Houzhi Cai

Monte Carlo simulation for microchannel plate framing camera

A microchannel plate (MCP) framing camera is simulated by using the Monte Carlo method. Considering that the laser pulse width has effect on the experimental exposure time, we simulate the experimental exposure time varying with the laser pulse width. The gain characteristics of the MCP framing camera are simulated while the MCP is applied with very short electrical gating pulses. A new method is presented using a nongain MCP instead of a gain MCP.

Demonstration of 11-ps exposure time of a framing camera using pulse-dilation technology and a magnetic lens

Abstract. A framing camera with high temporal and spatial resolution is demonstrated using pulse-dilation technology and a magnetic lens. The magnetic field of the magnetic lens is simulated using LORENTZ-3EM software, and the magnetic field distribution on-axis is similar to a Gaussian function. The temporal and spatial resolutions of the instrument are measured using light at the wavelength of 266 nm from a frequency tripled femtosecond laser. The measured exposure time of this camera is ∼11  ps, and the spatial resolution is better than 100  μm.

Note: Non-gain microchannel plate gated framing camera

An x-ray framing camera using a non-gain microchannel plate (MCP) is reported in this article. The advantage of the non-gain MCP is the less transit time spread. The non-gain MCP gated framing camera has four microstrip line cathodes with 6 mm in width. The time domain reflectometry curves of the four microstrip lines are measured, which show that the characteristic impedance of each microstrip line is about 17 Ω. While the photocathode is driven by the gating electrical pulse with width of 125 ps and amplitude of -1.48 kV with -400 V bias, the measured exposure time of this camera is about 72 ps.

Dilation framing camera with 4 ps resolution

A framing camera using pulse-dilation technology is reported in this article. The camera uses pulse dilation of an electron signal from a pulsed photo-cathode (PC) to achieve high temporal resolution. While the PC is not pulsed, the measured temporal resolution of the camera without pulse-dilation is about 71 ps. While the excitation pulse is applied on the PC, the measured temporal resolution is improved to 4 ps by using the pulse-dilation technology. The spatial resolution of the dilation framing camera is also measured, which is better than 100 μm. The relationship between the temporal resolution and the PC bias voltage is obtained. The variation of the temporal resolution with the gradient of the PC excitation pulse is also provided.

MCP gated x-ray framing camera

A four-frame gated microchannel plate (MCP) camera is described in this article. Each frame photocathode coated with gold on the MCP is part of a transmission line with open circuit end driven by the gating electrical pulse. The gating pulse is 230 ps in width and 2.5 kV in amplitude. The camera is tested by illuminating its photocathode with ultraviolet laser pulses, 266 nm in wavelength, which shows exposure time as short as 80 ps.

Observation of electron beam moiré fringes in an image conversion tube

An image conversion tube with a magnetic lens was designed to observe electron beam moiré fringes. Electron beam moiré fringes result from the interference between the photocathode and the anode meshes. The photocathode had a strip line structure with a spatial frequency of 10L/mm. The anode mesh had a fixed spatial frequency of 10L/mm, and could be rotated around the axis of the image tube. The changes to the fringe direction and the spacing as a function of the rotation angle between the photocathode and the anode mesh were examined. The experimental results agreed with the theoretical analysis. Moiré fringes with a modulation of ~20% were obtained using a 3keV electron beam.

The simulation for the temporal characteristic of the microchannel plate

Framing cameras based on the gated microchannel plate (MCP) are versatile diagnostic tools for fast Z-pinch and inertial confinement fusion experiments. In order to understand the temporal characteristic of the MCP used in such camera, the MCP is simulated using the Monte Carlo method. By simulating the electron cascade in the MCP, the relationship between the MCP temporal resolution and the ratio of its thickness to the channel diameter (L/D ratio) is obtained. The variation of the temporal resolution with the voltage applied on the MCP is also provided. The transit time and the transit time spread (TTS) simulations of the MCP are presented. The simulated results show that the transit time and TTS of the MCP are increased with L/D ratio increasing while the channel diameter of the MCP is 10 um.

Electron beam moiré fringes imaging by image converter tube with a magnetic lens

An image converter tube with a magnetic lens was used to obtain static images of moire fringes formed by electron beam. These moire fringes are formed due to the interference between the anode mesh and the photocathode containing slits of various spatial frequencies. Moire fringes are observed at an accelerating voltage of 3.5 kV requiring the magnetic excitation condition of ∼550 ampere-turns. Not only the features of the fringes are analyzed but also the change of fringe spacing as a function of the rotation angle is investigated. The experimental results are found well in agreement with the theoretical analysis. By changing the rotation angle or adjusting the excitation condition of the magnetic lens, we were able to record parallel moire and secondary moire fringes too. The secondary moire fringes can be observed in the rotation angle range of −39.5° to −50.6°. The theoretical analysis indicates that the secondary moire is formed by the interference between the photocathode slits and the 2-D periodic structure of the anode mesh. Combining our proposed moire method with the pulse-dilation technique may potentially open the door for future applications, in various fields including, but not limited to, ultrafast electrical pulse diagnostics.

Monte Carlo simulation for x-ray detector

X-ray detector based on the gated microchannel plate (MCP) is a powerful diagnostic tool for laser-driven inertial confinement fusion and fast Z-pinch experiments. In order to understand the behavior of the MCP used in such detector, the X-ray detector is simulated using the Monte Carlo method. By simulating the electron cascade in the MCP, the relationship between the MCP gain and voltage is obtained. The time, position and energy of the electrons at the MCP output surface are calculated. The transit time distribution, the electron-channel wall collision number distribution and the time distribution of the electrons travel from the MCP to the phosphor screen are given. Spatial resolution simulations of the MCP-based detector are also presented.

The MCP dynamic characteristics with different bias voltage

Microchannel plate (MCP) is a core part of the X-ray framing camera. Studying the dynamic characteristics of the MCP is critical to understanding the data obtained by the framing camera. The dynamic characteristics of the MCP with different dc bias voltage are simulated by using the Monte Carlo method. The relationship between the theoretical exposure time and the MCP bias voltage is obtained. A MCP gated X-ray framing camera is developed. The measured exposure time increase 9 ps while the MCP bias voltage is -300 V comparing to -200 V. The simulation and experimental results show that the exposure time and gain of the MCP are both increased while the negative bias voltage is increased.