Plasma and Fusion Research: Regular Articles

Volume 11, 2402046 (2016)

Studies of AM He Atomic Magnetometer for Fusion Plasmas∗)
Hiroyuki KIYONAGA, Yoshinobu YAMADA, Yusuke IIDA and Takeshi FUKUDA
Graduated School of Engineering, Osaka University, A1-214, 2-1Yamadaoka, Suita, Osaka 565-0871, Japan(Received 28 November 2015 / Accepted 22 February 2016)
Amplitude modulated helium atomic magnetometers (AM He AOM) are proposed as a new method, whichallows spatially resolved magnetic field measurement in the inner regions of burning plasmas without heating
beams nor impurity atoms. In order to understand the fundamental properties of AM He AOM, we have developed
a prototype, and Helium discharge lamp as well as its power supply has been also elaborated as a light source.
1083 nm AM emission was successfully obtained in the frequency range between 40 kHz and 500 kHz. We have
performed an exploratory experiment of test magnetic field measurement with the developed AM He AOM. For
the test magnetic fields: 2.6 µT and 5.0 µT, possible signs of magnetic field have been observed.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: atomic magnetometer, optically pumped magnetometer, optically driven spin precession, burning
plasma diagnostics, magnetic turbulence
DOI: 10.1585/pfr.11.2402046
Table 1 Values of magnetic ratio for He, Rb and Cs.

1. Introduction
An atomic magnetometer (AOM), also called as an
optically pumped magnetometer, has ultra-high sensitivity, which is comparable to SQUID, and can work without
cryogenic cooling or under radiation environment. Therefore AOM is employed for space magnetometry or detection of biomagnetic fields [1]. In this paper a new application of AOM for fusion plasmas is suggested.
The detailed measurement of magnetic field is above
all fundamental in fusion research, and various approaches,
as represented by the Faraday rotation and motional stark
spectroscopy techniques, have hitherto been extensivelydeveloped.
An atomic magnetometer, in particular, amplitude
modulated helium atomic magnetometer (AM He AOM)
allows spatially resolved magnetic field measurementseven in the inner regions of burning plasmas, where helium
ions are produced from the D-T fusion reaction and part ofthem exist as meta stable 4 He (23 S1 ) atoms suggested as
probing targets in this work. Accordingly neither heating
beam nor impurity atoms is necessary. The principle ofAM-AOM is following.
Atomic magnetometers take advantage of the transmission property of light through spin polarized atoms; alkali atoms and helium atoms are often employed. Magnetic moments of these atoms precess around the magnetic
field direction and its frequency is fL called as Larmor
frequency or Zeeman transition frequency, which equals
γB. Here γ is gyromagnetic ratio and Table 1 shows γvalues of He and alkali metals. γ is determined by nuclear spin momentum and 28 GHz/T for 4 He [2]. Theseprecessions change their refractive indices periodically at
author’s e-mail: kiyonaga.hiroyuki@eb.see.eng.osaka-u.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

Fig. 1 Phase modulation of Larmor precessions through lightabsorption is maximized when intensity peak of AM lightbeam injects to the cell. After several peaks inject, inphase precessions are produced.

the frequency fL and modulate transmitted light property.
The maximum modulation is obtained when precessions
are synchronized, which leads to resonant properties of thetransmission rate. In the case of AM He AOM, these synchronized precessions are produced through the opticallypumping whose intensity modulated at fL and this process
is illustrated in Fig. 1. Target atoms in a glass cell absorb
injected circularly polarized light together with its angular momentum, which modifies their magnetic moments topoint in a certain direction (left in Fig. 1). When amplitudeof the injected light is modulated at the frequency fL , intensity of these modification as well as absorption is also2402046-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 2402046 (2016)

modulated at the same frequency, and accordingly in-phase
precessions are produced. The basic principle herein employed is referred to as Bell and Bloom techniques [3].
Figure 2 shows a schematic of the AM He AOM system. The pump and probe beams are injected into a He
discharge cell in which probing targets, meta stable 4 He
(23 S1 ) atoms, are produced. The wavelength of these lightbeams is 1083 nm (He D1 line). An amplitude of pumpbeam is modulated at frequency fpump and circularly polarized for the spin-polarization of the He atoms, whilst
the probe beam is only circularly polarized. When fpump
equals the Larmor frequency fL (= γB), Larmor precessions of He atoms in the discharge cell are synchronized
and the resonant property is detected in terms of transmission rate. From the resonant frequency, magnitude of themagnetic field at the cell is evaluated. In order to understand the fundamental properties of the proposed AM HeAOM, we have developed the prototype and conducted exploratory experiments of measuring test magnetic fields.

prototype, He discharge lamp and its power supply havebeen designed and elaborated in the lab and it produces
AM light emission (discussed in detail later). Collimatedbeam from the light source is injected into He discharged
cell (probing targets) after circularly polarized. The transmitted light intensity is detected by a photo diode amplifier circuit. Furthermore the signal is amplified with
Lock-in amplifier whose reference is synchronized withthe frequency of amplitude modulation. He discharge cellis installed inside a magnetic shielded box made of PC
permalloy (µ = 20000 H/m), which is a cube of 150 mmside length and 1.5 mm thick. This shielded box decreases
low frequency (< 20 Hz) magnetic noise produced by thesurrounding electronics from 30.6 µT to 0.94 µT (these are
RMS values and measured by Gauss Meter DSP425). Inside the box, a helmholtz coil which generates test magnetic field is also fixed as shown in Fig. 4. This test magnetic field is perpendicular to a beam direction, which maximizes resonant signals [4].

2. Experimental Setup

3. Development of a Light Source

Our prototype of AM He AOM is shown in Fig. 3. For
simplifying the setup, single beam configuration has been
adopted, with which magnitude of the magnetic field on
the beam pass can be measured. As a light source for thisA hollow cathode type He discharge lamp shown inFig. 5 (left) has been developed in the lab. During discharge, oxygen is released from the surface of an Al hollow
cathode, and it degrades He D1 emission intensity throughthe Penning eﬀect shown below [5].

Fig. 2 Schematic of AM He AOM System, which can measureabsolute magnitude of magnetic field at the cross point oftwo beams.

Fig. 4 Arrangement inside the magnetic field.

Fig. 3 Schematic of Experimental setup. In measuring magnetic field, high frequency Lock-in amplifier, SR844 is employed. In order
to investigate the property of the light source, instead of SR844, light chopper and low frequency Lock-in amplifier, SR510, areutilized for evaluating DC light intensity.

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He + electron → He∗ (23 S, 21 S) + electron,∗∗+He (2 S, 2 S) + O2 → He + O + O + electron.
31For extending a lifetime of the discharge lamp, an oxygen absorber (Fe) is enclosed inside the lamp as shown inFig. 5 (right). Cooling this absorber with Liquid nitrogen isalso adopted aimed at improving the eﬃciency of oxygen
absorption. These techniques have improved the lamp’s
lifetime successfully as indicated in Fig. 6 (left).
In a preliminary experiment, emission intensities ofFig. 5 (left) A manufactured hollow cathode lamp. (right) An
oxygen absorber inside the discharge lamp.

Fig. 6 (left) Time development of 1083 nm emission intensity,
which was measured with NIR2b-14. (right) Comparison
between emission intensities of diﬀerent diameter lamps:
12, 18, 30 mm.

diﬀerent diameter lamps (12, 18, 30 mm) were evaluated.
As depicted in Fig. 6 (right), smaller diameter lamp had thelarger emission intensity. Therefore 12 mm diameter lamp
has been adopted as a light source.
Power supply of the discharge lamp has been also designed and built. The circuit diagram is shown in Fig. 7.
This power supply can produce frequency tunable AC voltage with DC voltage oﬀset which is high enough to discharge the lamp. Fig. 8 (left) shows the output waveform
at 100 kHz. The input rectangular signal from the function
generator, whose amplitude is 8 V, is amplified to about
80 V with the emitter amplifier circuit. Figure 8 (right) indicates AM emission intensity of this light source. The intensity was measured in the optical system shown in Fig. 3with the Lock-in amp, SR844. Though the intensity decreases in high frequency region possibly due to the restriction of discharge process, frequency tunable AM He D1
emission has been successfully obtained in the frequency
range between 40 kHz and 500 kHz.
Whilst AC component of the injected light into a helium discharge lamp (probing targets) synchronizes spin
precessions of atoms, DC component destroys these syn-Fig. 8 (left) The output waveform of the developed DC+ACpower supply at 100 kHz. (right) AC emission intensity
versus frequency.

Fig. 7 Circuit diagram of the developed DC+AC power supply depicted with LT-SPICE. V1 is a function generator, E3612A and V2 is a
DC power supply, 33120A. V3 is a double voltage rectifier circuit manufactured in the lab.

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Fig. 10 Transmission rate versus AM frequency.
Fig. 9 AC/DC ratio versus AM frequency.

chronized precessions. Therefore higher ratio of AC to DCcomponent (AC/DC ratio) is required for improving resonant signals. The AC/DC ratio for the developed lightsource was calculated from AC emission intensity depictedin Fig. 8 and DC emission intensity, which was measured
in the optical system shown in Fig. 3 with the light chopper and the Lock-in amp, SR511. As shown in Fig. 9, ourdeveloped light source has an about 7% AC/DC ratio at
100 kHz.

4. Results of Magnetic Field Measurement
With the developed AM He AOM prototype, we have
performed an exploratory experiment of test magnetic fieldmeasurement. The sweep rate of AM frequency is fixed at
1/3 kHz/s. In the experiment, transmitted light intensity ismeasured for activated probing targets (discharged absorption cell) together with non-activated targets, and transmission rate is calculated from these values.
Figure 10 shows the transmission rate versus AM frequency. Around 70 kHz (left) and 130 kHz (right), thesesignals seem to have their peaks, which correspond to2.5 µT and 4.6 µT respectively. These values roughly equal
to magnitude of test magnetic field: 2.6 µT (left) and 5.0 µT(right). Consequently possible signs of magnetic field havebeen observed with the developed prototype. The left signal has another peak around 140 kHz. However this is presumably a noise related with instability of the discharge
lamps.

5. Summary and Discussion
Amplitude modulated helium atomic magnetometers(AM He AOM) are proposed as a new method to measuremagnetic field in burning plasmas. In order to understandfundamental properties of AM He AOM, the prototype has
been developed in the lab. With the developed tunable frequency AM light source, whose wave length is 1083 nm(He D1 line), AM light emission has been successfully obtained between 40 kHz and 500 kHz, and its AC/DC ratio
is about 7%.
An exploratory experiment with test magnetic fieldhas been conducted. Through this measurement, possible signs of magnetic field have been observed for a few
micro tesla test fields: 2.6 µT and 5.0 µT. These resonant
signals are degraded possibly due to impurity atoms, inparticular, oxygen which is released from electrodes in ahelium discharged lamp. Therefore we suppose that thenon-electrode absorption cell, which takes advantage of inductively coupled helium plasma, is eﬀective for improving the signal quality.
For application to fusion plasmas, the amount of probing targets (He atoms) in the core must be considered.
Most of helium atoms produced from D-T reaction exist asions in the inner regions due to extremely high temperatureof burning plasmas. Therefore the quantitative relation between probing target’s density and signal intensity of AM
He AOM should be investigated in order to examine possibility of this application. In addition to that, since hitherto
AOM is generally employed for relatively small magnetic
fields measurement (under dozens of micro tesla), research
toward measurement of a few tesla magnetic fields, whichcorresponds to confinement magnetic fields, with AM He
AOM is also required.
[1] D. Budker and D.F. Jackson Kimball, Optical Magnetometry, Chp.15, 16 (Cambridge university press, Cambridge,2013).
[2] I.M. Savukov et al., Phys. Rev. Lett. 95, 063004 (2005).
[3] W.E. Bell and H.L. Bloom, Phys. Rev. Lett. 6, 280 (1961).
[4] V. Schultze et al., Optics Express 20, 13 (2012).
[5] M. Kogoma, J. Plasma Fusion Res. 79, 10 (2003).

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