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Abstract
An all-optical framing camera has been developed which measures the spatial profile of photons flux by utilizing a laser beam to probe the refractive index change in an indium phosphide semiconductor. This framing camera acquires two frames with the time resolution of about 1.5 ns and the inter frame separation time of about 13 ns by angularly multiplexing the probe beam on to the semiconductor. The spatial resolution of this camera has been estimated to be about 140 μm and the spectral response of this camera has also been theoretically investigated in 5 eV-100 KeV range. This camera has been applied in investigating the imploding dynamics of the molybdenum planar wire array Z-pinch on the 1-MA “QiangGuang-1” facility. This framing camera can provide an alternative scheme for high energy density physics experiments.
© 2017 Optical Society of America
1. Introduction
Framing cameras play an important role in investigating ultrafast phenomenon in the field of high energy density physics [1–4]. In order to observe the dynamics of the ignition targets with several ps time resolution, an all-optical framing camera has been developed by K. L. Baker’s group [5,6], which has the ability to acquire two sequential images with 1 ps time resolution and 5 ps inter-frame separation time. This framing camera, whose principle is different from conventional framing cameras based on the microchannel plate (MCP) [7], measures the spatial profile of x-ray flux by utilizing a laser beam to probe the refractive index change in the semiconductor. Based on this all-optical principle, G. Gao's group has developed a framing camera which can provide six frames with ~3 ps temporal resolution [8]. These two research groups demonstrated that the framing camera based on this new all-optical principle had outstanding performances. However, no applications of this new type of framing camera have been reported.
In the pulsed power area, framing cameras are always based on MCPs. However, the MCP based systems have many disadvantages. The fasted MCP based systems are ~1 ns, which are not suitable for diagnosing faster plasma effects, such as zippering effect, plasma turbulence and x-pinch imploding. Though the microstrip lines technology can be introduced to increase the time resolution to ~30 ps [7], such devices need more pinholes which put difficulties on alignment and induced the inherently different view angle of each captured frame. In addition, MCP based systems have a limited spatial resolution of ~50 μm due to spatial dispersion of electrons through the MCP [7]. Meanwhile, they are easily influenced by the radiation and electromagnetism. Compared with MCP based systems, the all-optical framing camera has a high temporal and spatial resolution of ~1 ps and 10 μm, respectively [5,8]. Moreover, They have the inherent ability to resist the interference of radiation and electromagnetism. Therefore, this all-optical camera should be introduced into the pulsed power applications.
In this article, we designed a framing camera based on this all-optical principle for investigating the imploding dynamics of the Z-pinch on the 1-MA “QiangGuang-1” facility. The framing camera can obtain two frames with 1.5 ns time resolution and 13 ns inter-frame separation time by angularly multiplexing the probe beam on to an indium phosphide (InP) semiconductor. The spatial resolution and spectral response of this framing camera have been investigated and the application for recording images of the imploding dynamics of the planer wire array Z-pinch has also been presented.
2. Experimental setup
The experimental setup is shown in Fig. 1(a). The all-optical framing camera mainly consists of an image converter, probe beams, optical paths and a charge coupled device (CCD) recorder shown in Fig. 1(a). The image converter was based on a 300 μm thick InP substrate, which was coated with a 100 nm thick aluminum (Al) film on the side facing to x-rays. The Al film was used to reflect visible light but to allow the passage of x-ray. A transmission grating with equal width bars and troughs was constructed on the Al film. This grating has an overall pitch of 30 μm, 15 μm of which is a 10 μm thick gold bar. The probe beam was generated by an Ekspla SL300 laser, which had the features of 1064 nm wavelength and 1.5 ns full width at half maximum (FWHM) duration time. Its pulse shape is shown in Fig. 1(b). This probe beam was divided into two beams by a nonpolarizing beamsplitter, both of which were delayed by mirrors and broadened by an extender lens. These two beams were incident on the image converter at different angles, which is shown in Fig. 1(c). During the imploding of Z-pinch, the emitted x-rays were incident on the image converter and caused the transient grating formation in the InP substrate. The probe beams were diffracted by the transient grating. As the diffraction efficiency of the transient grating was determined by the incident x-ray flux, the x-ray image could be reconstructed from the image of the diffracted light. The diffracted beams passed through a glass window, a beamsplitter, a narrow-band pass filter, a lens and a Fourier filter and relay imaged onto a CCD recorder. Two + 1 order diffracted beams were spatially separated at the focus of the lens. An optical wedge was placed in front of the right holes in the Fourier filter to spatially shift one frame image on the CCD film. The dimension of the Fourier filter holes is about 2 mm × 2 mm.
Fig. 1 (a) Experimental geometry used to record the x-ray images of Z-pinch at two separate times. (b) The probe beam pulse shape measured by a Hamamatsu R1328 detector with a rise time of 60 ps and fall time of 90 ps and a TEK DPO71254 oscilloscope with a bandwidth of 12 GHz. (c) The multiplexing probe beams incident on the image converter and their diffracted orders.
All of the experiments were performed on the “QiangGuang-1” facility, which can provide a peak current of 0.9-1.3 MA and a 10%-90% rise time of 50-60 ns [9]. The load was a molybdenum (Mo) planar wire array, which had 10 wires with the wire diameter of 10 μm and the equidistant wire spacing of 1 mm. The radiation power was estimated by an x-ray vacuum detector with a 2.5 μm terephthalate (Mylar) filter, which was placed 2.2 m from the load and sensitive to the x-rays with the photon energy > 100 eV. The x-ray spatial profile of Z-pinch was imaged onto the image converter by a 0.5 mm diameter pinhole. The pinhole was placed at the distance of 230 mm and 110 mm to the load and the image converter, respectively, indicating that the magnification factor is 0.48. The load, pinhole and image converter were placed in a vacuum chamber with a shot pressure below 4.0 × 10-2 Pa. All experiments were carried out at room temperature.
3. Results and discussions
3.1 Spatial resolution
In order to evaluate the spatial resolution of the framing camera, a 50 μm thick copper with a pattern shown in the inset of Fig. 2(a) was placed in front of the image converter and a raw image of + 1 order diffracted light with two sequential frames was acquired in one shot under the experimental configuration of Fig. 1(a) with removing the pinhole. The raw image with two frames spatially separated on the CCD film is shown in Fig. 2(a). The edge spread function along the cut in Fig. 2(a) is shown in Fig. 2(b), which was acquired by averaging 20 samples in the vertical direction relative to the cut. The spatial resolution was estimated to be about 140 μm indicated by the distance between the lateral positions with intensities of 10% and 90% [10]. The fringe existed in each frame of the result, which deteriorated the quality of the image. However, the reason for the fringe is not clear.
Fig. 2 (a) The obtained raw image of + 1 order diffracted light. The inset of Fig. 2(a) is the photograph of the 50 μm thick copper with a pattern. (b) The edge spread function which is used to estimate the spatial resolution of the framing camera.
3.2 Spectral response
The spectral response of the framing camera has been evaluated by analyzing the photon-induced phase shift in the InP substrate, for the phase shift determines the diffraction efficiency of the image converter, which is proportional to the recorded image signal. The phase shift, Φ, is described as Φ = (2π⋅Δn⋅2L)/λ, where Δn is the refractive index change excited by photons in the InP substrate, L is the thickness of the refractive index change region in the InP substrate, and λ is the probe wavelength. The refractive index change is determined by the carrier density and the probe wavelength [11,12]. The carrier concentration, ΔN, can be described as when dInP≥1/αInP and when dInP<1/αInP, where E is the flux of the excitation photons on the image converter, dAl and dInP are the thickness of the Al and InP, respectively, αAl and αInP are the absorption coefficients for Al and InP, respectively, whose relations with the excitation photon energy are shown in Fig. 3(a) [13]. ε is the electron-hole pair creation energy. Since the photons with energy lower than 5 eV can hardly penetrate 100 nm thick Al film [14], the selected spectral range for numerical analysis was 5 ev-100 KeV. In this spectral range, ε is supposed to be 3 times of InP bandgap energy [15,16]. The refractive index is calculated based on the probe wavelength and the carrier concentration according to the theory of Bennett [12], which consists of three effects: band-filling effect, band-gap shrinkage effect and free-carrier absorption effect. The carrier concentrations and the phased shift in InP generated by photons with 5 mJ/cm2 intensity at a specific photon energy varying from 5 eV to 100 KeV were calculated, which are shown in Fig. 3(b). The results indicate that this image converter can operate over a wide range of photon energies. The sensitivity of the image converter is mainly determined by the absorption spectra of Al and InP. When the photon energy is higher than about 33 KeV, the photon penetration length will exceed the thickness of InP substrate, leading to the decrease of the sensitivity. The sensitivity of this image converter for measuring the Mo planar wire array Z-pinch was also evaluated by analyzing the 0 order image under excitation [17]. The results show that photons with about 16 mJ/cm2 intensity from Mo planer wire array Z-pinch emission will generate about 1.24 radians of phase shift in the InP substrate, which roughly agree with the results of the numerical analysis.
Fig. 3 (a) Absorption spectra in 5 eV-100 KeV range for Al and InP. (b) Carrier concentration and phase shift in InP substrate, which are induced by photons with 5 mJ/cm2 intensity at a specific photon energy varying from 5 eV to 100 KeV.
3.3 Applications on Z-pinch experiments
The all-optical framing camera was applied for investigating the imploding dynamics of the Mo planar wire array Z-pinch under the experimental configuration in Fig. 1(a). The photograph of the Mo planar wire array load is shown in Fig. 4(a). The observing direction is parallel to the wire array plane. The load current, the radiation power (>100 eV) and their time relations with the camera shutters are shown in Fig. 4(b). During the experiment, the pulsed electronic current is loaded on the planar wire array. Then the planar array will rapidly implode under the J × B force, leading to a stagnating column on axis that radiates strongly. The dynamics of this column is on a short timescale and is measured by the all-optical framing camera with 1.5 ns temporal resolution and 13 ns inter-frame separation time. The raw results obtained by the framing camera are shown in Fig. 4(c), in which the background signals have been removed. The median filtering was performed on the raw results to remove the fringes. Then, the obtained results were raised to the power of 0.71 to provide the photons flux profile incident on the image converter, for the refractive index change, Δn, is proportional to the power of 0.7 of the carrier concentration, N [12]. The final results are shown in Fig. 4(d) showing the dynamics of the Mo planar wire array Z-pinch. The width of the x-ray source is estimated to be about 1 to 2 mm. The arc pattern in the second frame indicates the instability during Z-pinch implosion [3,18]. Two bright spots with the size of about 1.5 mm are seen in the second frame of Fig. 4(d) indicating the plasma density distribution at the stagnation stage [3].
Fig. 4 (a) Photograph of the Mo planar wire array load. (b) Load current and x-ray power (>100 eV) and their time relations with the shutters of two sequential frames. (c) Raw results of two sequential frames. The markers of Cath and An represent the cathode side and anode side, respectively. (d) Two sequential images of Z-pinch dynamics, which are obtained by performing median filtering on the results of Fig. 3(c) and then raising them to the power of 0.7.
4. Conclusions
An all-optical framing camera has been developed and applied in investigating the dynamics of Z-pinch. This framing camera acquires two frames with the time resolution of about 1.5 ns and the inter frame separation time of about 13 ns by angularly multiplexing the probe beam on to an InP semiconductor. The spatial resolution of this camera has been estimated to be about 140 μm and the spectral response of this camera has also been theoretically investigated in 5 eV-100 KeV range. This camera has been applied to investigate the imploding dynamics of the molybdenum planar wire array Z-pinch on the 1-MA “QiangGuang-1” facility. Two sequential frames have been acquired by this camera, which indicate the instability and distribution of the Z-pinch plasma. This framing camera can provide an alternative scheme for high energy density physics.
Funding
National Natural Science Foundation of China (Grant No. 11505139, 11505141).
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