Plasma and Fusion Research: Regular Articles

Volume 14, 1402128 (2019)

Fast Tangentially Viewed Soft X-Ray Imaging System Based on
Image Intensifier with Microchannel Plate Detector on QUEST
Canbin HUANG, Kazuaki HANADA1) , Kengoh KURODA1) , Shinichro KOJIMA1) ,Hiroaki FUJIYOSHI1) , Hiroki MIURA1) , Tomoki YAMADA1) , Hiroshi IDEI1) ,Makoto HASEGAWA1) and Takumi ONCHI1)Interdisciplinary Graduate School of Engineering and Sciences, Kyushu University, Kasuga 812-8580, Japan
1)
Research Institute for Applied Mechanics, Kyushu University, Kasuga 812-8580, Japan(Received 15 April 2019 / Accepted 11 June 2019)
A two-dimensional (2D) soft X-ray (SXR) imaging system for obtaining tangential views of plasmaproduced SXR has been installed on the QUEST tokamak. The system comprises a chevron micro-channel
plate (MCP) and a phosphor screen, which together serve as an SXR detector and image intensifier, along with ahigh-speed video camera for capturing and storing image data. The system can be used in imaging mode to obtainhigh spatial- and time-resolution images or in photon counting mode at an ultra-high framing rate of 100 kHz toobtain SXR energy spectra. Here, we propose a method of in situ energy calibration based on the transmission
characteristics of a 25-µm Be filter. The proposed method was used to obtain SXR spectra in the energy range
1.5 - 3.5 keV with a core electron temperature in agreement with electron temperatures measured by the Thomson
scattering method. Furthermore, oscillations based on the modification of plasma parameters and rapid increases
in impurity radiation were detected in imaging mode. This dual-mode-compatible method for obtaining 2D SXRdiagnostics is useful for the study of high temperature plasmas.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: soft X-ray imaging, MCP, photon counting
DOI: 10.1585/pfr.14.1402128

1. Introduction
Soft X-ray (SXR) diagnostics have been commonlyused to study a number of high-temperature plasma
phenomena, including magnetohydrodynamic activity [1],plasma shaping control [2], and impurity contamination[3]. One-dimensional scintillator-based arrays [4] anddiode-based arrays [5, 6] have been widely used in the tomographic two-dimensional (2D) reconstruction of X-ray
emission images; however, the spatial resolution of SXR
arrays is limited. Another problem with SXR measurement is the diﬃculty of identifying the physical meaningsof signals. The characteristics of SXR emissions dependon the plasma temperature, density, and impurity content
and can be used to capture variations in the plasma, but it
remains diﬃcult to clarify which plasma parameters havevaried.
Direct 2D SXR imaging with high spatial and time
resolutions has recently been made possible through theuse of charge coupled device (CCD) cameras sensitive to
SXR [7] or image intensifiers comprising a microchannel
plate (MCP) detector and a phosphor screen coupled to ahigh-speed video camera [8, 9]. The image intensifier ofan MCP detector can measure SXR signals with higher
gain [10] than SXR cameras, enabling their use in lowdensity plasmas. In this study, we attempted to exploit
author’s e-mail: huang@triam.kyushu-u.ac.jp

these diagnostic abilities to propose a mode-compatible application for MCP detectors. The time resolution of an
MCP-based diagnostic system is limited by that of thehigh-speed video camera, which generally has the capability of taking data every 10 us. The camera can be used in
one of two modes—photon counting or imaging mode—
which are selected by setting the time resolution of thehigh-speed camera. The diagnostics of photon counting
mode can be used to obtain an SXR energy spectrum corresponding to an accurate electron temperature; in imaging
mode, the high-speed camera output can be used to produce a time-integration SXR signal as a very high-spatialresolution 2D SXR image that can be used to measurerapid variations in the plasma.
In this study, we looked at how mode selection could
be applied in measuring the characteristics of the hightemperature plasmas produced by QUEST and resolved anumber of diﬃculties in applying the respective modes.
Typical examples of our results are discussed in this paper to demonstrate how the compatible use of modes can
extend the potential of SXR measurement to obtain further
understanding of the properties of plasma.

2. Experimental Setup
The Q-shu University Experiment with Steady-state
Spherical Tokamak (QUEST) is a medium-size spherical

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Fig. 1 Schematic of FOV of the tangential SXR imaging system on QUEST. (a) 3-D view; (b) poloidal section with movable FOV (redcolor circle in imaging mode and green color circle in photon counting mode). Fundamental and second harmonic resonance layersof 8.2 GHz ECW at Bt = 0.13 T are denoted by red and green lines.

tokamak (ST) device [11] with major and minor radii of
R = 0.64 m and a = 0.4 m, respectively, and major limiter radii of 0.22 and 1.32 m on the mid-plane. All of the
QUEST discharges discussed in this paper were produced
in an inboard limiter (IL) configuration [12]. Two electron cyclotron wave (ECW) systems operating at 8.2 and28 GHz are used for current start-up and current driving,
respectively [13–15]. A center solenoid (CS) coil is used
for Ohmic heating [16].
The SXR imaging system was installed at a tangential
port on QUEST. The system watches the core region of
QUEST plasma tangentially and the ECW harmonic resonance layers at the typical toroidal magnetic field. When
Bt is fixed at 0.13 T at the center of the vacuum vessel(R = 0.64 m), the fundamental and second harmonic resonance layers of 8.2 GHz ECW are located at Rres1 = 0.29 m
and Rres2 = 0.58 m, respectively. Figure 1 (a) shows 3-D
field of view (FOV) of the system. The viewing area is arange of 36-cm circle at the poloidal section of QUEST,
which is changeable at radial direction by moving the rearend of the system. Figure 1 (b) shows the typical FOV at
imaging mode (R = 0.26 - 0.62 m) and photon counting
mode (R = 0.48 - 0.84 m) in this paper.
Figure 2 shows a schematic of the QUEST 2D SXRdiagnostics. The incoming SXR flux can be adjusted bychanging the size of the pinhole (between 1, 2, 4, and6 mm) using a slider. A 30-mm-thick tungsten mask with aV-shaped hole is mounted at the front of the pinhole plate
as a shield against hard X-rays. A 5-µm-thick Al filter
mounted on a shutter is used to prevent visible and ultraviolet (UV) photons from entering the imaging system. The
second, 25-µm-thick Be filter mounted on the plate can beused to cover half of the four pinholes. The two filters
can be used independently for detection of SXR emissions
above 600 eV or together for energy calibration in photon
counting mode. The details of these methods are describedin Sec. 3.
The core component of the SXR imaging system is achevron detector assembly (PHOTONIS USA, INC) comprising a two-stage MCP assembly and a fiberoptic P46
phosphor screen mounted to a 6.0 vacuum flange. The
MCP serves as an SXR detector and image intensifier
with an SXR detection eﬃciency of 5 - 15% in the energy range 0.02 - 10 keV [17]. SXR photons incident on
the MCP produce photoelectrons that induce an MCP electron cascade via a secondary emission process. These secondary electrons are transformed by the phosphor screen
into visible images that are recorded by a fast camera(PHOTRON, INC. Model FASTCAM SA-X). The specified gain of the MCP is up to 1 × 107 at an operational voltage of 1.8 kV; gains of this magnitude have been shown
to be useful in monitoring the relatively low-density and-temperature QUEST plasma. The gain performance ofthe MCP/phosphor assembly was tested experimentally using MCP and phosphor voltages of 1.5 and 3.0 kV, respectively, as references. As shown in Fig. 3, increasing the
MCP voltage from 1.5 to 2.0 kV increased the image intensity by a factor of 11; by contrast, increasing the phosphor voltage from 3.0 to 5.0 kV increased the intensity by
only a factor of 2.5. According to the gain model in [10],the gain of an MCP has an exponential dependence on the
MCP voltage. As shown in Fig. 3 (a), the gain begins toexponentially increase at MCP voltages above 1.8 kV as aresult of current saturation in some of the MCP channels.

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Fig. 2 Schematic of the SXR imaging system on QUEST. A 25-µm-thick Be filter mounted on 1 and 6-mm pinhole plates is displayed;
another set of 2- and 4-mm pinhole plates is also available.

Fig. 4 (a) Photon counting frame. The red dash circle denotes
the imaging area of the MCP assembly. (b) Pixel information of yellow rectangular region in left image. Somephotons occupy more than one pixel.

Fig. 3 Variation of image intensity during bench test voltage
scan of: (a) MCP and (b) phosphor screen. The upperand lower limit data are obtained from the intensity variation over 200 frames. Exponential and linear fitting wereapplied to the MCP and phosphor data, respectively.

In addition to high gain, the MCP has ultra-high time(< 100 ps) and spatial resolutions (12-µm-diameter channels with 15-µm center-to-center spacing); the decay time
constant of the P46 phosphor screen is also very short(about 70 - 80 ns) [18], which allows the system to measure
SXR emissivity with high spatial and time resolutions inimaging mode. The MCP is also capable of single-photon
detection and can operate in photon counting mode using
the 100 kHz maximum framing rate available to the fastcamera.

3. Photon Counting Mode Application
The MCP detector can obtain QUEST plasma SXR
spectra in photon counting mode. Figure 4 show a typical photon counting image taken at the fast camera framing rate of 100 kHz. X-ray photons are detected at a rateof seven counts per 256 × 256 pixels in each frame. This
photon flux is low enough to achieve a complete photon
counting mode in which no double X-ray photons are detected in a given pixel. It is seen that some photons occupy
two or more pixels in a manner similar to the split events
produced in SXR CCD camera images [7] as a result of
spreading of the finitely sized electron clouds produced byindividual X-ray photons into neighboring pixels. To determine the energy of such X-ray photons accurately, thephoton intensity of electron spreading to two or more pixels is integrated.

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The energy of an SXR spectrum is typically calibrated
against that of an X-ray source [7,19]. However, the gain of
the SXR imaging system varies with the applied MCP and
phosphor voltages, making conventional calibration diﬃcult. Here, we propose a new in situ calibration method
based on the transmission curve of a Be filter. Over the
MCP and phosphor voltage ranges discussed in the preceding section, we performed photon counting over two
consecutive shots with the same plasma parameters but different filters—an Al filter and an “Al+Be” filter—applied.
The ratio of SXR photons detected using the respective filters is given by
R(In) =

NAl+Be Ψ p · (T Al · T Be ) · T MCP=
NAlΨ p · T Al · T MCP= T Be (hν),(1)

where Ψ p is the SXR photon flux from the plasma, NAl+Be
and NAl are the photon numbers obtained by the MCP detector with the “Al+Be” and Al filters, respectively, T Al and
T Be are the transmission rates of the Al and Be filters, respectively, T MCP is the quantum detection eﬃciency of the
MCP, In is the intensity of an image produced by an individual incident photon, and hν is the SXR photon energy.
The energy calibration (i.e. the relationship between
In and hν) is determined in the following process. According to [20], the Be filter transmission can be approximatedusing the following equation⎛⎞⎜⎜⎜ E03 ⎟⎟⎟
⎜
⎟⎠ ,
(2)
T Be = exp ⎝−[hυ]3where E0 = (log 2)1/3 Ec,50% , and the 50% cutoﬀ energy of
25-µm Be filter is Ec,50% = 1588.4 eV. Combining Eq. (2)with R(In) = T Be (hυ), photon energy can be expressed as
hν =

E0
E0
=
.
[− log(T Be )]1/3 [− log(R)]1/3(3)

Finally, the energy calibration function is determined by alinear fitting of In and hν data using least squares method.
The data and results of the energy calibration are
shown in Fig. 5. Figure 5 (a) shows the photon counting
data of NAl and NAl+Be , and Fig. 5 (b) shows the ratio data
R and calibrated T Be curve. It should be noted that for
In < 25, the ratio R is about 0.1∼0.2, higher than what we
expect from the Be transmission curve. The data in low
energy part is curious and remove from the fitting. The
several candidates are considered, but we could not haveany conclusion yet. Therefore, this part is removed in theenergy calibration. For In ≥ 60, the ratio R reaches 0.8
and starts to fluctuate around 1.0. This is because T Be is
close to one, so NAl+Be is close to NAl . The error bar in R
is larger because the photon number is smaller with large
error. We used 35 points in 25 ≤ In < 60 for energy calibration, which gives the energy calibration function:hυ = 24 × In + 590 [eV].

(4)Fig. 5 (a) Photon counting number with Al filter and “Al+Be”
filter. (b) Left-hand and lower axes: ratios of photon
numbers with “Al+Be” and Al filters to photon intensity,
In. R = NAl+Be /NAl . Data obtained from 260,000 frames
in photon counting mode are divided into 26 groups. The
error bars denote the standard deviation of data. Righthand and top axes: transmission curve of 25-µm Be filter
as a function of photon energy, T Be (hν).

Here the Be filter transmission of the intercept 590 eV is
T Be < 1 × 10−5 , close to 0. The coeﬃcients with 95%
confidence bound in the fitting is 24 ± 2, and the intercept
with 95% confidence bound in the fitting is 590 ± 90.
Figure 6 shows the SXR spectra in the energy range
1.5 - 3.5 keV following calibration and correction. This energy range is free of the characteristic radiation lines ofmetal and light impurities that are produced in other tokamak devices [19]. Here, we compare the energy spectra
produced by the left and right halves of the viewing area.
In the shots, which were conducted using a limiter configuration, plasma currents of 6 kA were sustained noninductively by an 8.2 GHz, 40 kW ECW power supply withthe value of Bt fixed at 0.13 T. The calculated bulk plasma
temperature (lower component) for each half is about 120 140 eV, a result consistent with Thomson scattering measurements for an inboard limiter configuration. It is seenthat the SXR spectra have a temperature component thatis much higher than the bulk temperature, clearly indicating the presence of higher-energy components in the RF
current-driven plasma that might play an essential role in
non-inductive current driving. Although this information ispotentially useful in understanding how plasma current canbe driven by RF, the associated physical analysis exceeds

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Fig. 7 Waveforms of: (a) plasma current, I p , and toroidal field coil current, IT F ; (b) Ohmic heating by CS coil and ECW heating power;(c) hydrogen Balmer line radiation, Hα ; and (d) SXR intensity. (e - h) Zoomed waveforms of: (e) I p ; (f) loop voltage and Hα ; (g)oxygen-2 line radiation OII and carbon-3 line radiation CIII; and (h) SXR intensity. I1–I3 denote the timing of SXR images inFig. 8.

4. Imaging Mode Application
4.1 Enhancing SXR impurity radiation via
slide-away electronsFig. 6 SXR energy spectra corrected using the quantum eﬃciency of MCP detector and filter transmission. The
imaging area of the MCP assembly is divided into a left
half (R = 0.48 - 0.66 m) and a right half (R = 0.66 0.84 m). Core plasma temperatures of 120 and 140 eV
is derived from the fitting. The error bar of temperatureis estimated using the 95% confidence bound in energy
calibration.

the scope of this paper. Because the second harmonic resonance layer lies in left half (high field side), as shown in
Fig. 1 (b), the measured bulk electron temperature in thishalf is higher than that in the right half (low field side).

Non-inductive current ramp-up has previously beenobtained in QUEST through the use of ECW [13, 14], andthe generation and confinement of energetic electrons has
been shown to play an important role in driving plasma
current [12]. Normally, the presence of energetic electrons
is indicated by the hard X-rays produced when the electrons hit a wall. Here, we propose an SXR imaging systembased detection method that measures the eﬀects producedby slide-away electrons.
We used an 8.2 GHz electron cyclotron current drive(ECCD) to inductively drive a 10-kA plasma current during the period t = 1.2 - 1.35 s (Fig. 7), with Bt increased
from 0.13 T at 1.35 s to 0.26 T at 1.8 s. Figures 7 (d) and
8 (a) show imaging system measurements of the SXR radiation from the hydrogen plasma at around t = 1.2 - 1.35 s.
The SXR intensity signals in Figs. 7 (d) and (h) were calculated by summing the SXR image intensities in each frame.
After inductively increasing to about 50 kA, the plasma
current began to decrease as the ECW power and Ohmic
heating were turned oﬀ. After the ECW power was stopped
at 1.3 s, the CS coil current was decreased to drive and decelerate the plasma current, which began to decrease from
1.5 s. As shown in Figs. 7 (e - h), the drop in plasma current created an enhancement of the loop voltage applied

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Fig. 8 Images of SXR emissivity (false color) in shot 22069: framing rate is 1 kHz. The timing of (a) I1, (b) I2, and (c) I3 images areshown in Figs. 7 (d) and (h). The white dashed circle in (c) denotes the imaging area of the MCP assembly. Spots outside of the
MCP area are caused by penetrating HX emission. The range of color map is [0, 40]. The 8.2 GHz resonance layer is outside (atthe left side) of the observation window.

Fig. 9 Waveform of: (a) plasma current, I p ; (b) 8.2- and 28-GHz ECW heating (the 28-GHz power is much larger than 20 kW); (c)hydrogen Balmer line radiation, Hα ; and (d) SXR intensity. (e - h) Zoomed waveforms of: (e) I p , (f) ion saturation current on
upper divertor probes, (g) Hα and (h) SXR intensity of the three areas shown in Fig. 10.

to the plasma, which eﬃciently accelerated the electrons
driving the plasma current. Visible inspection revealed increases in the carbon and oxygen impurities as a result of
the bombardment of slide-away electrons on the plasmafacing wall.
The eﬀect of enhanced impurity ejection as a result ofslide-away electron impact was also captured by the SXR
imaging system, as shown in Fig. 7 (h). Figures 8 (b) and(c) shows SXR images before and after the emission burst.
The time response of impurity events is very fast. The duration is about 1∼2 ms as detected by SXR images and visible measurement. Unfortunately, the framing rate of 1 kHz

is too slow to catch up the details of spatial intensity evolution. In future work, the framing rate can be increased toinvestigate the behavior of impurity transport using images
with high spatial and temporal resolution.

4.2 Low-frequency plasma oscillation
The SXR imaging system can also be applied toplasma behavior monitoring. Here, we report on the phenomenon of ∼20-Hz low-frequency plasma oscillation andits observation using the SXR imaging system. A typical
waveform is shown for shot 22069 in Fig. 9. The plasma

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Fig. 10 Images of SXR emissivity (false color) during one cycle of shot 22069. 15 out of 50 frames in one cycle are shown. The timingof frame 1 to frame 22 is 2.712 s to 2.733 s, with a framing rate of 1 kHz. Three circular areas chosen to analyze spatial variation
GHzGHz= 0.32 m and R8.2
= 0.54 m are indicated in frame 10 by cyan and red dashed lines, respectively.
are shown in frame 1; R28
res2
res1
The range of color map is [0, 12].

oscillation occurs from 2.3 - 2.9 s under the co-applicationof 8.2 and 28 GHz ECWs. As shown in Fig. 9 (a), the amplitude of I p oscillation increases with time. This lowfrequency oscillation behavior is also seen in the Hα andSXR radiation, which are in phase with each other. Although the oscillation is dominant in the plasma core, it
can also aﬀect the plasma boundary, as indicated by theion saturation current signal on the upper divertor probes.
Although the cause of this plasma oscillation remains unknown, it is similar to the oscillation phenomenon reported
in [21], although only an 8.2 GHz ECW was applied in thatstudy.
The shape and spatial evolution of a plasma oscillation
can be monitored in the imaging mode. Figure 10 showsparts of the plasma oscillation cycle as SXR images. It isseen that the oscillation activity is related to spatial movement in the radial direction. The value of Bt is fixed at
0.26 T during the oscillation and the resonance layers of
the 8.2- and 28-GHz ECWs are present within the observation window. The fundamental harmonic resonance layerGHz= 0.54 m andof the 8.2-GHz ECW is located at R8.2
res1
the second harmonic resonance layer of the 28-GHz ECW
GHz= 0.32 m. To analyze the spatial variation,is at R28
res2
the SXR intensities of the left, middle, and right circles inframe 1 of Fig. 10 are compared in Fig. 9 (h), from which
it is seen that the oscillation aﬀects only the left and middle parts of the view region, corresponding to R < 0.52 m.
Estimation of the velocity of radial motion from the images reveals a forward (or outward) velocity of 18 m/s anda backward (or inward) velocity of 40 m/s.

5. Discussion and Conclusion
Plasma continuum radiation, which primarily comprises free-free and free-bound bremsstrahlung and recom-

bination radiation, is described in [4] using the following
formula:

−hν
dP
−1/2
2
≈ T e γ(T e , Z)Ze f f ne × exp
,
(5)
dhν
kT e
where P is the radiated X-ray power, hν is the SXR photon
energy, T e , ne , and Ze f f are the electron temperature, electron density, and average plasma ion charge, respectively,
and the factor γ(T e , Z) is the enhancement of the radiatedpower from recombination radiation, which in QUEST
arises predominantly from ionized carbon and oxygen impurities. As seen in Fig. 7 (d), the intensity of SXR radiation from the impurities is about one order higher than thatfrom the hydrogen plasma. Nevertheless, because of recombination radiation it is diﬃcult to calculate Ze f f fromthe enhanced SXR radiation.
As shown in Fig. 6, T e can be calculated from theenergy dependence of the continuum radiation in Eq. (5).
Note that, although all of the photons along the line of sightare collected in the photon counting process, the calculated
value of T e shown in the figure is the core plasma temperature, not the line-averaged temperature. This is because, athν  T e , the outer, cooler regions of the plasma contributelittle to the X-ray emission because of the steep exponential fall-oﬀ of the spectrum. Thus, at high photon energies
one sees only the hottest part of the plasma.
In this study, an SXR imaging system that produces
tangential views was installed on QUEST. The system iscapable of the rapid imaging of SXR emissions in imaging
mode or obtaining SXR spectra and core electron temperatures in photon counting mode. Our planned upgrades
to the system include replacing the current “bare” MCP
with a CsI-coated MCP for higher detection eﬃciency (upto the 50% reported for CsI photocathodes [22]). Another1402128-7

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Volume 14, 1402128 (2019)enhancement will involve replacing the shutter and Al filter with a vacuum Be window to enable placement of the
MCP much closer to the QUEST plasma without incurring
arcing from QUEST gas injection. These improvements
will extend the viewing area into the edge region, which
will enable the calculation of the SXR spectral profile andelectron temperature using Abel inversion.
[1] S. Ohdachi et al., Rev. Sci. Instrum. 74(3), 2136 (2003).
[2] J. Mlynar et al., Fusion Eng. Des. 66-68, 905 (2003).
[3] R.D. Gill et al., Nucl. Fusion 40(2), 163 (2000).
[4] L.F. Delgado-Aparicio et al., J. Appl. Phys. 102(7), 073304(2007).
[5] K. Hanada et al., Plasma Phys. Control. Fusion 32(14),1289 (1990).
[6] K. Tritz et al., Rev. Sci. Instrum. 83(10), 10E109 (2012).
[7] Y. Liang et al., Rev. Sci. Instrum. 71(10), 3711 (2000).

[8] K. Tritz et al., Rev. Sci. Instrum. 74(3), 2161 (2003).
[9] F. Zhou et al., Rev. Sci. Instrum. 88(7), 073505 (2017).
[10] J.L. Wiza, Nucl. Instrum. Methods 162(1-3), 587 (1979).
[11] K. Hanada et al., Nucl. Fusion 57(12), 126061 (2017).
[12] S. Tashima et al., Nucl. Fusion 54(2), 11 (2014).
[13] M. Ishiguro et al., Phys. Plasmas 19(6), 062508 (2012).
[14] H. Idei et al., Nucl. Fusion 57(12), 126045 (2017).
[15] K. Hanada et al., Plasma Sci. Technol. 13(3), 307 (2011).
[16] K. Hanada et al., Plasma Fusion Res. 5, S1007 (2010).
[17] G.W. Fraser, Nucl. Instrum. Methods Phys. Res. 195(3),523 (1982).
[18] A. Baciero et al., J. Synchrotron Radiat. 7, 215 (2000).
[19] W. Jin et al., Nucl. Instrum. Methods Phys. Res. A 674, 15(2012).
[20] L. Delgado-Aparicio et al., Rev. Sci. Instrum. 81(10),
10E303 (2010).
[21] T. Onchi et al., Phys. Plasmas 22(8), 14 (2015).
[22] M.P. Kowalski et al., Appl. Opt. 25(14), 2440 (1986).

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