to clarify the localization of
the heat deposition near the MOU. The total absorbed
power on the waveguides (except miter-bends, coupling
components, and MOU) was about 25 kW which wasestimated by connecting the temperature of the waveguides
smoothly where measured temperature was not obtained.
The adiabatic condition was also assumed. This power
corresponds to about 6% of the transmitted power. In
practice, the temperature should be affected by the coolingat the miter-bend, radiation cooling, and the heat capacitiesof the miter-bend housings and waveguide couplers, andthe adiabatic condition was not satisfied. Thus, the above
estimation gives lower limitation.
Nevertheless, it was clarified that the waveguide atleast 5 m from the miter-bend was needed to be cooled
externally for long pulse transmission at 1 MW in thiscondition. Since this condition is close to that expected forthe transmission line in the gyrotron room in JT-60SA,
many part of the waveguides in the gyrotron room require
the external cooling for the use of 31.75 mm waveguides.
The requirement of flow rate of the cooling water for the
waveguide with a transmission loss of 6% is about
43 L/min even for a continuous wave operation at 1 MW
assuming the water temperature rise of 20 oC. Moreover,
the JT-60SA is pulsed operation of 100 s with duty cycle of
1/18. Therefore, the flow rate of the cooling water is not
major issue in the gyrotron room where the water flowlines for an ion cyclotron range of frequency heating
system (8400 L/min) and a lower hybrid current drive366

T. Kobayashi et al., Development of the Long-Pulse ECRF System for JT-60SA

injection angle is obtained with a reasonable beam width[16], e.g. the beam radius of less than 10 cm is required forneoclassical tearing mode stabilization at the poloidal
injection angle of around 40o, which is almost comparable
to the beam width in JT-60U. Moreover, mock-up mirrors
made of aluminum were fabricated for evaluating theoptical characteristics by low-power measurements. The
curvature of M2 is 700 mm and M1 is flat mirror in thismock-up antenna. The width and height of M1 and M2 are
120 mm x 150 mm and 400 mm x 275 mm, respectively.
Figure 5 shows an example of a comparison between
measured millimeter-wave power distribution andcalculated one at the poloidal angle, D, and the distancefrom the port edge of D = 0° and z = 500 mm,
respectively. It was clarified that the measured power
distribution was well consistent with the calculated one,
except the interference pattern on the measurement due toreflection from the measurement system. Therefore, it was
confirmed that the design based on the numericalcalculation [13] can be adopted for the optimization of theantenna design for JT-60SA.
The next important step on the launcher is mechanicaldesign of the launcher and its testing. In order to move theM1 by around 400 mm in front of the waveguide end, along-stroke bellows, bearing and support are required.
Moreover, additional movable axis is required for altering
the beam angle in toroidal direction. At present, an idea has
been proposed. In addition to the long-stroke shaft(main-shaft for linear motion), another shaft (sub-shaft for
rotation) is connected to the main-shaft as shown in
Figure 6 with T-shape. By turning the sub-shaft on the side
vertical to the main shaft, the main-shaft rotates around its
linear motion axis and resulting in the rotation of M1. The
M1 rotates around the axis of the main-shaft, not aroundwaveguide axis in this concept. The cooling water for M1
inputs from the top of the sub-shaft and through themain-shaft, the waver cools the M1 and backs through themain and sub-shafts again. The mechanical design has
been started in the middle of 2009 and the coolingcapability of those mirrors will be evaluated by the use ofcommercially available Finite Element Method (FEM)
code. The support structure will be designed after the basic
design of the launcher is carried out.

system (2870 L/min) in JT-60U can be used for the ECRF
system in JT-60SA. Further experiments for confirming the
localization of the temperature rise near the MOU and thetemperature distribution on the longer transmission line areplanned in near future for discussing the availability of the
31.75 mm waveguides in the torus hall.

5. Launcher/Antenna

Since maintenance of the components in the vacuum
vessel is difficult due to higher neutron fluence in JT-60SA
than that in JT-60U, highly reliable cooling and driving
mechanism are required for the antenna which is used foraltering the ECRF beam direction in both poloidal andtoroidal directions. A newly proposed Linear
Motion-antenna (LM-antenna) [15] using a linearly-driven
small mirror (M1) and a large fixed-curved mirror (M2)
has an advantage of reducing risks of water leakage andfault of the driving mechanism in the vacuum by
eliminating flexible tubes for coolant supply and linkmechanisms for rotating mirror to alter the poloidal beam
direction in the vacuum vessel.
Optical characteristics of this antenna have to beclarified because the curvature of M2 is not designed for
obtaining focused beam in a plasma but designed foraltering beam angle in wide poloidal range. In order toclarify this issue, a preliminary designed was carried out.
The calculated result showed that a wide range of poloidal

Figure 5. Comparison between low power measurement andcalculation

for

the

mock-upantenna.

(a)

shows

an

experimental setup. (b) shows measured (left) and (calculated)
power distributions at 500 mm from the port edge and a = 0o,Figure 6. Conceptual drawing of a toroidal rotation mechanism ofand (c) shows comparisons at the horizontal (left) and verticalthe movable mirror. The rotation of M1 is realized by(right) cross sections of them at the center of each profile.

turning the sub-shaft vertically to the main shaft.

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T. Kobayashi et al., Development of the Long-Pulse ECRF System for JT-60SA

6. Control and Data Acquisition

waveguide will give a capability of a long pulse operation
at 1 MW. On the other hand, the use of a 31.75 mm
diameter waveguide at a part of the transmission line has
also been considered for reducing construction cost. A
high power experiment for evaluating temperature rise ofwaveguide components was carried out and it wassuggested that the waveguide at least 5 m from themiter-bends needed to be cooled externally. For theantenna/launcher, a confirming an optical characteristicof a newly proposed antenna concept of the LM-antenna
and a designing of a mechanical structure were important
issues. A low power test using mock-up antenna mirrors
was carried out for confirming the reliability of thenumerical design and the measured profile was agreed
with the calculated profile. The next important step innear future is a designing the mechanical structure of thelauncher including its support structure, and the toroidal
rotation mechanism was proposed. For the control anddata acquisition system, a concept of the pre-programmed
control system developed in JT-60U and additional
feedback/pre-programmed system will be needed in
JT-60SA for achieving long pulse with high efficiency.
The control system and data acquisition system will be
designed in detail and fabricated at two or three years
before operation of the system in JT-60SA.

Since an ECRF system is an essential tool rather thana system for proof of principle, a control system is more
important in JT-60SA than that in JT-60U. From the view
point of long-pulse gyrotron operation, the gyrotron shouldbe always operated with a high total efficiency of around50%. Even for a fixed power operation, pre-programmed
and/or feedback controls of heater current and anodevoltage will be required because the beam current
decreases due to cathode cooling by electron emission
resulting in the changing of the oscillation condition of the
gyrotron during a shot. These control techniques have beensuccessfully developed using the JT-60U ECRF system [4]and the control units will be upgraded for JT-60SA.
Moreover, it is expected that the real-time (feedback
and/or pre-programmed) control of the cavity magnetic
field, cathode, anode and acceleration voltages can be used
for the slow output power modulation during a shot if suchan operation is required for feedback control of electron
heating power and driven current. Response times ofvoltages, magnetic field and cathode heater are roughlyorders of 1 ms, 1s, and 10 s, respectively, except the caseof fast power modulation for the neoclassical tearing modestabilization.
The data acquisition system will be newly designed toobtain operation data at long pulse of 100 s or longer. Since,
however, the longest acquisition time of the ECRF system
in JT-60U was 65 s, the expansion of the pulse length to
100 s does not cause significant difficulty.
Techniques and devices for control and dataacquisition systems will be improved every year.
Consequently the detailed design and fabrication of thesystem will be done at two or three years before it is usedin JT-60SA.

Acknowledgements

The authors would like to thank Drs. K. Sakamoto, A.
Kasugai, K. Takahashi, K. Kajiwara and Y. Oda in JAEA for
useful discussion on the ECRF system in JT-60.

[1]
[2]

7. Summary

[3]
[4]
[5]
[6]
[7]
[8]

The requirements of improvements on the ECRF
system and the present status of the design anddevelopment for achieving a pulse length of 100 s at
1 MW per line in JT-60SA were shown. For the power
supply which is fabricated and installed by EU, selectionof a topology is a major topic but the achievement of
100 s at a nominal operation parameter will be possibleby each topology. For the gyrotron, reduction of thediffraction loss in the gyrotron and improving theoscillation efficiency are required. In order to reduce
diffraction loss in the gyrotron, improved gyrotron
equipping a newly designed mode convertor was
fabricated and the conditioning operation has been started.
The pulse length reached 12 s at 1 MW (July, 2009)
without significant trouble. The higher oscillation
efficiency is expected by means of newly designed power
supply for an ideal case which is favorable from the view
point of the failure cycle of the collector. For thetransmission line, the use of a 60.3 mm diameter

[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]

368

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