| X-ray framing camera which makes use of image converter tube to obtain photoelectric conversion,pulse gating and image enhancement,is a highly sensitive diagnostic tool with a sub-nanosecond temporal resolution and 2-D spatial resolution.It is widely used in many important fields such as inertial confinement fusion(ICF),Z-pinch,etc.At the final stage of ICF,the process persists only 100 ps,which puts forward a higher request on the diagnostic equipment.Thus,the X-ray framing camera should have a 2-D spatial resolution of micrometers with a temporal resolution better than 30 ps,and moreover a large active area.The traditional MCP gating camera cannot meet the requirements.In 2010,the pulse-dilation camera was successfully designed and produced.The camera couples the pulse-dilation technique with the MCP gating technique,and obtains a temporal resolution of 5 ps.The invention of the pulse-dilation camera will provide a new equipment for ICF research on a higher temporal resolution level.In this paper,for the requirement of the ICF research in our country,a pulse-dilation framing tube using short magnetic focusing is designed and developed.The imaging characteristics and transient performance of the tube are intensively analyzed and experimentally studied.The main aim of this paper is to explore an effective way to improve the spatial and temporal resolutions for large-format X-ray framing camera,and moreover to provide a high-performance ultrafast diagnostic tool for ICF and Z-pinch researches of our country.The main work of this paper includes:1.By using Matlab programming,a theoretical model of the pulse-dilation framing tube with a short magnetic-focusing system is built.The imaging characteristics and the transient performance are theoretically analyzed.2.The static imaging of the image tube is numerically simulated.When a magnetic lens is used at an imaging ratio of 1:1,the photocathode(PC)is applied with-3k V DC,and the anode mesh is at ground potential,the spatial resolutions of the centre,6mm and 12 mm off-axis points are 16.2,7.3 and 2.5lp/mm;and when the imaging ratio is 2:1,the results of these points are 7.1,4.6 and 2.1lp/mm,respectively.The spatial resolution can be improved by combined imaging.Using double lenses or trible lenses with an imaging ratio of 2:1,it is reported that the resolution of the 6mm off-axis point is improved more than one time,that of the 12 mm off-axis point is improved 2 to 3 times,and the resolution of the positions within 18 mm from the centre is higher than 5lp/mm at an imaging ratio of 2:1.The aberration analysis suggests that the coma,astigmatism,field curvature and total distortion of combined imaging system is obviously reduced as compared with the single-lens system,and the image quality will be evidently improved.3.The transient performance of the image tube is numerically simulated.When the PC is applied with-3k V DC and a single lens is used at an imaging ratio of 1:1,the transit time and transit time spread(TTS)of the photo-electron pulse in the image tube is calculated to be 15.5ns and 3.4ps.The TTS in the accelerating region is calculated to be 447 fs,which can be mainly attributed to the initial energy spread.In the drift region,the width of the electron pulse is rapidly broadened by the initial energy spread and space-charge effect.When the PC is pulsed by a dilation electrical pulse with a constant slope,the broadening ratio of the electron pulse is found to increase with the decrease of PC bias and the increase of the value of the pulse slope.The excitation function of PC is derived for linear broadening of the electron pulse and proved by simulation.Using trajectory tracing simulation,when the PC bias is-3k V,and the pulse slopes are 10V/ps,7.0V/ps and 2.1V/ps,the technologic temporal resolutions are estimated to be 2.2ps,3.2ps and 6.6ps,and the total temporal resolutions are 4.0ps,4.7ps and 7.4ps,respectively.The dynamic spatial resolution within 2mm from centre is better than 10lp/mm,and that of 5mm off-axis region is about 6-7 lp/mm.4.Resolution plates photoetched on the PC micro-stripe are adopted to test the spatial performance of the image tube.When the accelerating voltage is 3k V and the imaging ratio is 1:1,the static resolutions of single-lens system are measured to be 9.2lp/mm,4.8lp/mm and 1.8lp/mm for 6mm,9mm,and 15 mm off-axis points,respectively.The spatial resolution of the combined imaging system is measured to be distinctively higher than that of the single-lens system,and the distortion is much smaller than the latter one.At an imaging ratio of 2:1,the resolution is measured to be better than 5lp/mm within 9mm from centre of the double-lens system,and 20 mm from centre of the trible-lens system with an imaging area on PC of off-axis distance 30 mm.Field curvatures in single-lens and double-lens system is measured by adjusting the excitation conditions.The curvature radii in the centre of image surface for the single-lens and the double-lens system are measured to be 6.0 mm and 9.6mm,respectively.A method is adopted to reduce the field curvature of the trible-lens system by applying higher voltage on both the upper and lower PC microstrips than the middle one,as a result,the spatial resolutions of off-axis ~25mm points on the two microstrips reach or approach 5 lp/mm.Additionally,the imaging features of electron beam moiré fringes is also investigated in the image tube.5.The transient performance of the system is measured and influence of the main factors is discussed.When the PC bias is-3k V,The transit time of the photo-electron pulse in the tube is indirectly measured to be 16.0 ns using a high-speed sampling oscilloscope.The temporal resolution and the dynamic spatial resolution of the system are measured by a fibre-optical bunch device.When the PC is not pulsed,the measured temporal resolution of the camera without pulse-dilation is 78 ps.When the excitation pulse is applied on the PC,the measured temporal resolution is improved to 5 ps by using the pulse-dilation technology.Using a magnetic lens at an imaging ratio of 1:1,the dynamic paraxial spatial resolution is measured to be better than 10 lp/mm.The main innovations of this paper are as follows:1.Under the condition of large active area,a method is proposed for improving the image quality of off-axis points by using combined magnetic lens.A theoretic model for large format image tube is established,and the simulated spatial resolutions of the tube are obtained.The experiments show that the field curvature is partially corrected by the combined magnetic-lens system.At an imaging ratio of 2:1,the curvature radii in the centre of image surface for the single-lens and the double-lens system are measured to be 6.0 mm and 9.6mm,respectively.2.Electron beam moiré fringes are systematically investigated in the image converter tube,and an experimental result is achieved that the imaging conditions are in accordance with those of the classical optical moiré. |
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