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Abstract
An x-ray framing camera based on pulse-dilation technology is reported. This camera first dilates the electron signal generated from a pulsed photo-cathode (PC), and then detects the dilated electron pulse by using a gated microchannel plate (MCP). While the PC is only applied with a direct current (DC) voltage, the camera’s temporal resolution without pulse-dilation is about 81 ps. It is the gated MCP’s temporal resolution. While an excitation pulse is applied on the PC, the electron pulse’s temporal width is dilated, and the resolution is improved to 14 ps. Furthermore, the camera’s temporal resolution uniformity is measured and simulated. The results show a 3.5 × drop in temporal resolution along the pulse propagation direction, due to the 5 × decrease of the PC excitation pulse gradient.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Gated microchannel plate (MCP) based x-ray framing camera is a valuable diagnostic tool in inertial confinement fusion (ICF) experiment due to its two-dimensions spatial resolution and high temporal resolving ability [1–5]. In the ICF experiment, x-ray framing camera is currently used to measure the shape of the peak x-ray emission. The measured results are beneficial to improve the dynamic symmetry and performance of indirectly driven ICF capsule [6]. Such cameras have been developed over the last several decades and contain four basic components, the pinhole array, the MCP imager, the MCP gating pulse generator, and the charge-coupled device (CCD). The pinhole array is placed between the object and the camera, producing an array of x-ray images. Then, the x-ray images are converted into electron images and amplified at different time to achieve time-dependent information by using a gated MCP. The electrons outputting from the MCP are accelerated and then hit the phosphor layer to produce enhanced visible images corresponding to the incident x-ray images. Typically, the temporal resolution of such a camera is 35-100 ps, limited by the transit time and transit time spread of electron multiplication process taking place in the MCP [7,8]. The ICF implosion typically lasts around 100 ps [9]. Therefore, the detailed time history of the burn phase of implosion cannot be captured by this traditional camera. A new camera with better temporal resolution is needed.
More recently, a dilation x-ray imager (DIXI) with temporal resolution of 5 ps is developed by coupling pulse-dilation technology with a traditional gated MCP [10–13]. In the DIXI, the incoming x-ray is converted to electrons at a transmission photo-cathode (PC). The PC is applied with a negative DC high voltage overlapped by a pulse voltage, and the anode mesh is at ground potential. Then, there is a time varying electric field between the PC and the anode mesh. While the x-ray is synchronized with the rising edge of the PC pulse, the electric field strength will be decreased in time during the electron generation process. The electron beam is accelerated by this electric field while it transit from PC to anode mesh. Thus, the electrons born early can obtain a larger energy and transit faster through the drift space between the anode mesh and the MCP than those coming later. The temporal width of the electron beam will be increased gradually as it travels through the drift space. Finally, the gated MCP captures a slice of the incoming signal, which has been temporally magnified up to 50 times [12]. Therefore, the temporal resolution of DIXI is better than that of the traditional framing camera.
In this paper, a dilation x-ray framing camera (DIXFC) is presented, and its temporal resolution uniformity is measured. Comparing with DIXI, our DIXFC has following different points [10,12]. Firstly, an axially symmetric non-uniform magnetic field produced by two large aperture magnetic lenses is used to image the electrons in DIXFC, and a uniform coaxially guiding magnetic field is achieved by four magnet coils in DIXI. The magnetic lens here is typically used in electron microscope and streak camera to improve the focusing ability and imaging quality of optical electronic [14–16]. Secondly, an image de-magnification of three times is obtained by a large uniform axial magnetic field electron lens in DIXI [10,12]. Here, the electrons from the PC onto the MCP are imaged by the magnetic lenses and the image ratio is 2:1. Moreover, the size of the DIXFC is smaller than DIXI. In DIXI, there are four 40 cm diameter coils and the axial length of each coil is 8 cm. The gap between each adjacent two of them is 15 cm [10]. In the DIXFC, two magnetic lenses with outer diameter of 256 mm, inner diameter of 160 mm, and axial length of 100 mm are used. Furthermore, the temporal resolution measurement methods are also different. In DIXI, the temporal resolution is acquired by six image frames with different shots [10]. The time between two neighboring images is 1.66 ps owing to a step of 0.5 mm varied optical path. However, this time is unstable because of the shot-to-shot timing jitter, such as the PC excitation pulse jitter, the MCP gating pulse jitter, or the laser jitter, which leads to a measurement error. In this work, the temporal resolution is measured by using a fiber bunch composed of 30 fibers with different lengths. The gating image of the fibers can be obtained in single shot, and the measurement error from shot-to-shot timing jitter could be avoided.
2. DIXFC description
The DIXFC is based on six main components, three transmission photo-cathodes, an anode mesh, two large aperture magnetic lenses, a MCP imager, a pulse generator, and a CCD. The schematic diagram and photograph of the DIXFC are shown in Fig. 1.
The transmission x-ray PC is shown in Fig. 2. Each PC is coated with 80 nm Au on a 90 mm diameter C8H8 film. Each PC with width of 12 mm and a gap of 10 mm between two neighboring photo-cathodes are achieved. A tapered transmission line etched onto the printed circuit board is utilized to guide the excitation pulse from input side connector to PC. And that is also used in the output side. The length of the input or output side tapered transmission line is 55 mm. The PC and tapered transmission line are connected by a gold foil. A 1.5 mm accelerating distance between PC and anode mesh is formed. The electroformed nickel mesh with 10 lp/mm is grounded, while the PC is applied with a negative DC high voltage overlapped by an excitation pulse.
Fig. 2 Photograph of the transmission x-ray PC. The labels A, B and C represent different positions on the middle PC. Point A is 25 mm from the left-most of PC, point B is the center of the PC and is 20 mm from A, point C is 20 mm from B.
The MCP imager is made up of three microstrip lines, a MCP, and a phosphor screen. The MCP has a 0.5 mm thickness, a 56 mm diameter, a 12 μm microchannel diameter, and a 6◦ bias angle. The three microstrip lines are deposited on the MCP input surface with 500 nm Cu overlaid by 100 nm of Au. The whole plane of MCP output surface is deposited with the same thickness of Cu and Au, and used as a ground plane. The width of each microstrip line is 8 mm, and the gap between two neighboring microstrip lines is 3 mm. The phosphor screen placed 0.5 mm from the MCP output surface is coated on a fiber-optic faceplate. A cooled CCD in contact with the faceplate output is used to capture the visible images. The MCP output surface is grounded, while the phosphor screen is applied with a 4 kV positive DC voltage.
The drift distance from the mesh to the MCP is 50 cm. The electrons moving through the drift space are guided by the magnetic field, with electrons born early moving faster than those coming later. Thus, a temporal magnified of the electron pulse is created due to the electron velocity dispersion. The electrons emitted from PC are imaged onto the MCP by two identical large aperture magnetic lenses and the image ratio is 2:1. The center of the first magnetic lens is placed 12.5 cm from the PC and the second one is 9 cm to the MCP. The currents of the first and second lens are 0.198 A and 0.31 A respectively. The annular magnetic lens consists of a soft iron frame and a 1320-turn copper coil, shown in Fig. 3(a). It is 256 mm in outer diameter and 160 mm in inner diameter, with an axial length of 100 mm and a circular slit width in the inner cylinder of 4 mm. Thanks to this slit, the magnetic field can leak from soft iron to drift space to form a magnetic lens. And an axially symmetric non-uniform magnetic field is produced by this magnetic lens. The magnetic field distribution along the axis is measured, shown in Fig. 3(b). The magnetic field has maximum strength of 4.8 mT. In the drift space, the on-axis distribution of the magnetic field strength produced by each magnetic lens is similar to a Gaussian distribution.
Fig. 3 (a) Schematic diagram of the magnetic lens. (b) The measured magnetic field strength distribution on axis in the drift space.
The pulses are obtained by first using avalanche transistors with a Marx bank configuration to generate high voltage fast step pulses. This circuit creates two pulses, one of which is utilized to drive the PC, and the other is shaped by using avalanche diodes to produce a MCP gating pulse [17]. The wave form of the PC excitation pulse is shown in Fig. 4(a), the gradient of the pulse is approximately 3.1 V/ps. The MCP gating pulse with width of 225 ps and amplitude of −1.8 kV is achieved and shown in Fig. 4(b). The x-ray or ultraviolet (UV) laser pulse should be synchronized with the rising edge of the PC excitation pulse to produce pulse dilation. At the very beginning of the excitation pulse applied to the PC, the gradient is small. Therefore, the input laser pulse is applied at about 200 ps later the very beginning of the PC excitation pulse.
Fig. 4 (a) PC excitation pulse wave form with gradient of 3.1 V/ps. (b) MCP gating pulse wave form with width of 225 ps and amplitude of −1.8 kV.
In the limit of short accelerating distance and ignoring any emitting energy spread, the photoelectron entering the drift region at time ti will arrive at MCP in the moment [10,11]
Here, L is the length of the drift space, m is the electron mass, e>0 is the magnitude of the electron charge, is the difference potential between PC and anode mesh, is the PC bias voltage, is the gradient of the PC excitation pulse. The temporal magnification factor M between two time steps isThen, the temporal resolution of the dilation x-ray framing camera can be given by [11]
where TMCP is the MCP gate width at the end of the drift region.The temporal magnification factor M is increasing with the increasing PC excitation pulse gradient. Then, the temporal resolution will be improved while the gradient of the PC excitation pulse is increased. However, the PC excitation pulse is subject to voltage loss as it propagates along the transmission PC which leads to the decrease of the PC excitation pulse gradient. And the temporal resolution will be worse gradually along the PC excitation pulse propagation direction. This work adapts a transmission line loss model to calculate the voltage loss in the PC. A loss-less PC can be modeled as a distributed network with inductance L and capacitance C per unit length [18]. Loss is included by adding resistance R in series with L and conductance G in parallel to C. The voltage at the point away from PC pulse input port with x cm can be given as [18]
where V0 is the voltage at the PC pulse input port. The exponential loss coefficient is given by [18]The loss coefficient α is measured by introducing a PC excitation pulse into one of the input ports shown in Fig. 2 with it connected to the PC and output port. We lack the high-impedance contact voltage probe (one of this probe is manufactured by GGB industries called a Picoprobe), so the actual voltage pulse applied to the tapered transmission line or PC stripline could not be measured while the excitation pulse travels in the tapered transmission line and PC. The PC excitation pulse is measured while it outputs from the pulse generator, and its wave form is shown in Fig. 4(a). Then, the excitation pulse travels in the PC and the signal from the output port is captured by a high speed oscillograph. It is worth noting that the loss coefficients in the PC and in the tapered transmission line etched onto the printed circuit board are different. Firstly, a printed circuit board with whole surface is used to measure the loss coefficient α1 in the tapered transmission line. The center of the printed circuit board is not cut, the input and output circuits are constructed on the printed circuit board, and the distance between them is 20 cm. The signal from the output port is captured. The loss coefficient is calculated by fitting the peaks to e-αx. The measurements are made multiple times to evaluate experiment error, the measured results show that the loss coefficient α1 in the tapered transmission line is about 0.02/cm. Then, a circle with diameter of 90 mm in the center of printed circuit board is cut, and a 90 mm diameter C8H8 film with three transmission photo-cathodes is placed in the center. The microstrip line between the PC and the printed circuit board is connected by a gold foil. The measured loss coefficient α2 in the PC is about 0.18/cm. Then, the voltage or gradient of the excitation pulse at each point on PC could be calculated by Eq. (4) and the temporal resolution by Eq. (3).
3. Measurement and simulation results
The temporal resolution is defined as the full width at half maximum (FWHM) of the gain versus time curve. The experimental setup of the temporal resolution measurement is shown in Fig. 5(a). We lack x-ray source for the resolution measurement. Therefore, the resolution is characterized using UV light pulse, which is also used by other researchers [8,10]. The temporal resolution is almost the same as that obtained by using x-ray source. The incident x-ray or UV pulse is converted into photoelectrons. The UV light does not produce exactly the same electron energy distribution as x-ray [8]. However, the initial photoelectron energy distribution is much less than the energy distribution achieved by the pulsed PC, and so it has little influence on the temporal resolution. The temporal resolution is mainly determined by four factors, the PC bias voltage, the gradient of the PC excitation pulse, the drift length, and the MCP gate width [10–12]. A third harmonic of a Quantronix Integra-C Ti-sapphire laser system is used. There are two laser beams with wavelengths of 266 and 800 nm outputting from the laser system. The 266 nm UV laser with 130 fs pulse width is used to excite the photoelectrons from the gold-coated cathode. The 800 nm laser beam illuminate the p-i-n detector to produce a trigger signal. The 266 nm UV laser beam is reflected by mirror M2, and then illuminates the fiber bunch composed of thirty different length fibers. The lengths of the thirty fibers are measured, which is increased by a step of 2 mm with a length error less than 10%. The thirty laser pulses output from the fiber bunch at different time and the delay time between each two adjacent fibers can be acquired. The measured delay time of 10 ps is obtained by using a streak camera. Then, the thirty laser pulses are imaged on the PC by the lenses L1 and L2 to excite photoelectrons with different birth time. The array of the fiber bunch output port is shown in Fig. 5(b).
Fig. 5 (a) Schematic diagram of the temporal resolution measurement setup. (b) The array of the fiber bunch output port.
The fiber bunch has three rows with ten fibers in each row. The shortest fiber is labeled number one. While the fiber number adds by one, the delay time will be increased by 10 ps. The spatial distance of each two neighboring fibers is 0.2 mm. The long side of the fiber image is vertical to the traveling direction of the PC excitation pulse. The laser beam with 800 nm wavelength is reflected by mirror M1 and illuminates the p-i-n detector to generate an electrical signal. Then, the electrical signal is delayed by a delay circuit and used to trigger the pulse generator to output PC excitation and MCP gating pulses. The delay circuit is adjusted precisely to synchronize the UV laser pulses with the rising edge of the PC excitation pulse. Then, the photoelectrons excited from PC will be accelerated by a time varying electric field between the pulsed PC and the grounded mesh, and an electron velocity dispersion will be achieved. The electron signal arriving at MCP will be dilated. Finally, the MCP gating pulse is timed to gate the dilated electron signal.
The excitation pulse is traveling from the left side of the PC to the right. While the UV laser pulse excites the photoelectron from the gold cathode at point A (shown in Fig. 2) which is 25 mm from the left-most of PC, the measured static image, gating image without an electron pulse-dilation, and gating image with a pulse-dilation technology are shown respectively in Figs. 6(a)–6(c). The fiber image at the bottom left has the shortest delay time and there is a spatial distance of 0.25 mm between each two adjacent images. The static image in Fig. 6(a) is measured with a static DC bias of −3 kV applied to the PC, and a static DC bias of −700 V on the MCP. The gating image in Fig. 6(b) is measured with a static −3 kV DC bias on the PC, and a gating pulse plus a −400 V DC bias on the MCP. The gating image in Fig. 6(c) is measured with a DC bias of −3 kV overlapped by a 3.1 V/ps gradient excitation pulse on the PC, and the voltages applied to the MCP are the same as for Fig. 6(b). Figures 6(a)–6(c) are original images obtained by CCD. The signals out of the gating images in Figs. 6(b) and 6(c) are shown in Fig. 6(d). The final results in Fig. 6(d) are calibrated by the static results from Fig. 6(a). The solid points in Fig. 6(d) are the experimental results, and the solid lines are the Gaussian fitting curves for the points. It can be seen from Fig. 6(d) that the temporal resolution is about 80 ps, while the camera is without pulse dilation. It is the temporal resolution of the gated MCP. And it will be improved to 14 ps by using the pulse-dilation technology. The temporal resolution might be varied slightly among different shots because of the less than 50 ps shot-to-shot timing jitter.
Fig. 6 (a) Static image, while −3 kV DC bias without excitation pulse is applied on the PC, and the MCP is applied with −700 V. The fiber image at the bottom left has the shortest delay time and there is a spatial distance of 0.25 mm between each two adjacent images. (b) Gating image without pulse-dilation, the PC is applied with a −3 kV DC bias only, and a DC bias of −400 V overlapped by a gating pulse are applied on the MCP. (c) Gating image with pulse-dilation, the PC is applied with an excitation pulse plus −3 kV DC bias, and the voltages applied on the MCP are the same as for (b). (d) Signals out of the gating images in (b) and (c).
While the UV laser pulse exciting point is 45 mm from the left-most of PC, which is the center of the PC (point B in Fig. 2), the measured gating image without pulse-dilation and gating image with pulse-dilation are shown respectively in Figs. 7(a) and 7(b). The voltages on the PC and MCP are the same as for point A. The signals out of the gating images in Figs. 7(a) and 7(b) are shown in Fig. 7(c). It shows that while the UV laser pulse exciting point is point B, the temporal resolution is about 81 ps without pulse dilation, and it is 19 ps while using pulse-dilation technology.
Fig. 7 (a) Gating image without pulse-dilation, while the photoelectron emitting point on the PC is point B. (b) Gating image with pulse-dilation on point B. (c) Signals out of the gating images in (a) and (b).
While the UV laser pulse exciting point is 65 mm from the left-most of PC (point C in Fig. 2), the measured gating image without pulse-dilation and gating image with pulse-dilation are shown respectively in Figs. 8(a) and 8(b). The signals out of the gating images in Figs. 8(a) and 8(b) are shown in Fig. 8(c). It can be seen from Fig. 8(c) that the temporal resolution in point C is about 82 ps without pulse dilation, and it is 28 ps with pulse dilation technology.
Fig. 8 (a) Gating image without pulse-dilation on point C. (b) Gating image with pulse-dilation on point C. (c) Signals out of the gating images in (a) and (b).
It can be seen from Figs. 6(d), 7(c) and 8(c) that the temporal resolution without pulse dilation has a little variety while the photoelectron emitting point on the gold cathode is different. The variations of the temporal resolution are within 2.5% while the variations of the photoelectron emitting point on the PC are less than 40 mm. The image ratio of the electron from PC onto MCP is 2:1. Therefore, the variations of the dilated electron signal hitting point on the MCP are less than 20 mm. The amplitude of the MCP gating pulse is reduced as it propagates along the MCP microstrip line, which leads to the small variations of temporal resolution without pulse dilation.
Figures 6(d), 7(c) and 8(c) also show that the temporal resolution with pulse dilation has obvious variety while the photoelectron emitting point is different. The temporal resolution doubles while the distance of the photoelectron emitting point is 40 mm. The amplitude of the excitation pulse is reduced as it travels along the PC, which leads to the decrease of the PC excitation pulse gradient. Therefore, the temporal magnification will be decreasing gradually along the PC excitation pulse propagation direction and the temporal resolution will be worse. The gradient of the excitation pulse at each point on PC is calculated by Eq. (4) and the temporal resolution by Eq. (3). The temporal resolution and the PC excitation pulse gradient are plotted versus the photoelectron emitting point on the PC in Fig. 9. There is a 3.5 × drop in temporal resolution along the PC pulse propagation direction and the variations in PC excitation pulse gradient are 5 × .
Fig. 9 Temporal resolution and PC excitation pulse gradient versus photoelectron emitting point on the PC.
4. Conclusions
A DIXFC is developed to acquire image with better temporal resolution than that of the previous gated MCP framing camera. The camera has three 12 mm-wide transmission photo-cathodes. A fiber bunch is used in the temporal resolution measurement and the gating image of the fibers can be acquired in single shot. This method can avoid measurement error introduced by shot-to-shot timing jitter. While the PC is applied with −3 kV DC bias only, and a DC bias of −400 V overlapped by a gating electrical pulse applied on the MCP, the temporal resolution without pulse-dilation has a little variety, and its average value is 81 ps. While the excitation pulse is applied on the PC, the temporal resolution with pulse-dilation has obvious variety among different photoelectron emitting points. The excitation pulse gradient is reduced as it travels along the PC, which leads to the worse temporal resolution. The temporal resolution uniformity of the camera is measured and simulated. The results show that there is a 3.5 × drop in temporal resolution along the pulse propagation direction and the variations in PC excitation pulse gradient are 5 × . One of the influencing factors for the temporal resolution uniformity is the thickness of Au coated on the C8H8 film. The PC excitation pulse gradient is reduced much more rapidly while the thickness of Au is smaller, which lead to a worse temporal resolution uniformity. However, the thicker Au will affect PC quantum efficiency. Future work is studying the thickness of Au to obtain acceptable temporal resolution uniformity and PC quantum efficiency simultaneously.
Funding
National Natural Science Foundation of China (NSFC) (11775147); Science and Technology Program of Shenzhen (JCYJ20170302153912966, JCYJ20160608173121055); Natural Science Foundation of SZU (2017015).
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