J. Plasma FusiorL Res. SERIES, Vol.5 (2002) 337-341

Recent Progress of the High Power ECRF System

on JT-60U
SEKI Masami, MORIYAMA Shinichi, KAJIWARA Ken, IKEDA Yoshitaka,
SAKAMOTO Keishi, IMAI Tsuyoshi and FUJII Tsuneyuki
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute,Naka. Ibaraki 311-0193. Japan(Received: ll December 2001 / Accepted: 12 November 2002)

Abstract
Ttre Electron Cyclotron Range of Frequency (ECRF) system having four I MW - gyrotrons has
been successfully operated on JT-60U to locally heat a plasma and drive a plasma current. Aftermodification by suppressing a parasitic oscillation, the gyrotron can generate -1.1 MW power for 3.2 s.
High RF powers generated by four gyrotrons are transmitted to two independent antennas consisting ofsteerable mirrors via four long transmission lines with moderate transmission efficiency of 70-80 Vo.In
2001, injected power reaches up to -2.8 MW for 2.8 s after optimizing operational parameters. In suchhigh power injection, a substantial increment in the central electron temperature from 5 keV up to -13
keV is observed in the Reversed-Shear (R-S) plasma with the Internal Transport Barrier (ITB).

Keywords:ECRF, JT-60U, plasma heating, current drive, gyrotron, high power injection

obtained on the main DC power supply with stability of

1. Introduction+l 7o by keeping high stability of +0.5 Vo in theElectron r:yclotron (EC) waves play important anduseful roles of heating (ECH), current drive (ECCD) andstart-up assist not only in tokamaks but also in otherfusion devices. Especially a local current drive by EC

acceleration voltage. The diamond window of thegyrotron also enabled the gaussian output mode, which
was effective for conversion to low loss HE11 mode in
the transmission line [4]. In 2000, three gyrotrons werewaves can stabilize MHD instabilities. Moreover, EC
waves have arr advantage of propagating in vacuum, so
the antenna can be set far away from plasma.
Development of gyrotrons at a frequency of over
100 GHz has made good progress in the past several

operated to ECH / ECCD at the injection power level of

1.5-1.6 MW [5]. In 2001, the ECRF system in JT-60U
has been completed to be a 4 MW-l10 GHz system fora wide variety of ECH/ECCD investigations such as on/years. The (lollector Potential Depression (CPD)

off-axis and co/counter injections.
This paper gives an overview of the high power

technology for the gyrotron has improved efficiency [1].
A diamond window enabled steady state operation of
the gyrotron due to its low RF loss and high thermal
conductivity [2]. One ECRF unit, which was composedof a ll0 GHz CPD gyrotron, power supply system, atransmission line and an antenna, was constructed in1999 [3]. A I MW oscillation of the CPD gyrotron wasECRF system and recent progress through experience ofoperation in Section 2 and3, respectively. Section 4 and5 show the typical result of high power heating and thesummarY.

@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Research337

Seki M. er a/., Recent hogress of the High Power ECRF System on JT-60U

2. Overview of the ECRF System on JT-60U
The ECRF system consists of four independentunits as shown in Fig. l. Each unit can generate a highas shown in Fig. l. The total length from the gyrotron tothe antenna is about 60 m. The transmission line isevacuated to avoid RF breakdowns in it. The gaussianmode from the gyrotron output window is converted tothe low-loss F1E11 mode in the transmission line using
two phase-corrected mirrors in a Matching Optical Unit
(MOU). The transmission line is mainly composed of acorrugated waveguide with a diameter of 31.75 mm.

output power around 1 MW by the high power gyrotron
at a working frequency of 110 GHz. The gyrotron ineach unit is featured by the Collector Potential
Depression (CPD) technology. The CPD technique is torecover the kinetic energy ofthe spent electron beam bydepressing the voltage between the body and collector
section of the gyrotron. The CPD technology also allows
a stable operation of the gyrotron by the combination of

There are directional couplers to detect RF power at theoutlet of the gyrotron and at the inlet of the antenna. A
pair of polarizers is used to change the polarization of

the transmitted wave. A I MW x 3 s dummy load isinstalled for conditioning the gyrotron. The diamondwindow is installed near the antenna for the vacuum

the main DC voltage supply system and the stableAcceleration Power Supply (APS). The APS can give anacceleration voltage between the cathode and the bodysections up to 100 kV, 300 mA within high stability of+O.5 Vo. Since the APS is applied in floating to thecathode voltage fed by the main DC power supply (-60kV), the acceleration voltage (-+85 kV) between thecathode and body section can be constant even if themain DC voltage is fluctuated. The gyrotron oscillation
mode of TE22,6 is converted to a gaussian mode. Theboundary.
One of the major aims of the ECRF system is tolocally drive a plasma current and to control its position

within about -0.1 m for suppression of MHD
instabilities. Therefore an injection angle of the RFbeam should be controlled. There are two antennas on
the system, so called antenna-A and antenna-B.
Antenna-A is connected with three units to control thepoloidal injection angle at the fixed toroidal injection
angle of -20o. Antenna-B is connected with unit 4 to

output window employs a single CVD diamond disk
with an outer diameter of 96 mm, allowing it tocontinuously transmit the gaussian beam at the power
level of 1 MW.
There are four transmission lines, each of which
has basically the same combination of RF components

control the injection angle in toroidal/poloidal directions

with two steerable mirrors [6].

| [f,]V, I IOGHZ Gyrotrons

Wall of the forus Halllntenna-Btor unlt 4#1(toroldal/poloHal sc6n)
antenna-A for

unlt I -3(pololdal scadI

Ilanrondwlndow\Dumnry/pola rtzers-6Om trnnsrnlsslon llnes

Du|ilry

Fig. 1 Layout ofthe ECRF system on JT-60U.
338

RFnpnltor

Seki M. er al., Recent Progress of the High Power ECRF System on JT-60U

3. Progresses of the EGRF System
3.1 Optimization of Start-up Speed of APS

inlet of the antenna. In 2000. the transmission

The CPD gyrotron worked on the combination ofthe main DC voltage power supply system and the APS,
however, a large fluctuation was observed in the CPD
voltage (Vcpo = %"" + %uooa") when the start-up speed

were measured and found to be -75 Vo, -70 Vo and -60
Vo for unit 1,2 and 3, respectively. A misalignment ofefficiencies including the mode conversion loss in MOU

moment when the CDP voltage was beyond the critical

l0 mm (displacement)/10 m (in length) was locallyfound in the transmission line of unit 3. Suchmisalignment seemed to degrade the transmission
efficiency by undesired RF loss due to tilt effect. In
2001, re-alignments (less than -2 mml 10 m) of thelevel (-30 k!'). When V6pp reached the level, spenttransmission lines have been performed for unit 2 and 3,

electrons seemed to flow back to the body much more
and as a result the over-current interlock acting in theAPS. The over-current limited the APS voltage for highpower operation. To suppress the over-shoot in V6pp, thestart-up speed of the APS was optimized from 10 to 50
ms. Thus the stable oscillation has been realized at thehigh APS voltage of 85-90 kV.

then the re-alignments improve the transmission
efficiencies to a level from -70 Vo to -75 Vo and from-6O 7o to -80 Eo, respectively. These moderatetransmission efficiencies were close to the optimal of8l Vo.

of the APS'*,as set at -10 ms as shown in Fig. 2.
Breakdown in the gyrotron often occurred at theOne of the main issues for long pulse operation is
the over-heat of the transmission components such as
miter bends, diamond windows and corrugatedwaveguides. The temperature was measured using
thermocouples and was consistent with the prediction,
which indicates the RF components is capable oftransmitting a power of 0.8 MW (power density: -1000
MWm2) for long pulse operation.

3.2 Increment of Transmission Efficiency
The RF loss of the long transmission line wasmeasured by installing a calorimetric dummy load at the100

3.3 lmprovement of the Gyrotron
One major problem in the previous gyrotron

80

operation was the increment of beam current duringoscillation. Typical waveforms of the beam current are
shown in Fie. 3. The oscillation was terminated bv the60

4020Fcs.

0-20

3oo-40-with RF absorber
w/o RF absober

j

co-60

ni

ts-80

o*o{J=40E

CL

IE

40

lL

S20

rtfi

RF-20

oo'040time lmsl

80120

0.5 1.0 1.5 2.O 2.5
time [s]Fig. 3 Typical time evolution of beam current /*.- andRF power with / without RF absorber. The beam
current /0"". increases in the case without RF ab-Fig. 2 Stabilization of the APS at start-up. Overshootwas observed in the ycpD for the start-up time constant of 1Oms compared with that of 50 ms.

sorber.
339

Seki M. er al., Recent Progress of the High Power ECRF System on JT-60U

interlock of the beam over-current in this shot. It was
found that the abrupt increment of the beam current wasbrought by the temperature rise of cathode surface due
to a parasitic oscillation in beam tunnel. Moreover, theparasitic oscillation damaged a ceramic of the electrongun. After several shots of 3-second pulse, a vacuum
leakage occurred in the gyrotron due to a crack in theThe parasitic oscillation has been successfully
suppressed by installing an SiC ceramic (RF absorber)on the inner wall of the beam tunnel [7]. The beam
current does not increase and rather slightly decreases.
Stable oscillation continues up to 3.2 s at output powerof 1.1 MW even for such slightly decreasing beam
current. The output power increases with the beam
current at this stable phase as shown Fig. 4. The output
power is expanded to -1.3 MW (for 0.1 s) with the RFabsorber while it was limited to I MW for 2 s withoutceramic.

the RF absorber. The performance of improved
gyrotrons has been extending through adequate

F r.s3

conditioning.

vvith RF absorber

1.0roEoo.

5IL 0.soo=

4. Typical Result of High Power Heating
A typical result of heating experiments using theECRF system is shown in Fig. 5. An RF power of,4r"

ECRF is injected into a reversed-shear plasma with ITB
initiated by the Lower Hybrid (LH) current drive during
a plasma-current ramp-up. In this shot, an ECH powerof -2.8 MW is injected to obtain high temperatureelectrons, so that the central electron temperature within
rla -O.3 measured by the polycrometer ECE increases
from -5 keV to -13 keV by on-axis ECRF heating. The/(,;,h0,o.0.,0204060

beam current tAI

YAG Thomson measurement also shows the same
efficient electron heating. In the same shot, Ion
Cyclotron (IC) wave is launched to investigate theinteraction between the fast waves of IC and highFig. 4 Relation of the RF output power with the beam
current. The RF output power increase with thebeam current.

temperature electrons produced by EC heating, and a15E40633

Neutral Beam (NBI) is injected to measure ion
temperature for charge exchange recombination

6spectroscopy.

TenelkeVJ[10tsrn+10Pr-H4

The high power ECRF system was constructed on
JT-60U, having a capability of high output power up to4 MW for several seconds using four CPD-gyrotrons at
110 GHz. By optimizing the start-up speed for the APS
from 10 to 50 ms to suppress over-shoot in the V6pp, RFoscillation becomes stable at a high power level of morethan I MW. The gyrotron was improved by installing an
RF absorber in the beam tunnel, and the output power
and pulse duration are enhanced up to 1.1 MW for 3.2 s
because of suppression of the parasitic oscillation, while
the output power was limited at I MW for 2 s without
the RF absorber. After re-alignment of transmission
lines, moderate transmission efficiencies from gyrotrons

02,"t oPrc,ruar
IMWI

0P=",
IMW

5. Summary-0;

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time [s]Fig. 5 Electron heating experiment by high ECRF power
injection in R-S plasma. The LH power is injected
for formation of the R-S plasma. lC power is in-to antennas are obtained around 70-80 Vo. The
capability of the RF components equipped in thejected to interact with high temperature electrons
produced by EC heating.

transmission line has been also demonstrated at a Dower340

Seki M. er al., Recent Progress of the High Power ECRF System on JT-60U

References

density of -1000 MWm2.
In conclu:rion, the improvements of power supply

tll K. Sakamoto et al., J. Plasma Fusion Res. 71, 1029
(1995).

operation, gyrotrons and transmission line alignment
have enabled ur to obtain the design values of the ECRF
system, -2.8 MW for 2.8 s, and central electron
temperatures of -13 keV are observed by a high power
ECRF electron heating experiment in an R-S plasma
with ITB.

t2l K.Sakamoto et al., Review Sci. Instrum. 70, 2O8(1999\.

t3l Y. lkeda et al., Fusion Eng. Des. 53, 351 (2001).
t4l A. Kasugai et a/., Fusion Eng. Des. 53,399 (2001).
t5l K. Kajiwara et al., J. Plasma Fusion Res. SERIES,Vol. 3, 372 (2000).
t6l S. Moriyama, et al.,Proc. l4'h Top. Conf. Appl. ofRF Power to Plasmas, May, 2001, Oxnard.
t7l H. Shoyama et al., J. J. Appl. Phys. 40, L906

Acknowledgement
The authors would like to express their thanks tothe members of RF Facilities Division who have(2001).

contributed to the operation of the ECRF system.

34r