Numerical analysis of electron optical system with microchannel plate

Current Generation II Gallium Arsenide (GaAs) image intensifier tube technology requires that the tube microchannel plate (MCP) component have a thin dielectric coating on the side facing the tube's photocathode component. This protective coating substantially reduces the release from the MCP of ions and neutral species, particularly when the image intensifier is operated. The prevention of MCP outgassing is necessary in order ot prevent the poisoning of the Cs:O surface on the GaAs photocathode. Many authors have experimented with omitting the MCP coating. Such experiments have consistently led to an intensifier with a significantly reduced lifetime, due to contamination of the Cs:O layer on the photocathode. Unfortunately the MCP film acts as a scattering cneter to electron transport within the intensifier and effectively reduces the photoelectron detection efficiency. Substantial enhancement of the image intensifier operating parameters is the motivation for the removal of the MCP film. Removal of the MCP film promises to simplify MCP fabrication and enhance the intensifier parameters related to Electro-Optical performance and image quality. This paper presents results showing for the first time that it is possible to fabricate a long lifetime image intensifier with a single unfilmed MCP and achieve improved imaging and performance characteristics.

Created electronic optical system model is based on finite element method. Libraries of electronic optical system electrodes and presumable electron trajectories were made. Analysis of finite element mesh creation was completed and finite element mesh spacing was determined in the zones of presumable electron trajectories. Tetrahedral mesh was created. Partial electrical field calculations and electrical field calculations with real potential values were performed. Macrocommands for overall electronic optical system construction assembly from separate electrodes and for writing electrode potential and electric field strength values to files were created. These files will be later used in MATLAB program for calculation of electron trajectories in electric field and for solving electron focusing problems. Ill. 9, bibl. 4 (in English; summaries in Lithuanian, English and Russian).

A compact charge coupled device (CCD) camera system has been designed and characterized for use in the six inch manipulator (SIM) at the Nova laser facility. The camera system has been designed to directly replace the 35 mm film packages currently used in SIM-based x-ray imaging diagnostics. The unit’s electronic package has been constructed for small size and high thermal conductivity which reduces the overall camera size and improves its performance when operated within the vacuum environment of the Nova target chamber. Measurements of the x-ray imager’s contrast transfer function (CTF) were made under a variety of operating conditions on a static x-ray Manson source using both the CCD and Kodak T-Max 3200 film as recording media. The CTF data were converted to an equivalent modulation transfer function (MTF). The MTF plots show that the microchannel plate has a uniform response within our measurement accuracy along its strips. In a direction normal to the strip, however, the MTF is reduced due to the slant angle of the pores in the MCP. The measurements show that the CCD camera has a lower MTF response than T-MAX film for all spatial frequencies and configurations measured. However, data obtained from the film exhibited reciprocity failure and border effects that are not observed in the CCD data. Measurements indicate that the signal-to-noise ratio for the CCD data is four to six times larger than that obtained with film and higher photon flux levels were recorded. The CCD-based diagnostic offers immediate access to the data, improved dynamic range, and reduced turnaround time, while eliminating the need for film development, digitization, equipment, and personnel.

The research on key techniques of wide field extreme ultraviolet( EUV) spherical micro-channel plate( MCP) photon-counting imaging detector which will be used in space probe in China have been done.Firstly,the secondary electron yield model of photocathode material was built,and the secondary yield of alkali halide and MCP in soft X-ray and EUV band were calculated using the model. The measurement equipment of MCP's quantum detection efficiency also was set up. The calculated fomula of MCP' s quantum efficiency was introduced,and MCP's quantum efficiency vs. wavelength and incidence angle in soft X-ray and EUV region were measured. secondly,the instrument of solid core spherical MCP fabrication was established,a set ofspherical MCP with 150 mm radius were made using the instrument. The 48 mm induced charge wedge and strip anode in effective diameter was made by the use of traditional UV lithography,and the photon-counting imaging detector was integrated based on the anode and spherical MCP stack in Z configuration. Thirdly,the position readout electronics including analog frent-end and digital processing circuit and the software with image distortion rectification and real-time collection were developed. Finally,the measurement instrument of spherical MCP detector's spatial resolution and image linearity was built,and the specifications of the detector were measured using the instrument,showing that all technical specificaions of the detector satisfy the design requirements.

Some fundamental properties of 18-mm-diam gated proximity-focussed microchannel-plate (MCP) image intensifiers used as fast image shutters in the 1 to 10 ns range have been identified and studied. Light pulses (≈ 5-ps-wide) from a modelocked dye laser optically sample the gated MCP. Shuttering is achieved by applying a forward-biasing electrical gate pulse to the quiescently reverse-biased photocathode-MCP interface. Variable delay (≈ 30-ps jitter) between the gate pulse and the laser pulse permit tracing the MCP's optical response. Gating speeds, turn-on and turn-off patterns, the asymmetric spatial dependence of the MCP optical response, and resolution effects as functions of gate pulse width and photocathode-MCP bias have been characterized. Shutter times of ≥ 750 ps and ≤ 5 1p/mm resolution with the MCP fully on were observed. Variations in the intensity profiles of the phosphorl.s spatial response for uniform photocathode illumination are measured with a calibrated silicon-intensified-target (SIT) focus projection, scan (FPS) television camera and a high-speed video digitizer while photomultipliers (PMTs) monitor the laser pulse and the phosphor's spatially integrated output intensities. The characterization system, gating and biasing circuits, and experimental results will be presented.

Low Noise Proximity Focused Image Intensifiers

To derive a theoretical model for the modulation transfer function (MTF) of the complete imaging chain of a cardiovascular x-ray system. A sufficiently accurate MTF model may be used in quantitative coronary arteriographic applications to decrease the systematic overestimation of small coronary vessels. The model could be used to restore blurred images in image restoration applications and also may be used for image quality assurance measurements. Digital images of a step wedge phantom were acquired at different modes (5-, 7-, and 9-inch) of the image intensifier and at different kilovolt levels of the x-ray generator. From these images the MTFs that were used to estimate the parameters of the model were assessed. The total MTF of the cardiovascular x-ray imaging chain consisting of image intensifier, video camera, optical elements, and analog/digital converter can be modeled by the simple formula MTF = e-(w/f)R, where w is the frequency in linepairs per millimeter. The constants frequency f and the device index n are parameters that can be derived for an individual x-ray system using the phantom. In this formula, the contribution from the focus of the x-ray source has been excluded. The mean square error (MSE) between the measured values and those assessed from the theoretical model of the MTF for the horizontal direction was found to be equal to 0.00003 for the 5-inch mode of image intensifier, and for the vertical resolution at the 9-inch mode this MSE was equal to 0.00038. In addition, we have demonstrated that the contributions of the video camera and the image intensifier to the total MTF can be replaced by an imaginary electron-optical device. An accurate representation for the MTF of a cardiovascular x-ray system has been derived. This MTF model can be used in x-ray image quality assurance measurements and in quantitative image processing applications.

Gated image intensifier tubes (IITs) with microchannel-plate (MCP) may be used for the diagnostics of very fast transient phenomena such as predischarge development in gases. The gating pulse is usually applied to the cathode of the IIT while the voltages across the MCP and the anode remain constant. Due to the transparency of the cathode, which is in the range of 10<SUP>-5</SUP>, and the low emission energy of the MCP-material, which can be below 2 eV, a small portion of radiation is able to pass the cathode during its non-gated state and release electrons directly from the MCP-material. These electrons are amplified by the MCP which may result in an aliased picture. Therefore the amount of radiation which can be applied to the IIT during its non-gated state is limited. In order to increase the gating efficiency of the IIT combined gating of the cathode and the MCP is used. This allows us to take pictures from predischarge phenomena with very low luminosity followed by the intensive radiation of the high current arc. Similar problems exist during investigations of the recovery phase of high current switches. In addition gating of the MCP reduces all kinds of noise caused by the MCP during the relatively long integration time of the video-camera usually connected to the IIT. A single frame high speed camera with gated photocathode, gated MCP, and variable exposure times between 700 ps and 5 ns has been developed. The transparency could be decreased from 1E - 5 of less than 1E - 13.

A theory of synthesizing electrostatic electron optical system in three-dimensional electrostatic fields has been put forward in this paper. The general approach followed is to start with a suitable curvilinear coordinate system (u, ν, w) in which ν = constant and w = constant lines, are desired trajectories. The mathematical conditions, under which principal rays, i.e. ν = constant and w = constant lines, become dynamical trajectories with uniformity in focal properties of narrow pencils focusing on these principal rays, are obtained by writing down dynamical equations in various forms. These mathematical conditions when substituted in Laplaces equation for electrostatic potential distribution lead to the nature of the potential field required and the specific nature of the electron trajectories needed for constructing the curvilinear coordinate system for any electron optical system. The analysis shows that the principal rays of any electron optical system should be of type x = νf(z), where f(z) is a su...

Algorithm and program was created in MATLAB environment by using splines and which is intended for interpolation of electric field potential created by electronic optical system (EOS). Initial electric potential distribution may be known at the nodes of regular or irregular coordinate mesh and potential values can be found at any selected point of space, at the nodes of regular coordinate mesh and in other similar cases with the help of interpolation. Electric potential distribution in regular coordinate mesh is used during modeling of electron trajectories when designing color TV-tubes. Created algorithm and program can be used in order to interpolate not only theoretical potential or potential created by EOS, but also to solve other interpolation problems. Interpolation errors are estimated using modeling data

As shown in [ 1, 2] , the use of a microchannel plate (MCP) as an electromagnetic radiation photoreceiver has certain advantages over the use of semitransparent metal photocathodes. First, MCPs feature a significantly higher level of radiation conversion, particularly in case of hard quanta. Second, the spectral sensitivity range of MCPs is much wider running from vacuum ultraviolet to the 17-domain. These properties make an image converter tube (ICT) with an MCP photocathode a promising device for registration of high speed processes in a wide spectrum range.

Image intensifier tubes using microchannel plate amplification stages and phosphor output stages are being increasingly used in various detection applications. In this paper, measurements of the decay times of what are attributed to be the P20 phosphors in various image intensifiers are reported. It is found that the long tail on the decay curve of the phosphor following illumination can be a limitation for certain observations. In addition, the background level of phosphor glow (which is seen by the subsequent detection system as a light signal) continues to build with continuing illumination.

The presence of photon reflection in the input surface of the microchannel plate causes photon scattered noise, which reduces the signal-to-noise ratio of the micro-light image intensifier. The input surface of the microchannel plate was covered with ion barrier film, which had a great impact on the input surface photon reflection. In order to study the effects of ion barrier film Al 2 O 3 on the photon reflection characteristics of microchannel plate input surface, this paper uses the spectrophotometer system to test the 4 groups filmed and un-filmed microchannel plate input surface reflectivity in the 200-800nm band range.The results show that the reflectivity of the microchannel plate with ion barrier film can be kept below 4% and about 2.0% lower than that of the un-filmed microchannel plate; and in the short wave interval, The amplitude of the reflectivity of the filmed and un-filmed microchannel plate is larger,the variation of the long wave interval is relatively gentle.That is valuable to reduce photon scattered noise of microchannel plate.

A two-dimensional (2D) soft X-ray (SXR) imaging system for obtaining tangential views of plasma-produced SXR has been installed on the QUEST tokamak. The system comprises a chevron micro-channel plate (MCP) and a phosphor screen, which together serve as an SXR detector and image intensifier, along with a high-speed video camera for capturing and storing image data. The system can be used in imaging mode to obtain high spatial- and time-resolution images or in photon counting mode at an ultra-high framing rate of 100 kHz to obtain SXR energy spectra. Here, we propose a method of in situ energy calibration based on the transmission characteristics of a 25-µm Be filter. The proposed method was used to obtain SXR spectra in the energy range 1.5 - 3.5 keV with a core electron temperature in agreement with electron temperatures measured by the Thomson scattering method. Furthermore, oscillations based on the modification of plasma parameters and rapid increases in impurity radiation were detected in imaging mode. This dual-mode-compatible method for obtaining 2D SXR diagnostics is useful for the study of high temperature plasmas.

The spatial resolution that can be obtained with image intensifiers depends largely on the spatial resolution of the microchannel plate (MCP)-anode assembly. Existing models on MCPs never include the nonuniform anode field penetration into the channels. The objective of this article is to show that this field penetration leads to a lens effect which has a crucial influence on the electron trajectories and the spatial resolution that can be obtained. From a simple model the relevant parameters determining the spatial resolution are identified as the anode gap, the anode field, the end-spoiling penetration depth, and the channel diameter. These parameters are varied in a Monte-Carlo simulation in which 20 000 electrons are traced through a channel, the end-spoiling, and the anode gap. The spatial resolution as obtained from these simulations is shown to depend heavily on the nonuniform field at the end spoiling. Especially low-energy electrons generated at the end spoiling deteriorate the spatial resolution. The best resolution is obtained for a small anode gap (300 μm) and small anode voltage (3 kV). This indicates that, above a certain value of the electric field in the anode gap, the effect of the lens is dominant. Doubling the end-spoiling penetration depth from 10 to 20 μm increases the resolution especially at high spatial frequencies. Furthermore, the resolution is enhanced considerably by decreasing the channel diameter. These results can be fully understood from the simple model presented. In addition, results are presented from experiments in which the spatial resolution is measured for various anode gaps and anode voltages. The dependencies of the spatial resolution on these parameters are in good qualitative agreement with the simulation results.