Plasma and Fusion Research: Regular Articles

Volume 14, 3402107 (2019)

Measurement of the Lower Hybrid Wave Using RF MagneticProbes on the TST-2 Spherical Tokamak∗)Yongtae KO, Naoto TSUJII, Satoru YAJIMA, Yuichi TAKASE, Akira EJIRI, Osamu WATANABE,
Hibiki YAMAZAKI, Charles P. MOELLER1) , Yasuo YOSHIMURA2) , Yusuke YOSHIDA,Yusuke IIDA, Kotaro IWASAKI, Yuya KAWAMATA, Sho SAKAMOTO, Yuki AOI,Teruki OISHI, Kyohei MATSUZAKI and James RICEThe University of Tokyo, Kashiwa 277-8561, Japan1)
General Atomics, San Diego, CA 92186, USA2)National Institute for Fusion Science, Toki 509-5292, Japan(Received 10 January 2019 / Accepted 8 May 2019)
Polarization-resolved spatial distribution of waves in the lower hybrid frequency range was measured using
radio frequency magnetic probes (RFMPs) in the TST-2 spherical tokamak. Thirteen RFMPs were installed insideof the vacuum vessel, eight on the inboard-side, three on the outboard-side and two on the bottom-side. In TST-2,
the lower hybrid wave (LHW) is excited by the outboard-launch antenna and the top-launch antenna. Higher
plasma currents can be achieved using the top-launch antenna. Bottom-launch can be simulated by reversing thedirection of the toroidal magnetic field. Propagation and absorption of the LHW were investigated numerically foroutboard-, top-, and simulated bottom-launch at low and high plasma currents. Results of the wave measurement
revealed diﬀerent propagation characteristics for diﬀerent launching, which agree partially with the results ofnumerical calculation.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: spherical tokamak, lower hybrid wave, magnetic probeDOI: 10.1585/pfr.14.3402107

1. Introduction
Fully non-inductive plasma start-up and current drive
is a major issue for the spherical tokamak (ST) since theelimination of the central solenoid (CS) for inductive current drive is considered necessary for the ST-based fusion
reactor. On the TST-2 spherical tokamak, non-inductive
plasma current (Ip ) ramp-up using the lower hybrid wave
(LHW (200 MHz)) is being studied [1]. Two capacitivelycoupled combline (CCC) antennas, located on the outboard
side (outboard-launch) and on the top side (top-launch),
are used to excite the LHW. Furthermore, bottom launch
can be simulated using the top-launch antenna by reversing
the direction of the toroidal magnetic field Bt . According
to ray-tracing calculation using GENRAY [2], wave propagation and current drive characteristics are clearly diﬀerent among outboard launch, top launch and simulated bottom launch [3]. In order to identify the optimum scenario
for Ip ramp-up, the LHW and the fast wave (FW) were
measured by RF magnetic probes (RFMPs). In addition
to wave power, phase, and frequency spectrum, wave polarization can be measured by RFMPs [4]. In this study,
the global distributions of the LHW and the FW are measured using thirteen RFMPs installed inside the vacuum
author’s e-mail: ko@fusion.k.u-tokyo.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

vessel and compared with predictions of numerical calculation. Experimental data were generally consistent withnumerical calculation results of wave propagation and polarization.

2. Experimental Setup
Thirteen RFMPs were used in this experiment. Each
RFMP consists of a single-turn coil formed by connecting
the inner conductor to the outer conductor of a semi-rigid
coaxial cable, metal enclosure for shielding, and a slit oriented in either toroidal or poloidal direction to select theRF magnetic field with particular polarization.
On the inboard side, eight RFMPs (10 mm × 15 mm
rectangular loop) are arranged symmetrically about the
mid-plane z = 0, as shown in Fig. 1. The (R, z) coordinatesof RFMPs are MPupper : (+575, +100), MPmiddle : (+585, 0),MPlower : (+575, −100) on the outboard side, MPin : (+329,−450), MPout : (+535, −450) on the bottom side, and Apol :(+113, +327), Bø : (+113, +177), Cpol : (+113, +177), D:(+113, +27), Eø : (+113, −27), Fø : (+113, −177), Gpol :(+113, −177), Hpol : (+113, −327) on the inboard side.
RFMPs on the outboard side and the bottom side consistof a single-turn coil wound around a teflon bobbin, and a
metal enclosure with a slit in the poloidal direction. AllRFMPs are located in the poloidal plane at a toroidal angle
φ = −90◦ measured from the center of the outboard-launch

3402107-1

c 2019 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 14, 3402107 (2019)

Fig. 1 Poloidal cross section of the vacuum vessel and a radial
view of the inboard wall showing the locations of the limiters and RFMPs.

Fig. 3 Evolutions of typical non-inductive Ip ramp-up discharges. Red, blue and green lines indicate outboardlaunch, top-launch (with CW Bt ) and top-launch (withCCW Bt ), respectively. The last case simulates bottomlaunch.

Fig. 2 RFMP and antenna locations viewed from the top.

antenna (Fig. 2).
The RF signals are measured by two oscilloscopeswith bandwidths 500 MHz at 500 MSamples per second
with 1 MWord memory (2 ms observation window) aroundt = 60 ms where Ip reaches the maximum value. The signal intensity is evaluated by integrating the frequency spectrum from 199 MHz to 201 MHz. To measure the signals
obtained by all thirteen RFMPs, four reproducible shots
are required due to the limited number of the oscilloscope
channels.

3. Wave Measurement Results
The time evolution of the typical non-inductive Ip
ramp-up discharge, comparing diﬀerent cases of LHW
launching, is shown in Fig. 3. In this series of experiment,
low and high Ip case was investigated for each launchingcondition, though the Ip could not be controlled to be thesame for each launching condition, with plasma currentsof 5.5 kA to 11.7 kA for outboard launch (CW Bt ), 6.3 kA

to 19 kA for top launch (CW Bt ), and 6.8 kA to 14 kA for
simulated bottom launch (top launch with CCW Bt ). The
Hpol RFMP was out of order for the top-launch experimentwith CW Bt .
Figure 4 summarizes the RF signal intensities measured by the thirteen RFMPs. As shown on the right figure, the horizontal axis on the left figure is the distancesfrom the midplane on the inboard side defined as the origin, measuring downward, then radially outward throughRFMPs MPin and MPout , and finally vertically upward tothe topmost RFMP on the outboard side (MPupper ). The
vertical axis on the left figure is the measured RF signal
intensity normalized by the injected LHW power. The normalized RF signal intensities are plotted for low Ip andhigh Ip . The background noise level was measured in theabsence of RF power injection. The signal level is at least
two orders of magnitude greater than the noise level.
In the case of outboard-launch, the poloidal magnetic
field component is largest at MPlower and MPout . This result
suggests that the LHW injected from outboard side propagates downward. It has been observed previously that theRF magnetic field has a poloidal polarization around theselocations as expected from LHW launch [4]. After passing through outboard and bottom regions of the plasma,
the wave reaching the inboard side has a strong toroidal

3402107-2

Plasma and Fusion Research: Regular Articles

Volume 14, 3402107 (2019)

Fig. 4 Ip dependence of RF signal intensity for diﬀerent wave launching modes. The horizontal axis in the left figure represents thedistance from midplane downward, radially outward, then upward through RFMPs as shown on the right figure which also shows
the locations of RFMPs on the poloidal cross section. Symbols of circle and asterisk indicate the slit along toroidal and poloidal
direction, respectively. The vertical axis is the RF signal intensity normalized by the LHW injection power. The noise levelmeasured without RF power injection is shown for each probe.

polarization which now appears to correspond to the FW
polarization. The poloidally polarized component (corresponding to the LHW polarization) measured at the upper
inboard-side by Cpol and Apol becomes weaker at higher Ip .
This may indicate stronger damping of the LHW at higher
Ip predicted by the numerical analysis shown in the next
section.
In the case of top launch, the RF power intensity measured by RFMPs in the upper half of the inboard side isover 100 times greater than in the case of outboard launch.
The poloidal polarization is largest at Cpol . The RF power
intensity is two orders of magnitude less at Bø , compared
to Cpol which is right next to Bø . In the lower inboard region, toroidal polarization becomes larger. The measured
overall signal intensity becomes weaker as the distancefrom the top-launch antenna increases as expected naturally from the damping of LHW. The signal decreases athigher Ip which is consistent with stronger absorption predicted theoretically. Significant diﬀerences could not beseen between outboard launch and top launch by RFMPs
in the bottom and outboard regions.
Obvious diﬀerences can be seen for top launch when
the direction of Bt is reversed from CW to CCW (CCW Bt
corresponds to simulated bottom launch). For simulatedbottom launch, the degree of polarization is weaker, andthe RF signal intensity measured in the lower inboard region becomes larger than outboard launch and top launch.
This indicates that the absorption is substantially weaker
for CCW Bt compared to CW Bt . The RF signal intensity
decreases at lower half of the plasma by an order of magni-

tude at higher Ip , which suggests that stronger absorption
is recovered at higher Ip .

4. Comparison with Numerical
Calculation
Results of ray-tracing calculation using GENRAY areshown in Fig. 5. Each experimental sections performed inSec. 3 were simulated. The ray trajectories are projected
on a poloidal cross section, and the rays with 70% of themaximum power are shown. Figures 5 (a) and 5 (b) show
the outboard launch case with CW Bt with high Ip and low
Ip . The top launch case with CW Bt with high Ip and low
Ip and also CCW Bt with high Ip and low Ip are shown inFigs. 5 (c-f). The color bar indicates the degree of polarization towards Bz defined as


|Bz |
180
atan
θpol =,(1)
π|Bø |
where Bz and Bø are calculated from the dispersion relation.
For outboard launch case, rays have θpol = 60◦ polarization initially and propagate toward lower half of theplasma regardless of the Ip . This agrees with the experimental data shown in Sec. 3. However, the poloidal polarization becomes dominant close to the inboard side,
whereas in the experiment, toroidal polarization is dominant. This suggests that mode conversion of LHW to FW
is not accounted for properly in the numerical model. Further measurement and modeling are required to clarify thispoint.

3402107-3

Plasma and Fusion Research: Regular Articles

Volume 14, 3402107 (2019)

Fig. 6 Comparison of RF power which were measured experimentally and calculated numerically for top (CW) andtop (CCW) cases. The RF power is plotted in linear scale
and the ray power is normalized by the initial power.

reflected around z = +0.2 m propagate towards the plasma
center and to the outboard region, and are reflected at theoutboard edge again. At low Ip , rays which are reflectedaround z = +0.2 m propagate downward and fill the inboard bottom region more density compared to the high Ip
case as shown in Figs. 5 (e) and (f). These results agreewith the experimental observation that the RF signal intensity in the lower inboard region is an order of magnitude
higher at low Ip than at high Ip .
Fig. 5 Results of ray-tracing calculation using GENRAY for (a)outboard launch with CW Bt and high Ip , (b) with low Ip ,(c) top launch with CW Bt and high Ip , (d) CW Bt and low
Ip , (e) CCW Bt and high Ip , and (f) CCW Bt and low Ip .
The color bar indicates the degree of polarization towards
Bz (Eq. (1)).

For top launch with CW Bt , rays reach the upper inboard side around z = +0.2 m with dominantly poloidal
RF magnetic field polarization. In the range betweenz = +0.2 m and z = −0.2 m polarization becomes dominantly toroidal in the core, the rays turn around radially,and polarization becomes more poloidal near the inboard
wall. Toroidal, poloidal, and intermediate polarization exist in this region. This alternation of polarization direction
can also be seen in Fig. 4 for the top (CW) case. At low Ip ,up-shift of the parallel refractive index was weaker, resulting in weaker absorption. Therefore the RF signal intensity
is expected to be larger than high Ip .
For top launch with CCW Bt , at high Ip , the RF magnetic field polarization is dominantly poloidal in the upper inboard region. The wave power decreases as the rays
move downward. A comparison between the calculated raypowers and the measured RF powers are shown in Fig. 6
for top (CW) and top (CCW) cases. In this figure, the RF
power is plotted in linear scale and the ray power is normalized by the initial power. Damping of the ray power can beseen as rays travel downward. At high Ip , rays which are

5. Summary
Wave measurements were performed on TST-2 using
RFMPs for three diﬀerent modes of LHW excitation, outboard launch, top launch, and simulated bottom launch(top launch with reversed Bt ). In particular, RFMPs on
the inboard side could measure wave polarization. Experimentally, diﬀerent characteristics were observed depending on LHW excitation mode and Ip . Measured powers on
the outboard RFMPs and bottom RFMPs did not vary significantly among diﬀerent LHW excitation modes. In contrast, RF power distribution measured by inboard RFMPschanged drastically for diﬀerent excitation modes. RF
signal intensities detected by inboard RFMPs were muchstronger for top launch compared to outboard launch. RF
signal intensity and polarization were compared with results of numerical calculation.
For outboard launch with CW Bt and high Ip , resultsof ray-tracing calculation are partially consistent to experimental results that wave propagate in the lower plasma
region. On the other hand, the polarization of RF magnetic field on the inboard side did not agree with the raytracing calculation. Possibility of mode conversion to FW
needs to be investigated further. For top launch with CW
Bt and high Ip , results of ray-tracing calculation are consistent with experimental observation. RF magnetic fieldpolarization on the inboard side is dominantly poloidal in
the upper region, becomes more toroidal in the middle

3402107-4

Plasma and Fusion Research: Regular Articles

Volume 14, 3402107 (2019)

region near the midplane, and dominantly poloidal again
in the lower region. For top launch with CCW Bt , results of ray-tracing were again consistent with experimental data. The wave power is strong in the upper inboard
region. The comparison of ray powers shown in Fig. 6 implies that ramp up to higher current may be achieved forthe top (CCW) case than either the outboard case and thetop (CW) case since ray power damping is stronger. Rays
with down-shifted parallel wavenumber fill a broad rangeof edge plasma on the inboard side at low Ip , and the RF
signal intensity decreased as Ip increased. RF signal intensities detected by outboard and bottom RFMPs were similar for both top (CW) and top (CCW) cases.
In conclusion, spatial distributions of wave power,phase, frequency spectrum, and wave polarization weremeasured by thirteen RFMPs surrounding the plasma crosssection. In particular, wave propagation changed drastically depending on Ip except for the outboard launch case,but experimentally, the dependency of RF power distribution on Ip could not be seen apart from reduction in amplitude. In order to resolve this discrepancy and to makea more quantitative comparison, expansion of the RFMP
system for higher spatial and polarization coverage is necessary. This will contribute to identify optimum Ip ramp-up
scenario.

Acknowledgement
This work is supported by Grant-in-Aid for Scientific Research (S) (21226021), by National Institute for Fusion Science Collaboration Research
Programs NIFS14KOCR001, NIFS18KOAR022 andNIFS12KUTR078.
This work is also supported inpart by Japan/US Cooperation in Fusion Research andDevelopment and US DoE Cooperative Agreement
DE-FC02-04ER54698.
[1] Y. Takase et al., Nucl. Fusion 53, 063006 (2013).
[2] A.P. Smirnov et al., 1994 GENRAY Codewww.compxco.com/genray.html[3] S. Yajima et al., Plasma Fusion Res. 13, 1202093 (2018).
[4] T. Shinya et al., Plasma Fusion Res. 9, 3402040 (2014).

3402107-5