Anodes simulation software

Transcription

1 Anodes simulation software Henry Frisch, Jean-François Genat, Hervé Grabas, Guilherme Nettesheim University of Chicago August 17, Physics to simulate This software intends to simulate a part of a MCP-PMT (Micro-Channel Plates Photo-Multipliers) detectors. A MCP-PMT detector converts photons into an electric signal using the following principle: a photocathode converts a photon into an electron. This electron is then accelerated into a microchannel plate (MCP) where, by hitting the walls of the microchannels it emits secondary electrons. These secondary electrons hit the wall again and this iterative process leads to an exponentional amplification of the electrons. Around 10 6 of them are generated at the output of the MCP. These electrons are then accelerated towards an anode stripline plan across a gap. When hitting the stripline, this electron shower induces two pulses in the stripline traveling in opposite direction. The signal is read by electronics at both ends of a stripline. 2 Description of the software Of all the stages of the MCP-PMT detector, this software will simulate the last one: the signal development in the anodes striplines. This process takes place in three phases, all of which having to be implemented: Electrons acceleration from the bottom plate of the MCP to the anode plan. 1

2 Electrons hitting the anode plan. Signal development and propagation inside the stripline The simulation requires therefore to solve the Maxwell s equation with finite elements to compute the electromagnetic field and wave created by the electron shower in the gap and anodes. The result will be to display with a graphical interface the progression of the electrons in the gap, the field lines generated in the gap and anodes and voltage across the 50Ω terminaison resistors for each time step of the simulation. The user will be able to setup and launch a simulation using a graphical interface and store its results in a standardized output file. 3 Simulation model and data The system to simulate naturally separate itself in two entities with very different models : the gap, where the electrons are accelerated and the anodes striplines, where the electromagnetic pulses develop and propagates themselves. 3.1 The gap For the simulation of the gap, we have to consider that the MCPs from the previous stage are generating around 10 6 electrons with 6 parameters. All of them are afterwards accelerated in the gap with 6 degrees of freedom. Two options are therefore available: The first option is to consider every single electron coming from the MCP as a single entity in the simulation. Apply to them the laws of physics at each time step: acceleration due to the high voltage, repulsion from the other electrons. Calculate the field radiated by all the electrons for each time step. The requirements for the incoming electrons are: Electron spec Value Position x 0,y 0 Velocity v x0, v y0, v z0 Time of arrival t 0 Number of electrons

3 The second option is a tentative to reduce the complexity of the simulation by grouping all the electrons coming out from the MCP in the same time interval: all the incoming electron between [t and t+1ps] will be treated as one entity called batch. The speed and position of the batch will be the average of the speed and position of all its electrons. Only the high voltage will be applied to the batches, but they will be able to expand due to electron-electron repulsion. The radiated field will be computed at all point. The requirement for the incoming batches are: Batch spec Value Position x 0,y 0 Velocity v x0, v y0, v z0 Time of arrival t 0 Number of electron in batch From 0 to Number of batches Around The anode striplines The second part of the simulation is the anode stripline and signal development inside it. This problem is very independent from the gap simulation. Here, we will simulate how the electrons coming from the gap and striking the stripline are producing an EM wave in the line and how this wave propagates. In order to simulate that, two approaches are possible again: The first possibility is to solve the Maxwell equations throughout the stripline with the following excitations and boundaries: - Inside-out or inside-in pattern. - Radiated field from the electrons above the anode plan. - Deposited charge from the electron or the batches on the anodes. - 50Ω terminaisons at both side of the transmission line. This approach allows to fully calculate and understand the model beyond the anodes striplines: characterize the impedance, the crosstalk, the losses, and the reflexions. 3

4 Considering that implementing the Maxwell s equations in time domain for very high frequencies and small time step might reveal itself very complicated, a second approach could be developed: the transmission line would be modelized as a succession of LC cells. These LC cells would follow the same exact excitation and boundaries: - Inside-out or inside-in pattern. - Radiated field from the electrons above the anode plan. - Deposited charge from the electron or the batches on the anodes. - 50Ω terminaisons at both side of the transmission line. This approach is much simpler but suppose that a LC model of the stripline is precise and sufficient enough to accurately match the behavior of a real transmission line. In particular, the model will have to integrate the frequency dependencies, the losses, the strip-to-strip coupling, 3.3 Data compatibility requirement As a part of the MCP-PMT detector, the software will be required to be able to take as input the output of the previous stage (MCP) from a standardized input file, simulate it and store its results in a standardized output file. An idea of how such a file could look like is given: Standard Input Output File For MCP-PMT Simulation Head Job name : String (16 char) 4

5 Simulated module name : String (16 char) Software version : String (16 char) Creation date : dd/mm/yyyy Simulation id : Int Author : String (16 char) Description : String (32 char) Physical specification Photocathode description : String (16 char) Photocathode gap (in mm) : 4-digit number MCPs description : String (16 char) Number of MCPs : Int MCPs pore size (in mm) : 4-digit nb list separated by comma MCPs gaps (in mm) : 4-digit nb list separated by comma MCPs L/D : 4-digit nb list separated by comma MCPs Open area ratio (in %) : 4-digit nb list separated by comma MCPs Resistance (in Ohms) : 4-digit nb list separated by comma MCPs Pore angle (in degrees) : 4-digit nb list separated by comma MCP to Anodes gap (in mm) : 4-digit number Anodes description : String (16 char) Number of striplines : Int Stripline length (in mm) : 4-digit number Stripline width (in mm) : 4-digit number Stripline hight (in mm) : 4-digit number Stripline spacing (in mm) : 4-digit number High voltage (in Volts) : 4-digit nb list separated by comma Simulation specification Time of simulation (ps) : Int Nb of Photo-electrons : Int Result compression : Boolean PE1 physical properties : x1, y1, vx1, vy1, vz1, t1 5

6 PE1 physical properties : x2, y2, vx2, vy2, vz2, t2 PE1 physical properties : xn, yn, vxn, vyn, vzn, tn Simulation results after the MCP If Result compression = false Total nb of elctrons : Int E1 physical properties : x1, y1, vx1, vy1, vz1, t1 E1 physical properties : x2, y2, vx2, vy2, vz2, t2 E1 physical properties : xn, yn, vxn, vyn, vzn, tn If Result compression = true Total number of electrons : Int Nb of [t0,t0+1ps] : Int Nb of [t0+1ps,t0+2ps] : Int Nb of [t0+n-1ps,t0+nps] : Int Simulation results after the Anodes * If the Anodes are not simulated this section is empty. If not the * voltage across the 50 Ohms is given ps per ps. [t0,t0+1ps] : Int [t0+1ps,t0+2ps] : Int [t0+n-1ps,t0+nps] : Int 6