23P154-P
Measurement of two dimensional x-ray imaging by total reflection
framing camera in fast ignition fusion experiment
高速点火統合実験における全反射フレーミングカメラを用いた二次元Ｘ線画像計測
Yoshinori Ishii, Mayuko Koga, Hiroyuki Shiraga, Shinsuke Fujioka, and Hiroshi Azechi
石井 圭憲, 古賀 麻由子，白神 宏之，藤岡 慎介，疇地 宏
Institute of Laser Engineering, Osaka University
2-6, Yamadaoka, suita,Osaka 565-0871, Japan
大阪大学レーザーエネルギー学研究センター 〒565-0871 大阪府吹田市山田丘2－6
In laser fusion experiments, a large number of photons incident to detectors. High-flux irradiation in avery short time (~ps) causes the saturation effect of microchannleplate (MCP) devices; photomultipliers in
X-ray framing camera (XFC). And it is a serious problem for accurate measurement. Therefore, it isnecessary to know the linearity of the detector. In fast ignition, the experimental data suggested the output
saturation of the detector by high-flux x-ray irradiation. In this research, we developed a new MCP gainmodel for our XFC, which includes the saturation effect against intense input signals. The result shows
better agreement with the experimental data than the conventional MCP gain model without the saturation
effect.

1. Introduction
Pulse-gated X-ray framing cameras (XFC) havebeen deployed widely in laser-plasma experiments[1][2]. This type of camera has microchannel plate(MCP) devices as a photomultiplier, and Au
microstriplines on its surface which work as
photocathodes. The multiplication process of MCP
is described by a simple, well-known model, called“dynode model” [3].
However, the gain of MCP is not infinite, thus if
input photoelectrons increase extremely, output
signal is heavily saturated. In fast ignition
experiment, we also have to measure large numberof photons in a very short time (~500ps). Then, theMCP output probably saturate and the experimental
result is not consistent with the conventional MCP
gain model which does not take into account of thesaturation effect.
Therefore, in this research, we developed a new
MCP gain model for our XFC which includes thesaturation effect, and compared with theexperimental data.
2. Experiment
Figure 1 shows the schematic diagram of ourx-ray framing camera. Target images are projectedon Au photocathodes on the MCP surface, and thenchanged into electrons. While voltage pulses
propagate along photocathodes, the MCP becomessensitive only when voltage pulses exist. Thus, ourXFC has 80ps time resolution. We can get six
images at 360ps full-scale, by using the six pinhole

arrays and two voltage pulses which propagate with
200ps interval. After passing thorough the MCP,
electrons hit a fluorescent plate, and finallyrecorded with a CCD camera. We use the XFC
which has a pair of Pt total reflection mirrors toseparate soft x-rays from imploded core plasma andhard x-rays from hot electrons generated by theinjection of the heating laser.
Our XFC has two identical MCPs, one (MCP1)
acts for gating, and another (MCP2) acts forelectron multiplication. The Voltage pulse which
propagates along the striplines on MCP1 is
V=800V in a FWHM=200ps gaussian pulse. For
MCP2, we can change DC voltage, 500~900V. In
fast ignition experiment, we operated those at twovoltages, 800V and 900V. We show one example ofexperimental data in Figure 2. Then, we calculated
the average value of the intensity near the corecentroid, and plotted the time variation for two
voltages in Table I. Note that, even though wemeasure the same phenomenon, FWHM of two
profiles have the difference by 50ps, and the ratio
CCD camera
MCP
Voltage pulse
Pt mirror
PinholeSoft X-ray
Hard X-ray

Figure 1 The schematic diagram of XFC

080

160

experimental data quantitatively in this state.
Therefore, we have to adjust radiation intensity
somehow with filters or pinholes.

200280 360 [ps]01500 [CCD count]
Figure 2 X-ray images of imploded coreplasma (#34140)

3. MCP modelingWe describe the MCP gain model which
includes the saturation effect by high flux
irradiation. Some approaches are conducted about
the MCP saturation effect [4][5]. We developed anew MCP gain model for our XFC, based on theconventional dynode model [6].
Input photoelectrons are accelerated by theelectric field made by the voltage between theelectrodes. Then, they collide with the channel wallof the MCP, and generate more secondly electrons.
The gain of a MCP can be described by power law
relation,V  n   Z  VC k(1)where δn is the effective gain per stage in a internalcascade multiplication process, Vz is the axial
potential of electrons, Vc is the first crossover
potential, and k is a constant coefficient. The total
gain of MCP is,Table I Comparison of the experimental data andthe result of gain simulation
Voltage [V]900800 ratioCore FWHM [ps]445391
Max intensity[a.u.]1468 1215 1.21
Gain of conventional
10920 3041 3.59
model
Gain of this research
63.7
33.7 1.89
Output [photon/channel]of maximum values is 1.21.

Input [photon/channel]
Figure 3 Input vs. Output gain curves of theMCP, at 900V (solid), 800V (dotted) and 700V(dashed)

The mechanism of the gain saturation is anincrease of electrons stripped from channel walls.
More input photons strip more electrons fromchannel walls, and leads to a decrease of the voltage
between the electrodes. It is like an electric current
between the electrodes of a capacitor.

5. Summary
In fast ignition experiment, we measured x-ray
images of the imploded core plasma by using totalreflection x-ray framing camera. The data suggested
the output saturation of the MCP, therefore wedeveloped a new MCP gain model which includes
the saturation effect. The result showed better
agreement with the experimental data than theconventional gain model without saturation. We
also calculated the input-output gain characteristics
and obtained the operational linearity range for ourXFC.

4. Results & discussion
We show the result of the new gain model of our
MCP in Table I, for two types of voltages; 800V &900V. We note that the result of the new modelshows better agreement with the experimental data
than the conventional model without the saturation
effect. Figure 3 shows the gain characteristic ofinput vs. output electrons for some applied voltages.
At a high voltage, the saturation begins at the lower
number of photons compare to at the low voltage.
We estimate that the number of input photons isover 106 in fast ignition. We cannot evaluate the6. References[1] D. K. Bradley, et. al., Rev. Sci. Instrum. 66:
716 (1995)[2] M. Katayama, et. al., Rev. Sci. Instrum.
62(1) (1991)[3] E. H. Eberhardt, App.Optics, 18 1418(1979)[4] L. Landen, et. al., SPIE 1993 International
Sympo. Opt. Instrum. App. Sci. (1993)[5] C. A. Kruschwitz, et. al., Rev Sci. Instrum. 79,10E911(2008)[6] L. Giudicotti, et. al., Rev. Sci. Instrum. 65 (1)(1994)G      ・・・n

1 2 3n(2)n