Behavior of reflected extraordinary mode in the fundamentalelectron cyclotron heating and current drive in JT-60U

K. Kajiwara b,*, S. Moriyama a, K. Takahashi a, Y. Ikeda a, M. Seki a, T. Fujii a

a Japan Atomic Energy Research Institute, 801-1, Naka-machi, Naka-gun, Ibaraki, Japanb General Atomics, DIII-D National fusion Facility, 3483 Dunhill Street, San Diego, CA 92121-1200, USA

Received 30 January 2002; accepted 13 August 2002

Abstract

Unwanted X-mode behavior in the fundamental harmonic electron cyclotron (EC) wave launching from the low field

side with an oblique toroidal injection angle is experimentally studied on JT-60U by changing the poloidal injection

angle. It is found that there is a critical region in the poloidal injection angle to cause the rise in vacuum pressure at the

NBI port adjacent to the ECRF antenna port. The typical EC beam power is about 1 MW and the pulse length is 3 s in

this experiment. The ray-trace calculation indicates that the X-mode launched from the low field side is guided into the

NBI port after reflection at the cut-off layer for the critical poloidal injection angle. The results suggest that the O-mode

injection with quite high mode purity, that is, with a very low fraction of X-mode is required for the avoidance of the

overheat of the in-vessel components due to the reflected X-mode in the case of the EC beam launching from the low

field side, especially for long pulse operation.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Electron cyclotron heating; Extraordinary mode; Cut off; Neutral beam injection; JT-60U

1. Introduction

Electron cyclotron range of frequency (ECRF)

system has been successfully utilized for fusion

plasma heating and current drive. Besides the

plasma heating and current drive, ECRF system

has been recognized as a useful tool for plasma

production, plasma profile modification, magneto-

hydrodynamics control, and transport, study. In

International Thermonuclear Experimental Reac-

tor (ITER) 20 MW 170 GHz ECRF system is

planned and designed to inject from the low field

side (lfs) with fundamental harmonic EC wave [1].

The EC wave is injected into plasma with an

oblique toroidal injection angle for ECCD. In an

upper port, the poloidal injection angle is changed

by a steerable mirror for current profile control. In

this situation, the ordinary mode (O-mode) injec-

tion is necessary, since, the cutoff layer for the

extraordinary mode (X-mode) exists before the

resonance layer.

In some machines, the effect of the reflected EC

wave is investigated. In the JIPPT-2 stellarator [2],

* Corresponding author. Tel.: �/1-858-455-3960; fax: �/1-

858-455-4190

E-mail address: kajiwara@fusion.gat.com (K. Kajiwara).

Fusion Engineering and Design 65 (2003) 27�/32

www.elsevier.com/locate/fusengdes

0920-3796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 0 - 3 7 9 6 ( 0 2 ) 0 0 1 9 3 - X

there is no clear difference in the electron heatingbetween O-mode and X-mode launched from the

low field side. The result is explained by the

depolarization scattering of waves between the

cutoff layer and the vacuum vessel wall. The O-

mode is produced after multi-reflection, and then

most of the injected power may be absorbed. In

deed, in the FTU tokamak [3], the residual of EC

wave (injected from lfs, fundamental harmonic O-mode and X-mode) power after multi-reflection

between the cutoff layer and vacuum vessel is

detected by RF probes. However, the locality of

the reflected X-mode, which is controlled by the

injection beam angle, has not been studied in other

machine.

In JT-60U, a 110 GHz 3 MW ECRF system

[4,5] has been constructed to improve the plasmaperformance by its controllability of current,

profile and electron temperature profile. The

frequency is selected for fundamental harmonic

EC wave injected from the low field side at the O-

mode. This operational scheme is the same as that

in ITER. In JT-60U, a single pass absorption is

expected for the O-mode due to its high electron

temperature plasma. Moreover, the EC beam iswell focussed and its poloidal injection angle can

be controlled by a steerable mirror. Therefore the

precise study of the reflected X-mode is possible in

JT-60U. In this paper, we study the behavior of

the localized X-mode reflected by changing the EC

beam injection angle.

The experiments which confirm the O/X mode

control ability were done. The ratio of the O-modewas changed wide range. In these experiments, the

pressure increase of the NBI port which is adjacent

to ECH port was observed. It is caused by the

reflected X-mode. This is the start point of this

study.

2. Experimental set-up

JT-60U ECRF system consists of three 1 MW

gyrotrons, three transmission lines and an an-

tenna. Frequency of 110 GHz is selected for the

fundamental harmonic ECH/ECCD at the central

toroidal magnetic field of around 3.9 T.

The gyrotron with a energy recovery (CPD) is

designed to generate RF power up to 1 MW. The

gyrotron has a diamond window to transmit RF

power with Gaussian-like distribution, which is

easily coupled to the HE11 mode for corrugated

waveguide transmission. The waveguide (the inner

diameter: 31.75 mm) are aligned �/60 m in length

with nine miter bends. The transmission line is

evacuated to avoid the RF breakdown in it.

Another diamond window is installed at the inlet

of an antenna to isolate the transmission line and

the JT-60U tokamak.An antenna is installed at the outer side (upper)

port whose angle as shown in Fig. 1. The three

transmission lines are into the one antenna. The

antenna has focusing mirrors to minimize the EC

beam width and a flat steerable mirror to control

the poloidal injection angle of EC beams (c ) from

the plasma edge to the center with a speed of �/

108 s�1 during a discharge. The toroidal injection

angle is fixed as �/208 for current drive. The e-

folding radius of the EC beam is measured within

6.5 cm at the center of the vacuum vessel.In order to excite the oblique O-mode, the

elliptical polarization of the injected EC beam

for various plasma is required. A pair of miter

Fig. 1. Illustration of the ECRF antenna (P-17) and the upper

perpendicular NBI port (P-16) (poloidal and toroidal view).

The symbol c indicates the EC injection angle.

K. Kajiwara et al. / Fusion Engineering and Design 65 (2003) 27�/3228

bends with the different grooved mirrors areinstalled in the transmission line to change the

power fraction of O/X-mode. The effect of the

polarization in the electron heating is already

studied [6], where the electron temperature in-

creased with the increase of the O-mode fraction.

Eleven units of neutral beams injection (NBI)

with deuterium are available in JT-60U. The beam

acceleration voltage is 80�/85 kV and the injectionpower per unit, is 1.8�/2.2 MW. One of the NBI

units is located adjacent to the ECRF antenna in

the toroidal direction. The poloidal location of this

NBI port, is the same as the ECRF one. The size

of the cross-section of the NBI port is 0.58 m in

height and 0.66 m in width. Fig. 1 shows the layout

of the ECRF antenna and the NBI unit.

3. Experimental result

Injection of the EC beam by changing the X-

mode fraction had been performed in this experi-

ment. The plasma parameters were as follows:

plasma current, Ip�/1.5 MA, toroidal field Bt�/

3.65 T, major radius R�/3.22 m, minor radius a�/

0.80 m. A large pressure rise in the NBI port

adjacent to the ECRF port was observed in the

case of the X-mode fraction of 54% at the injected

power of 1 MW�/3 s. Fig. 2(a) and (b) show the

time evolution of the vacuum pressure in the NBI

port with and without EC beam injection, respec-

tively. The rise in pressure for 1 MW�/3 s EC

beam injection reached seven times as large as the

NBI injection phase, while the pressure kept the

low level in the case of NBI alone. This rise in

pressure might be occurred by the invasion of the

stray EC beam (reflected X-mode) into the NBI

unit. A decrease of liquid helium of a cryo panel

for the NBI port was also observed in the EC

beam injection case. The invaded X-mode heated

the cryo panel and, as a result, the decrease of

liquid helium in the cryo panel was occurred. Fig.

3 shows the decrease of the liquid helium level in

the cryo panel for the NBI port, as a function of

the injected X-mode energy (�/Injection power�/

fraction of X-mode�/pulse length). The figure

includes discharges at various EC pulse length

(B/3 s) and fraction of X-mode (15�/65%). The

average electron density (nc) and EC injection

angle are range of 2.0�/2.5�/1019 m�3 and of

19�/238, respectively. The EC power and the other

plasma parameters were almost the same as

discharge in Fig. 2. It is found that the decrease

of liquid helium level increases in proportion to the

X-mode energy. It was found that the X-mode

invasion into NBI port causes the decrease of the

liquid helium. The energy which needs evaporation

of liquid helium in Fig. 3 is 0.35% of injected X-

Fig. 2. Time evolution of vacuum pressure in the NBI port (a)

with EC beam (b) without EC beam.

Fig. 3. Dependence of the liquid helium level on the injected X-

mode energy. The vertical axis shows the decrease of liquid

helium level of the cryo panel in the NBI port. The average

electron density (/n̄/c) and EC injection angle are range of 2.0�/

2.5�/1019 m�3 and 19�/238, respectively.

K. Kajiwara et al. / Fusion Engineering and Design 65 (2003) 27�/32 29

mode energy. It shows a little RF may damage the

cryo panel.

The effect of the EC beam injection angle (c )

onto the amount of evaporation of liquid helium

for a cryo panel is studied. The ratio of energy of

evaporation for helium to X-mode energy shown

in Fig. 4. The vertical axis is the energy which

needs evaporation of liquid helium. It is normal-

ized by injected X-mode energy. Here, the electron

density was not so varied (2.2�/2.5�/1019 m�3)

and the other plasma parameters were based on

Fig. 2. There is a clear dependence on EC wave

injection angle. The evaporation of liquid helium

was large when the injection angle was around 218.This result indicates that the reflected X-mode is

localized.

The dependence of evaporation of liquid helium

on density is also studied as shown in Fig. 5. The

vertical axis is the same as in Fig. 4 and the

horizontal axis is average electron density (/n̄e): The

EC injection angle is around 198 and the other

parameters are the same as in Fig. 2. There is a

clear dependence on the electron density. The cut

off layer is shifted to outward by increasing the

density, then the direction, of reflected X-mode is

changed. As a result, the X-mode did not invade

the NBI port and the evaporation of liquid helium

was not observed in high density plasmas. It is

noticed that the EC beam of X-mode keeps its

locality even after the reflection at the cut-off

layer.

4. Discussion

The position of cut off layer for the fundamental

harmonic X-mode is determined by the densityand the magnetic field strength, i.e. vpe

2 �/v(v�/

vec)sinu , where vpe is plasma frequency, vec is

electron cyclolron frequency, v is the injected EC

beam frequency and u is the angle between the EC

beam and magnetic field line. Fig. 6 shows the

typical EC beam rays calculated by a 3D ray-

tracing code which is developed by Hamamatsu

[7]. In the case of No. 2 ray, the EC beam is guidedinto the NBI port after reflection at the cut off

layer, while the reflected X-mode do not invade

into the NBI port when the injection angle and the

plasma density are shifted from the critical condi-

tion as the case No. 1 and 3. The critical parameter

region where the reflected X-mode is guided into

the NBI port is obtained by the ray-trace calcula-

tion as shown in Fig. 7. The vertical axis is the ECbeam injection angle and the horizontal axis is the

center density ne0 which is defined as follows:

n(r)�ne0(0:04(1�r2)0:47�0:06) (1)

Here, the symbol r is the normalized minor

radius. The hatched region is the critical parameter

region where the reflected X-mode is guided into

the NBI port. The conditions for the ray-tracing

calculation (plasma configuration, density profile

Fig. 4. Dependence of evaporation energy for liquid helium to

X-mode energy on EC injection angle. The vertical axis is the

energy of evaporation of liquid helium for cryo panel which is

normalized by injected X-mode energy. The horizontal axis is

EC injection angle c which is defined in Fig. 1. The horizontal

bar indicates the range of EC injection angle changed during a

discharge. The electron density is not varied (/n̄/c 2.2�/2.5�/1019

m�3).

Fig. 5. Dependence of evaporation energy for liquid helium on

plasma density. The vertical axis is the energy of evaporation of

liquid helium for cryo panel which is normalized by injected X-

mode energy. The EC injection angle is around 198.

K. Kajiwara et al. / Fusion Engineering and Design 65 (2003) 27�/3230

and temperature profile) in Fig. 7 are based on the

experiments. The EC ray is moved 7.5 cm at the

inlet of the NBI port when the EC injection angle

is changed at 18. The e-folding radius of the X-

mode at the inlet of the NBI port is about 7 cm on

assumption of the Gaussian beam even after

reflection at cut off layer. The closed and open

marks show experimental results with and without

large decrease of liquid helium of a cryo panel of

the NBI unit, respectively. As shown in Fig. 7, the

closed circles only exist within the hatched region.

It clearly indicates that the local reflected X-mode

causes the decrease of liquid helium in the NBI

port. Here, the horizontal bar indicates the range

of EC injection angle when it is changed during a

discharge. In the discharges, which is shown as an

open triangle around injection angle of 198 with

bar, the slight decrease of the liquid helium was

observed. However, the decrease of the liquid

helium was not observed in the discharges with

open circle (angle of 358). The results of calcula-

tion indicates the reflected X-mode strays into

NBI port with keeping its locality.

In the case of high power and quasi-steady state

operation such as ITER, the reflected X-mode

may cause a serious problem as shown in this

study. For example, if 99% O-mode RF with

power of 20 MW is injected during 8.0 s, the

reflected X-mode reaches 1.6 MJ which is compar-

able with our case, The adequate polarization

depends on the EC injection angle and plasma

parameters (position, configuration and tilting of

the magnetic field at the plasma edge). Therefore,

an accurate real time control of the polarization

for 100% O-mode injection is required from the

Fig. 6. Results of the ray tracing calculation for X-mode (a) poloidal view and (b) toroidal view of cross section of the JT-60U. Ray

No. l, 2 and 3 are calculated with nc0 (defined in Eq. (1)) of 3.4�/1019, 2.5�/1019 and 2.3�/1019 m�3, respectively. The injection angles

are 19, 23 and 358, respectively.

Fig. 7. Critical region of X-mode invasion to the NBI port in

the c�/ne0 plane. The hatched region shows the condition where

the reflected X-mode is guided into the NBI port. The closed

and open marks show experimental results with and without

large decrease of liquid helium of a cryo panel of the NBI unit,

respectively. The bar indicates the range of EC injection angle

when it is changed during a discharge. The open triangles

indicate the discharge with the slightly decrease of the liquid

helium and open circles indicate the discharges for which the

decrease of liquid helium is not observed.

K. Kajiwara et al. / Fusion Engineering and Design 65 (2003) 27�/32 31

viewpoint of the avoidance of the harmful re-flected X-mode.

5. Conclusion

Unwanted X-mode behavior in ECH/ECCD has

been studied when the fundamental harmonic EC

beam launched from the low field side on JT-60U.The rise in vacuum pressure at the NBI port

adjacent to the ECRF port was observed when the

high power X-mode was injected. A decrease of

liquid helium of a cryo panel for the NBI port was

also observed. It is caused by the heating of the

cryo panel by invaded X-mode. The decrease of

the liquid helium depends on the EC beam

injection angle. These results indicate that thereflected X-mode keeps its locality. Moreover,

the ray-trace calculation shows that the injected

X-mode is locally reflected and guided into the

NBI port when the decrease of the liquid helium

was observed. This suggests that the high fraction

of the O-mode is required to inject not only for a

good absorption of EC wave but also for suppres-

sion of the harmful reflected X-mode when thefocussed EC beam is injected from low field side in

ECH/ECCD.

Acknowledgements

The authors gratefully appreciate to RF facility

Division, NBI facility division and JT-60U teamfor their support of this work. Especially, thank

you for the generous cooperation of NBI division

about the information of the liquid helium beha-vior of the cryo panel of NBI port.

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