Proceedings of ITC/ISHW2007

The development of potential measurementswith 6 MeV Heavy Ion Beam Probe on LHD
A. Shimizu, T. Ido, M. Nishiura, H. Nakano, S. Ohshima, I. Yamada, K. Narihara,
T. Akiyama, T. Tokuzawa, K. Tanaka, K. Kawahata, H. Igama, Y. Yoshimura, T. Shimozuma,
S. Kubo, K. Nagaoka, K. Ikeda, M. Osakabe, K. Tsumori, Y. Takeiri, O. Kaneko, S. Morita, M. Goto,
M. Yokoyama, S. Kato, M. Yokota, K. Tsukada, H. Ogawa, T. Inoue,
A. Nishizawa1), Y. Hamada and LHD Group
National Institute for Fusion Science, Oroshi-cho, Toki, Gifu,509-5292, Japan1)Tono Geoscience Center, Japan Atomic Energy Agency, Izumi-cho, Toki, Gifu,509-5102, Japan(15th Oct. 2007)
In order to measure the potential in Large Helical Device (LHD), 6 MeV heavy ion beam probe (HIBP) hasbeen developed. The temporal evolvements of potential in the central region of plasmas, which were produced byneutral beam injection (NBI) and additionally heated by electron cyclotron heating (ECH), were measured with ourHIBP system. The line averaged density of these shots were about 1 ~ 5 × 1018 m-3. At the time of ECH wasapplied, a rapid change of potential to positive direction was observed. In this ECH phase, negative pulses wereobserved in the potential signal, which were very similar to "pulsation" that was firstly found in Compact HelicalSystem (CHS), however, the time constant of these pulses were larger than in the case of CHS. In the NBI phase,the fluctuation was observed with HIBP, of which frequency was correlated with potential of equilibrium. It isconsidered that this frequency was influenced by E × B drift.
Keywords: Heavy Ion Beam Probe, Large Helical Device, helical system, plasma potential, electric field,Internal Transport Barrier

1. Inroduction
In helical magnetic configurations, fluxes are not
intrinsically ambipolar and the radial electric field is
produced [1, 2]. In the neoclassical theory, the constraintthat the ion and electron fluxes be equal determines theradial electric field. The theory gives two roots, so called"ion root" and "electron root". By the strong core heating
by ECH, the internal transport barrier (ITB) / the coreelectron-root confinement (CERC) [3], which is an
improved confinement mode, has been realized in various
helical devices [4-7]. The study of radial electric fieldand transport of ITB has been continued in these machines.
In order to investigate the physics of ITB in detail, we havebeen developing heavy ion beam probe (HIBP) system inlarge helical device (LHD) [8, 9].
By using HIBP, the potential in the inside of hightemperature plasma can be measured with good spatial andtemporal resolutions, without any disturbances to plasma.
Moreover, density fluctuation can be measured
simultaneously with potential fluctuation. Therefore flux
caused by electrostatic fluctuation can be estimatedexperimentally. In this article, we will report the present
status of HIBP system and recent results of potential
author’s e-mail: akihiro@nifs.ac.jp

measurements in LHD.

2. Heavy Ion Beam Probe System in LHD
Heavy ion beam probe diagnostics is based on theenergy conservation low. A beam of single charged
heavy ion is injected to plasma and the beam of doubly
charged ion arises by the collision with plasma. Theenergy of the beam of doubly charged ion coming fromplasma is measured in the outside. The change of energy
between singly charged ion and doubly charged ion
corresponds to potential at the ionized point. In order toinject heavy ion beam to the center of LHD plasma, theacceleration energy of 6 MeV is needed for the probing
beam of Au+ when toroidal magnetic field strength is 3 T.
In our HIBP system, to reduce the required voltage ofaccelerator, we use a tandem accelerator and decrease therequired voltage to be half (3 MV). In order to use
tandem accelerator, negative gold ions (Au-) are needed.
Au- ions are produced by an ion source of sputtering type.
Detail of our negative ion source system is shown in Ref.
[10].
The Au- extracted from this ion source ispre-accelerated up to 50 keV. It is injected to tandem
accelerator and accelerated by the voltage of 3 MV. In

Proceedings of ITC/ISHW2007
the gas cell located at the center of the accelerator, Au- ionis stripped two electrons, and changed to positive gold ion(Au+). Au+ is re-accelerated by the voltage of 3 MV, so
the 6 MeV Au+ beam is obtained. This beam is guided toplasma through several components: a charge separator, the4.8 m cylindrical deflector, the 7.8 degree deflector. At
the incident port, the beam incident angle can be changedby octapole deflector, by which the observation point iscontrolled. Injected Au+ is stripped an electron by thecollision with plasma, and Au2+ beam is produced. Here,
we call the former the primary beam, and the latter thesecondary beam. At the exit port of chamber, the ejection
angle of the secondary beam is controlled by anotheroctapole deflector to direct the beam to the energy analyzer.
We have 3 slit holes at the entrance of this analyzer, so thepotential of neighboring 3 points in plasma can bemeasured. For the energy analyzer, we apply the tandem
energy analyzer [11] to suppress the required voltage andcosts. For detecting beam, high gain detector, microchannel plates (MCPs) are used, by which a very small
amount of secondary beam current, order of a few pA, canbe detected. Detail of our HIBP system is shown in Ref.
[8,9].

The configuration of LHD is characterized by the strengthof toroidal magnetic field, Bt, the major radius of magnetic
axis, Rax, the pitch parameter γ, and the quadrupole
magnetic field Bq. In this experiment, these parameters
were as follows: Bt = 1.5 T, Rax = 3.6 m, γ = 1.254, Bq =100 %. The acceleration energy of beam was 1.5 MeV.
Temporal evolutions of potential at the center of plasmameasured with HIBP are shown in Fig. 1 (b). At thebeginning of discharge, the potential was negative. Whenthe additional ECH was applied, the abrupt increase of
potential was observed. It is considered that this positive
potential was generated by the creation of ITB in thecentral region.
Temperature profiles with Thomson
scattering diagnostics before and during ECH are shown inFig. 1 (c), low density case, and (d), high density case.
High temperature region in the core was due to the creationof ITB. The increase of temperature in the low density
case was larger than in the high density case. The ratio oftemperature increase between high density case and low
density case coincides with the ratio of potential increase
between these two cases.

4. Fast temporal change of potential
When the ITB appears and multiple states of electron
and ion root are allowed, the bifurcation phenomenon canbe seen and the fast change of potential is observed.
When plasma parameters satisfied some condition, thenegative pulses were observed in the potential. In Fig. 2,
the observed signal measured with HIBP is shown. Thissignal is very similar to the pulsation which was firstlyfound in Compact Helical System (CHS) [12]. The pulse
was repeated about a few ms. The typical time constantof the pulse was 90 µs at the potential drop phase and 500µs at the returning phase as shown in Fig 3. These are

3. Potential measurements of internal transportbarrier
As described above, strong core heating with electron
cyclotron heating (ECH) produces an improved
confinement region, ITB, which is related to an electronroot. Temporal evolution of density and heating, when
ITB was observed in LHD, are shown in Fig. 1 (a). The
plasma was produced by neutral beam injection (NBI)
heating, and ECH was applied at the time of 1 sec. In thefigure, two shots which had different density are shown.
1.56#70222#70224150.5NBIb)

2
Potential (kV)

0.945s
1.3s

132
1ECH

065

#70222#70224Te (keV)030-14d)#702240.945s1.3s

32
1-2-3c)#70222

4Te (keV)ne (1019 m-3)

a)

00.5

11.5Time(sec)

202.5

33.5
R (m)

44.5

Fig.1 (a) Temporal evolutionsof line averaged density andheating. (b) Temporal evolutions of potential measured withHIBP. (c) Temperature profiles of low density case(#70222), and (d), high density
case (#70224).

Proceedings of ITC/ISHW2007
larger than in CHS. This time constant is determined by
the neoclassical theory, we will compare this experimental
result with the theoretical prediction in a future.
The plasma parameters, in which the ITB andnegative pulses were observed, are compared with theneoclassical theory as shown in Fig. 4. In the theoreticalestimation, the electron and ion temperature profiles are
assumed as Ti,e(0) ( 1 - ρ2 ). And density profile isassumed as ne(0) ( 1 - ρ8 ). Here, ρ is a normalized minorradius. From the neoclassical theoretical prediction, Ti Te space is separated to three regions, in which ion root,
electron root and multiple roots are expected. In thefigure, boundary lines of these domains are shown.
Points show the experimental data, when negative pulses
were observed with ITB, no negative pulses with ITB, andno ITB was observed (NBI phase L-mode). In NBIphase, the parameters exist in the ion root region. Thepulses were observed in the region of the multiple roots.
As shown in Ref. [12], the pulses are considered to occur
in the region of multiple roots, so the experimental results

ITB w/o Pulses
ITB with Pulses
L mode (NB)

Te (keV)3

Potential (kV)0

Potential (kV)

0.4

NBI#20
1.2
1.4

ECH
1.6

NBI#3

1.8
2.0
Time (sec)

2.2

1.84
1.86
Time (sec)

1.88

0.0

2.4

Potential (kV)

4.0
3.5
3.0
2.5
2.0
1.80

1.82

1.90Fig.2 Negative pulses were observed in potential signal
measured with HIBP.

Potential (kV)

4.0

τ = 90 µs

3.0τ = 500 µs
2.5
2.0
1.845

1.846

1.847
1.848
Time (sec)

1.849

1.850

Fig.3 .Time constant of potential change is shown.

-3

0.2

1

3.5

0.5

1Ti (keV)

1.5

2in LHD are consistent with the neoclassical theory.

19ne (10 m )ne0

Fig.4 Electron temperature Te and ion temperature
Ti are compared with the neoclassical theory.
Boundary lines separate Ti - Te space to threedomains, electron root, ion root, and multiple
roots. Negative pulses in potential were observedin the region of multiple roots.

0.6

2

2

Ion root#75087

3Multipleroots

Electronroot

1、

4#754584

4. Fluctuation measurements with HIBPHIBP has a good temporal/spatial resolution in themeasurements, so if large S/N ratio is realized, the physics
related to various fluctuations, such as MHD instability
and micro instability, can be studied with this tool. WithHIBP, the measured fluctuation of potential is local one, sointerpretation is easy. However the density fluctuation
measured with HIBP includes an effect of fluctuation on
the beam path as well as at the observed point, due to socalled "path integral effect". This effect in LHD isconsidered in Ref [13] in detail, so we do not discuss about
it here.
In Fig. 5 (a), the temporal evolution of potential
fluctuation spectrum measured with HIBP in LHD isshown. The vertical axis shows frequency and the color
in the image shows power density of potential fluctuation.
The fluctuation, of which frequency was changed
temporally, can be seen in this figure. In Fig. 5 (b), thewave forms of line averaged density and heating are shown.
The frequency of fluctuation has a weak correlation withdensity, however it is not clear. The frequency correlates
more strongly with potential rather than density. In Fig. 5(c), the temporal evolution of frequency, at which thefluctuation power density has maximum, is shown. The
measured potential in the central region of plasma is alsoshown. The temporal changes of these signals are very

Proceedings of ITC/ISHW2007
similar except the slight time delay in the change offrequency at 0.75 sec. It seems that the rotation of E × B
drift makes an effect on the frequency of fluctuation. The
slight difference seen in Fig. 5 (c) may occur from thedifference between the potential and electric field. Inorder to make clearer this effect, we need to measure aradial electric field, namely radial profile of potential.
However, the profile could not obtain with a reliableaccuracy because of the problem of diagnostics. The
temporal character of current amplifier was also slow inthis experiment. The improvement of the amplifier andthe improved measurements of profile will be done in anear future.

with our system. The rapid increase of potential wasobserved in ECH phase. The fast negative pulses wereobserved in potential signal in ECH phase. It was very
similar to pulsation, which was firstly found in CHS. The
plasma parameter region, in which these pulses wereobserved, coincides with the region where multiple roots
are predicted from the neoclassical theory.
Thefluctuation of potential, of which frequency changedtemporally, was observed.
This frequency wasconsidered to correlate with E × B flow.

Acknowledgements
This work was supported by NIFS under contract No.
NIFS07ULBB505, 514, 515. The authors with to thankscientific and technical staff of LHD and CHS.

5. Summary
In LHD, heavy ion beam probe system has been
continued to develop.
The temporal evolution ofpotential, when ITB was created by ECH, was measured

References

Shot#:75462

30Frequency (kHz)25[1] L. M. Kovrizhnykh, Nuclear Fusion 24, 851 (1984).
[2] D. E. Hastings, W. A. Houlberg, K. C. Shaing, Nuclear
Fusion 25, 445 (1985).
[3] M. Yokoyama, H. Maasberg, K. Ida, C. D. Beidler, et al.,"Core Electron-Root Confinement (CERC) in HelicalPlasmas", IAEA, 2006.

(a)

2015
1050
0.4

0.5

0.6

1.2

1.4

(b)

0.4

ne-3ne (10 m )

190.8
1.0
Time (sec)

0.3
0.2

ECH

0.1

NBI

0.0
0.4

Potential (kV)0.8
1.0Time (sec)

1.2

1.4

#75462(c)40
Potential

220
Frequency

000.4

0.6

0.8
1Time (sec)

1.2

1.4

Fig.5 (a) Temporal evolution of spectrum ofpotential fluctuation measured with HIBP. (b) Lineaveraged density and heating. (c) Temporal
evolution of potential and frequency wherefluctuation power density has maximum.

Frequency (kHz)40.6[4] Maasberg et al., Phys. Plasmas 7, 295 (2000).
[5] T. Shimozuma, S. Kubo, H. Idei, et al., Nuclear
Fusion 45, 1396 (2005).
[6] T. Estrada, A. Lonso, et al., Plasma Phys. Control.
Fusion 47, L57 (2005).
[7] A. Fujisawa, et al., Phys. Plasmas 7, 4152 (2000).
[8] T. Ido, et al., Rev. Sci. Instruments 77, 10F523(2006).
[9] A. Shimizu, et al., Plasma Fusion Res. SERIES to bepublished (2007).
[10] M. Nishiura, T. Ido, A. Shimizu, et al., "Beamplasma interaction in high temperature plasmas for aheavy ion beam probe diagnostics" in thisconference.
[11] Y. Hamada, et al., Rev. Sci. Instruments 68, 2020(1997).
[12] A. Fujisawa, et al., Phys. Rev. Lett. 81, 2256 (1998).
[13] H. Nakano, T. Ido, A. Shimizu, M. Nishiura, et al.,"Consideration of density fluctuation measurement
using Heavy Ion Beam Probe on Large HelicalDevice" in this conference.