# Development of an x-ray imaging system for the Laser Megajoule (LMJ)

G. Turk, C. Reverdin, D. Gontier, S. Darbon, C. Dujardin, Gilles Ledoux, M.

Hamel, Vesna Simic, S. Normand

► To cite this version:

G. Turk, C. Reverdin, D. Gontier, S. Darbon, C. Dujardin, et al., Development of an x-ray imaging system for the Laser Megajoule (LMJ). Review of Scientific Instruments, 2010, 81 (10), pp.10E509. 10.1063/1.3475788. hal-02116947

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# Development of an x-ray imaging system for the Laser Megajoule (LMJ) a

G. Turk,¹ᵇ C. Reverdin,¹ D. Gontier,¹ S. Darbon,¹ C. Dujardin,² G. Ledoux,² M. Hamel,³ V. Simic,³ and S. Normand³

¹CEA, DAM, DIF, F-91297 Arpajon, France

²Université de Lyon, Université Lyon 1, CNRS, UMR 5620, LPCML, Villeurbanne, France

³CEA, LIST, Laboratoire Capteurs et Architectures Electroniques, F-91191 Gif-sur-Yvette Cedex, France

Presented 20 May 2010; received 15 May 2010; accepted 16 June 2010; published online 5 October 2010

This imaging system aims at recording images of the core size and shape of an imploding deuterium-tritium DT microballoon on LMJ inertial confinement fusion ICF experiments. Image acquisition is difficult due to the harsh surrounding created by the fusion reaction, which affects system specifications. This one is made of a scintillator, an optical relay, and a CCD camera shielded from the surrounding. The system was tested on different facilities at CEA/DIF, where a spatial resolution of 120 µm was achieved and gamma dose up to 20 rad effects were measured. Setup and performed test are described.

## I. INTRODUCTION

The proposed system consists of a scintillator, whose image is focused on a CCD camera through an optical relay system. The aim of this one is to record images of x-ray emission of the target interacting with a 1.8 MJ laser during ICF experiments on LMJ. The interaction between the laser and the DT target generates a plasma, giving rise to x-ray emission that can be imaged to evaluate the quality of implosion for symmetry, etc. Since DT fusion reactions yield a large number of neutrons, an x-ray imaging system for ICF experiments on LMJ must be carefully designed to operate in a harsh surrounding.

### A. LMJ radiative surrounding

A DT ignition shot yields 1016 neutrons in 4 sr, whose energy distribution has its maximum at 14 MeV. Due to the difference between X-rays and neutrons time of flight; the image has to be recorded in less than 100 ns1 before the neutrons reach the chamber wall. After 100 ns⁻¹, the dose rate of neutron and gamma rays created by neutron interactions with the chamber environment reaches instantaneously 108 radSi. s⁻¹ and generates a high noise level on the image. Despite the high dose rate, the integrated dose is only 2 rad(Si).

Such dose rates can at least blind the CCD or destroy it. Optical components are also sensitive: glass emits Cherenkov radiation and sometimes darkens. Because of this surrounding, each component of the imaging system has to meet particular specifications, which are detailed in the following paragraphs.

### B. Main specifications of the imaging system

The major part of x-ray emission corresponds to the maximum DT compression with a duration of 100 ps. Simulations showed that about 10 kJ of x rays are emitted by the target with energies above 10 keV. So, we want to record an image in the 10–30 keV range, with a spatial resolution less than 10 µm in the object plane. The most sensitive part of the system, the CCD camera, is located in a radiation-safe area shielded from the direct line of sight against neutron and gamma rays Fig. 1.

## II. DESCRIPTION OF THE IMAGING SYSTEM

### A. Scintillator choice

The most important parameters of scintillators are x-ray absorption, decay time, scintillation efficiency, and emission wavelength. The scintillator’s decay time has

<figure><img src="image_2.png" /><figcaption>FIG. 1. Imaging system design.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>FIG. 2. Light efficiency vs decay time of scintillators.</figcaption></figure>

$$ \eta \cdot \int_{0}^{100 \text{ ns}} \exp\left(-\frac{t}{\tau}\right) dt \cdot \left[ \int_{0}^{\infty} \exp\left(-\frac{t}{\tau}\right) dt \right]^{-1} \\ \geq 10^4 \text{ photons MeV}^{-1}, \qquad (1) $$
to fit with the 100 ns acquisition time during which scintillators must be as efficient as 10.000 photons MeV⁻¹ [see Eq. 1],
where η is the scintillation efficiency in photons MeV⁻¹ and τ is the scintillation decay time in nanoseconds.

<figure><img src="image_4.png" /><figcaption>FIG. 3. Normalized radioluminescence spectra of scintillators compared to CCD sensitivity and Cherenkov spectrum.</figcaption></figure>

The imaging performances depend mostly on scintillator x-ray absorption in our energy range. To achieve the best compromise of spatial resolution and image intensity, we have to adjust the scintillator thickness. Increasing thickness improves detection efficiency but degrades spatial resolution, and vice versa. For example, NE102 scintillator, which is fast and bright, has a low x-ray absorption, resulting in low intensity if not thick enough or blurred images. Among these scintillators, YAG:Ce (Y3Al5O12 :Ce3+) is the best compromise.

## B. Catadioptric optical relay system

The optical relay transports the scintillator image to the CCD camera. The relay length is about 20 m if the CCD camera lies in an LMJ bunker and about 5 m if in an LMJ dog house. The relay is composed of a field lens placed symmetrically between two Maksutov objectives see Fig. 1. The main problem is not the transport length, but the object field. For long working distances, a large field lens is necessary to focus the whole field of view on the CCD camera.

We want to image an x-ray plasma source size of about 100 µm, with 10 µm spatial resolution on object plane, in 1 mm field of view FOV. Standard diagnostics for plasma source imaging are the pinhole camera and x-ray microscopes. Plasma images are magnified with an M magnification factor on the scintillator. Therefore, the optical relay has to resolve M by 10 m. To minimize the Cherenkov effect and glass darkening, the relay system should contain the smallest amount of glass, which is possible. Therefore, mirror-based objectives, such as Maksutov configuration, are preferred. Additionally, such systems are simple and cheap, have no chromatic aberrations, and present low coma and astigmatism.⁴

To qualify the imaging system using a mirror-based relay, we used two MC Rubinar lenses 4.5/300.⁵ Figure 1 shows the imaging system design using such objectives. The optical performances of the relay are presented in Sec. III.

## III. FIRST RESULTS

The following paragraphs present measurements of our imaging system performances. The results concern spatial resolution of full imaging system, its behavior in hard gamma environment, and first x-ray plasma images.

### A. Spatial resolution measurements

The spatial resolution of the global imaging system was measured with a 160 kV x-ray generator at CEA/DIF. Measurements were achieved with two different optical relay lengths 2.4 and 4 m. We used a 1 mm thick YAG:Ce scintillator in front of the system. A 100 m thick lead test pattern target was placed on the scintillator surface, facing the incident x-ray beam. Resolution patterns were placed at the center of FOV to get points of the contrast transfer function Fig. 4. A resolution of 4.5 line pairs mm⁻¹ circa. 100 µm with a 50% contrast was obtained for a 4 m long optical relay.

<figure><img src="image_5.png" /><figcaption>FIG. 5. Experimental setup of hardening measurements.</figcaption></figure>

This imaging system, if working with a pinhole magnification of 10, allows 11 µm of spatial resolution. This result is promising since the optical relay system was designed only with commercial Maksutov objectives and a simple lens. Spatial resolution can be improved with optics of enhanced quality.

The scintillator must also emit red light to match with the maximum sensitivity of most CCD cameras. Red emission can also be well separated from Cherenkov blue light by means of optical filters. In Fig. 2 the merit curve of Eq. 1 is presented with a set of scintillators.

Most of the scintillators shown in Fig. 2 are inorganic ones, only NE 102 Nuclear Enterprises and MHA 28 homemade scintillator are organic ones. Organic scintillators have not only a very fast decay time but also a low emission intensity. Inorganic ones, such as NaI, YSO $Y2SiO5 :Ce3+$, and CsI, are brighter but have a slower decay.² Few scintillators are above the curve of merit, and most of them, such as YAP $YAlO3 :Ce3+$ and YSO emit

blue light. The only one to emit red light is MHA 28 homemade scintillator but isn't very bright. The emission spectrum of various scintillators is compared to the Cherenkov spectrum and to a backilluminated CCD camera sensitivity in Fig. 3.³

<figure><img src="image_6.png" /><figcaption>FIG. 4. Contrast vs spatial frequency for 4 m optical relay system with 1 mm thick YAG:Ce crystal.</figcaption></figure>
## B. Hardening measurements

The Cherenkov light levels in our imaging system were measured at the ELSA electron accelerator facility.⁶ Gamma rays are generated by interaction of 18 MeV electrons in tantalum target. This interaction results in a gamma spectrum from 1 to 18 MeV. Thus spectrum and intensity that can be delivered can be compared to LMJ gamma term. The experimental setup is shown in Fig. 5. All glass parts objectives and field lens were exposed to a low divergence gamma beam during irradiation without scintillator. The CCD camera was kept out of the beam with a 45° incidence angle mirror. Nevertheless, the CCD was exposed to downscattered gamma rays arising from interaction with Dural parts of the mounting supporting the CCD.

<figure><img src="image_7.png" /><figcaption>FIG. 7. Cherenkov intensity compared to scintillation levels.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FIG. 6. Light emitted by imaging system under gamma rays.</figcaption></figure>

Cherenkov intensities are picked up on images such as in Fig. 6. The numbered zones correspond to hidden zones on both sides of the field lens, allowing us to identify each contributor of Cherenkov light in our diagnostic. Comparison of zone intensities revealed that the lens generates the major part of Cherenkov light. We obtain the linear characteristic CCD light intensity versus gamma dose in Fig. 7. Superimposed are scintillation levels obtained from a previous experiment. The scintillators were excited by a 10–40 keV spectrum, with a fluence of 3*10⁸ photons/cm², by means of a 160 kV x-ray laboratory generator. The Cherenkov light level is equivalent to the organic such as MHA28, MHA30, and MHA 31 scintillating light intensity for doses of 10 rad and overpowers it for greater doses. For inorganic scintillators such as YAG:Ce and LYSO:Ce, scintillating light remains as bright as two decades above Cherenkov light for doses up to 20 rad.

These doses are largely greater than the ones estimated for fusion reaction, i.e., 2 rad in Sec. I A. Both organic and inorganic scintillation light production can dominate the Cherenkov one for deposited doses of about 2 rad. This imaging system can thus achieve a good signal-to-noise ratio for ignition shots.

## C. Plasma images

Penumbral plasma images were recorded at the Equinox facility.⁷ The plasma is generated by the interaction of a laser 200 mJ energy, wavelength of 800 nm, and 10 ps FWHM pulse with a copper target. The resulting plasma x-ray emission is in the 200 eV–1 keV range. Our diagnostic was composed of a YAG:Ce scintillator, a 2.4 m long optical relay, and a Pixis 2048 CCD camera. We recorded the elliptic plasma source shape successively through 1 mm and 200 m tantalum pinholes with a magnification factor of 10. The plasma ellipse of Fig. 8 is about 100 µm on its major axis and about 80 µm on the minor axis. Each pixel represents 5 µm on the object plane.

<figure><img src="image_9.png" /><figcaption>FIG. 8. Unfolded plasma image obtained at the Equinox.</figcaption></figure>

From prior x-ray measurements spectrum and intensity achieved at Equinox, we inferred imaging system sensitivity: one visible photon detected by the CCD camera corresponds to 550 visible photons emitted by YAG:Ce, arising from absorption of 11 fJ x-ray intensity emitted by the source. The acquired images were in good accordance with reference facility source images.

Those first measurements reveal a promising imaging system for ignition shots. Further tests are necessary to completely qualify the system, such as neutron aggression effect measurements and optical performance with a 10 m optical relay.

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