Plasma and Fusion Research: Regular Articles

Volume 11, 2402123 (2016)

Development of 2D Potential Profile Measurements Using theHeavy Ion Beam Probe on the Large Helical Device∗)
Akihiro SHIMIZU1) , Takeshi IDO1,2) , Masaki NISHIURA3) , Hiromi TAKAHASHI1) ,Hiroe IGAMI1) , Yasuo YOSHIMURA1) , Shin KUBO1,2) , Takashi SHIMOZUMA1) ,Shinji KATO1) , Masayuki YOKOYAMA1,4) and LHD Experiment Group1)
1)

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-0814, Japan3)
Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan4)
The Graduate University for Advanced Studies (Sokendai), 322-6 Oroshi-cho, Toki 509-5292, Japan2)(Received 30 November 2015 / Accepted 13 July 2016)
Two-dimensional potential fluctuation profiles were measured with the heavy ion beam probe (HIBP) on theLarge Helical Device. For two-dimensional measurements with HIBP, the probe beam energy had to be changed,and subtle adjustment of the beam trajectory along the long beam transport line (∼20 m) was required. An
automatic beam adjustment system was developed for the beam transport line, and used to easily adjust the probe
beam trajectory. Potential fluctuations in the high-frequency range (∼200 kHz) were measured, and the probe
beam energy was changed shot to shot. Two-dimensional potential phase structures were successfully obtainedby analyzing the coherence between potential and magnetic field fluctuation, which is used as a reference signal.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: heavy ion beam probe (HIBP), two-dimensional potential profile, helical device, the Large HelicalDevice (LHD)
DOI: 10.1585/pfr.11.2402123

1. Introduction
In magnetically confined toroidal plasmas, the radial
electric field Er is an extremely important parameter because it generally influences the plasma’s transport property. For example, in the 1/ν regime of the helical device,
the large particle flux, which can be predicted by neoclassical theory, is suppressed by ambipolar Er [1, 2]. The Er
is correlated with the plasma flow through the E × B drift,
and the shear flow structure can improve the confinementproperties of the plasma by reducing its turbulence [3], as
has been experimentally observed in the H-mode and internal transport barrier (ITB) [4, 5]. In recent theoretical research, the eﬀect of the mesoscale structure of the plasma
flow, such as zonal flows and streamers, has been considered important in accurately estimating the turbulent transport [6, 7]. More precise measurements of the flow structure are expected to compare the experimental results withtheoretical predictions in detail in order to study and clarify
the physics of the zonal flow and their eﬀect on anomaloustransport.
A heavy ion beam probe (HIBP) [8] is a useful tool
for studying the physics related to Er because it can measure directly the plasma potential with excellent temporaland spatial resolution. Usually, in HIBP diagnostics, theprobe beam energy is fixed and the incident angle of probe
author’s e-mail: akihiro@nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

beam is varied to measure a one-dimensional potential profile. If the probe beam energy is changed additionally, theobservable region is extended and a two-dimensional potential profile can be obtained. In the Large Helical Device (LHD), an HIBP system has been developed to study
Er formation physics [9, 10]. This system has a considerably long beam transport line (∼20 m), and thus, it isdiﬃcult to change the probe beam energy as many electrostatic deflectors and lenses on the beam transport line
must be adjusted to optimize the beam trajectory along thebeam transport line. If this beam adjustment is manuallyperformed, 20 - 30 min are required to optimize the beamtrajectory, with much time being consumed to adjust thevoltages of many electrostatic deflectors. To reduce thisadjustment time, a PC-based automatic adjustment system
was developed [11]. Using this system, the required time
for this optimization has been reduced to under 3 min;therefore, we can easily change the probe beam energy for
two-dimensional potential profile measurements. In recent
analysis, the two-dimensional phase structures of potential
fluctuations were obtained. The detail of the automatic adjustment system and experimental results are presented inthis article.

2. HIBP System on the LHD and theAutomatic Beam Adjustment System

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The detailed principles of the HIBP diagnostics werec 2016 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 11, 2402123 (2016)

reviewed in Ref. 8, and thus, only a brief explanation isprovided here. For the HIBP diagnostics, the primarybeam, mostly comprising singly charged ions, is injected
into the plasma. The ions of the primary beam are further ionized, and secondary beam of doubly charged ionsis created. The secondary beam ejected from the plasma
is guided to an energy analyzer to measure its beam energy. Based on the energy conservation law, the energy
diﬀerence between the injected primary beam and the detected secondary beam corresponds to the plasma potential
at the ionization point from the primary to the secondary.
Therefore, we can measure the local potential in the plasma
by analyzing the energy of the secondary beam. The current intensity of the detected secondary beam reflects theplasma density at the ionization point, although the path
integral eﬀect is contained since the beam is attenuatedalong the beam path. With HIBP, the density fluctuation
can be simultaneously measured with the potential fluctuation, which is an advantage for this diagnostics because
the particle flux driven by electrostatic fluctuation can beevaluated using the coherence analysis of the density andpotential fluctuations.
At the LHD, the required probe beam energy reaches
MeV order because the maximum toroidal magnetic fieldstrength is 3 T and the minor radius of the plasma is approximately 0.6 m. We use the tandem accelerator to generate such a high-energy probe beam. This beam is guided
from the accelerator to the injection port of the LHD
through the beam transport line, which is approximately
20 m in length. A schematic of the LHD-HIBP system isshown in Fig. 1.
The observation point of HIBP, which is called sample
volume, is usually varied by controlling the incident angleof the probe beam. This angle is changed by an octupoleelectrostatic deflector. In Fig. 2, the observation region ofthe LHD-HIBP is shown. The actual observation points
are arranged three-dimensionally; thus, the projections of
those points on the vertically elongated cross-section are
shown for the ease of understanding. In addition, the partto the right in Fig. 2 shows the projected vertical position
Z as a function of the toroidal angle, indicating the actual observation region. When the probe beam energy, Eb ,was fixed at 1.176 MeV, the observation point was scannedalong the line of 1.176 MeV by changing the incident angle of the probe beam. If Eb is fixed at another value, theobservation point is scanned along another line. Therefore,
the two-dimensional potential profile can be measured bychanging Eb in addition to the beam incident angle.
In the LHD-HIBP system, the length of the beamtransport line from the accelerator to the probe beam injection port is approximately 20 m, and we have many electrostatic deflectors and lenses to adjust the beam trajectoryalong the beam transport line. When Eb is changed, theseelectrostatic deflectors and lenses must be adjusted. Basically, the voltages of these deflectors are changed in proportion to Eb ; however, in the region near the LHD, theFig. 1 Schematic of LHD-HIBP system.

Fig. 2 The observation regions of LHD-HIBP system areshown. The right figure shows the projections of observation regions onto a vertically elongated cross-section
whereas left figure shows projected Z as a function of
the actual toroidal angle of observation region. Observation point can be scanned along the line of corresponding beam energy by changing the incident angle of probebeam.

leak magnetic field from the LHD influences the beam trajectory and this magnetic field cannot be changed with Eb
as its strength is usually fixed. Thus, subtle adjustments arerequired for electrostatic deflectors in the region near theLHD. To easily adjust the voltages of many high-voltage
power supplies, a PC-based automatic adjustment system
was developed. The displacements of the probe beam at
position i are defined as Δxi , Δyi . Here, the x and y axes areorthogonal on the plane perpendicular to the probe beam.
The changes in the voltages of the deflector j are defined as
ΔV x j , ΔVy j . Basically, the displacements of the beam linearly depend on the deflector voltages; therefore, the following relation can be obtained:2402123-2

Plasma and Fusion Research: Regular Articles⎛⎞ ⎛⎜⎜⎜ Δx1 ⎟⎟⎟ ⎜⎜⎜ A11⎜⎜⎜⎜ Δy ⎟⎟⎟⎟ ⎜⎜⎜⎜ A1 ⎟⎜⎜⎜
⎜ 21⎜⎜⎜ Δx2 ⎟⎟⎟⎟⎟ = ⎜⎜⎜⎜⎜ A31⎜⎝⎟⎠ ⎜⎝Δy2A41A12A22A32A42A13A23A33A43

Volume 11, 2402123 (2016)⎞⎛⎞
A14 ⎟⎟⎟ ⎜⎜⎜ ΔV x1 ⎟⎟⎟⎟⎜⎟A24 ⎟⎟⎟⎟ ⎜⎜⎜⎜ ΔVy1 ⎟⎟⎟⎟⎟⎟⎟ ⎜⎜⎜⎟.
A34 ⎟⎟⎟ ⎜⎜⎜ ΔV x2 ⎟⎟⎟⎟⎟⎠⎝
⎠
A44
ΔVy2

In this case, two displacements and deflectors are considered, and for more displacements and deflections, the sizeof the matrix would need to be enlarged. The matrix Ai j
represents the transport matrix. Beam displacements on
the transport line are measured with beam profile monitors(BPMs), which detect the beam current with helical rotating wires. Raw signals from BPMs are acquired to the PC
through the ADC board with a sample frequency of 1 kHz.
Beam displacements are analyzed from these raw signals,
and the required voltages are calculated to adjust the beamdisplacements from the center of the beam transport chamber to be zero in all BPMs. The required voltages are calculated using the inverse matrix of Ai j , i.e., ΔV j = A−1ij
Δxi . The calculated voltages are applied to high-voltage
power supplies through CAMAC DAC modules connectedto the PC with the general purpose interface bus (GPIB).
One cycle of this adjustment requires a few seconds. This
procedure is then repeated until beam displacement at all
BPMs becomes less than 1 mm. Visual Basic in the Windows operating system is used to develop the optimization
system on the PC. Using this system, the required time forthe beam adjustment can be reduced to less than 3 min. Inthe LHD, the period of the discharge cycle is 3 min; thus,
the system enables us to change the probe beam energy Eb
between each shot.

The parameters of the LHD’s magnetic configuration
are as follows: the toroidal magnetic field strength Bt is
1.375 T, the major radius of the magnetic axis, Rax , is3.75 m, the quadrupole component of the magnetic field,
Bq , is 100 %, and the pitch parameter, γ, is 1.254. The time
evolutions of typical discharge’s parameters are shown inFig. 3. The plasma is produced by balance NBI heating due
to NBI#1 (counter-injection) and NBI#2 (co-injection).
The short pulse of the ECH is applied with the durationof 4.5 - 4.9 s. At 5.3 s, NBI#1 and #2 are turned oﬀ andNBI#3 (counter-injection) is turned on. The line averaged electron density is approximately 0.4 × 1019 m−3 ,and the central electron temperature is approximately 1.5keV. The spectrogram of magnetic fluctuation from themagnetic probe is shown in Fig. 4 (a). Figure 4 (b) is anenlarged graphic of Fig. 4 (a). Although many coherent
modes can be observed, we focus on the high frequency
mode (∼200 kHz) indicated by an arrow in Fig. 4 (b) as

3. Experimental Results of TwoDimensional Potential Fluctuation
Measurements
The two-dimensional potential profiles were obtainedusing the automatic beam adjustment system. For the twodimensional equilibrium potential profile measurements,
experimental results have already been shown in Ref. 11.
Here, the experimental results of the two-dimensional potential fluctuation measurements are described.

Fig. 3 The temporal evolutions of various heating methods, thecentral electron temperature, and the line-averaged electron density are shown.

Fig. 4 (a) The spectrogram of magnetic field fluctuation from a magnetic probe, (b) the spectrogram of an enlarged graphic of (a), and(c) the spectrogram of potential fluctuation from the LHD-HIBP system.

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Plasma and Fusion Research: Regular Articles

Volume 11, 2402123 (2016)
improved, and a more detailed mode structure (e.g., theamplitude profile of the potential and density fluctuations)will be analyzed to understand the physical mechanism of
this mode.

4. Summary
An automatic beam adjustment system was developed
to easily change the probe beam energy in the LHD-HIBP
system. Applying this system to the LHD-HIBP system,
Eb could be changed within 3 min, which is an interval oftime in the LHD discharge cycle. In LHD experiments, Eb
was changed from shot to shot and the two-dimensional
potential fluctuation structure of a high-frequency mode(∼200 kHz) was measured. Coherence analysis between
the potential fluctuation and the magnetic field fluctuation
signals allowed the two-dimensional phase structure of potential fluctuation to be successfully obtained.

Acknowledgments

Fig. 5 The two-dimensional phase structure of potential fluctuation with a frequency of approximately 200 kHz. Magnetic surfaces under the vacuum condition are also shown
for references.

this mode is observed in the potential fluctuation signal
from the HIBP (Fig. 4 (c)). The incident angle of the probe
beam is scanned at a frequency of 10 Hz. The potential
fluctuation of this mode is localized in the central regionof the plasma. From coherence analysis between the HIBP
potential fluctuation signal and the magnetic field fluctuation signal (i.e., a reference signal), the two-dimensional
phase structures were obtained. To obtain the experimental result, data from six shots were used, with the probe
beam energy Eb scanned from shot to shot. In Fig. 5, an
experimental result of two-dimensional phase structure isshown with vacuum magnetic surfaces. The reproducibility of the shots was not as expected. However, the poloidal
mode number of this fluctuation is considered to be 2. The
two-dimensional phase structure of the potential fluctuation was successfully measured with our HIBP system, although the physical aspect of the mode has not been interpreted. In the future, the signal to noise ratio will beThis study was supported by NIFS under Contract
Nos. ULHH 020 and 23, and by JSPS KAKENHI Grant
Nos. 25820441, 15K06653 and 16K06946. The authors
are grateful for the continuous support from staﬀ in theLHD experiment group.
[1] M. Yokoyama, H. Maassberg, C.D. Beidler et al., Nucl.
Fusion 47, 1213 (2007).
[2] L.M. Kovrizhnykh, Nucl. Fusion 24, 851 (1984).
[3] K.H. Burrell, Phys. Plasmas 4, 1499 (1997).
[4] A. Fujisawa, H. Iguchi, T. Minami et al., Phys. Rev. Lett.
82, 2669 (1999).
[5] T. Ido, K. Kamiya, Y. Miura, Y. Hamada, A. Nishizawa andY. Kawasumi, Phys. Rev. Lett. 88, 055006 (2002).
[6] P.H. Diamond, S.-I. Itoh, K. Itoh and T.S. Hahm, Plasma
Phys. Control. Fusion 47, R35 (2005).
[7] A. Fujisawa, T. Ido, A. Shimizu, S. Okamura, K. Matsuokaet al., Nucl. Fusion 47, S718 (2007).
[8] T.P. Crowley et al., IEEE Trans. Plasma Sci. 22, 291(1994).
[9] T. Ido, A. Shimizu, M. Nishiura, A. Nishizawa, S. Katoh,
K. Tsukada, M. Yokota, H. Ogawa et al., Rev. Sci. Instrum.
77, 10F523 (2006).
[10] A. Shimizu, T. Ido, M. Nishiura, H. Nakano, I. Yamada,K. Narihara, T. Akiyama, T. Tokuzawa, K. Tanaka et al., J.
Plasma Fusion Res. 2, S1098 (2007).
[11] A. Shimizu, T. Ido, M. Kurachi, R. Makino, S. Kato et al.,Rev. Sci. Instrum. 85, 11D853 (2014).

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