Modelling of VPTs
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Transcript of Modelling of VPTs
Modelling of VPTs
Work in Progress
Ignacio Yaselli, Brunel University
Creating a Model for VPTs
• How fast are the VPTs?• What affects the performance of VPTs• Allows prediction of performance under
conditions which are not practical to test.• Development of technical data for use of VPTs
on other applications.• Compare results with previous studies and with
future experiments.
Electron Absorption
• Photons produce primary electrons at cathode• Some e- absorbed on Anode as primary signal • Some e- absorbed on Dynode• Secondary emissions of e- from Dynode (some then
absorbed by Anode, some return to the Dynode
VPT Mesh
RIE: 100 lines per mm, 50% transparent
Sample of SIMION Simulation
• Anode – Realistic Mesh at 1000 Volts
• Dynode – Electrode at 800 Volts
• Cathode
Realistic simulation of a production (RIE) VPT
Potential Arrays
1000 V
Anode
800 V
Dynode
0 V
Kathode
0 V
Aluminium Wrap
SIMION User Programs
• HP like programming• Separated on segments
– Each independent of each other– Each called at different stages of the ION flight
• Only the segment that are needed have to be defined• Access the behaviour of ION by use of reserved
variables• User Programs are linked to specific instances.
Therefore, SIMION relies on Instance hierarchy for running these programs.
• Determine which electrode is hit by electron.
• Recall sufficient data to: – Generate secondary electron– Construct a *ion file from these secondary
electrons– Generate a Signal file.
Implementation of SIMION User Programs
Pros and Cons of SIMION
PRO• Powerful 3D Ion
Optics Workbench• User Programs• Geometry files• 3rd party programs
cooperation• Allows data
recording• Used over many
years for electon optic design
CONS
• Requires 3rd party program for simulation of secondary emission
ION-ator Functionality
• Assume a Starting Ion File has been Created• Assume, that that SIMION has originated first Output • Load data files generated by SIMION
– Open Output file, Read it, and Close It– Determine which Ions Have Generated Previous Secondary Ion
• Generate Random New_Ions from each previously unused Ion– Generate a Poison Distributed Random Number– Parameterise characteristics of parent ion to generate
subordinates
• Select Ions With Sufficient Energy to Escape Electrode• Append Only the Ions Selected to the original Ion File.
Relation of SIMION and ION-ATOR
SIMION ION-ATOR
Sort hits on Anode from hits on Dynode
Sort hits on Anode from hits on Dynode
Record hits on Anode on signal file for analysis
Record hits on Anode on signal file for analysis
Store Secondary Electrons on *.ion file
Store Secondary Electrons on *.ion file
Generate secondary electrons from hits on Dynode with int N= Ep/m
Generate secondary electrons from hits on Dynode with int N= Ep/m
Reads *.ion file and loadElectrons into memory
Reads *.ion file and loadElectrons into memory
Load User ProgramLoad User Program
Fly ElectronsFly Electrons
Store Hit ListStore Hit List
Simple dynode model will be refined
Currently in Progress
• Refining Simulation Model (especially the secondary emission details)
• Testing Hypothesis• Analysis of simulation results: Time Delay,
pulse width, gain, etc. • Generating high statistics simulations
K
A
D
Dynode at 200V Dynode at 1005V
Dynode at 800V Dynode at 1015V
Secondary electron absorption with anode at 1kV and at a magnetic field of 0T
Measured VPT Gain at 0T
0
2
4
6
8
10
12
0 200 400 600 800 1000
Dynode Voltage
Gai
n
V(A)=1000V
V(A)=800V
Note that this is NOT data from a production VPT
Simple Dynode Model
• Dynode assumed noise free• Number of secondary electrons is strictly
proportional to incident electron energy• Gain is 25 for an electron energy of 1000
eV• Emission energy of secondary electrons is
fixed at 5 eV but the angle is random within +- 5 degrees of the normal to the dynode
Simulated Gain of VPT (0T)Hit contributions with Anode at 1000 V
0
200
400
600
800
1000
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1400
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200 400 600 800 900 950 980 1005 1005.1 1005.2 1005.25 1005.26 1005.3 1005.5 1006
Dynode Voltge
Hit
s o
r G
ain
%
Anode Dynode
Hit contributions with Anode at +200 V from Dynode
0
200
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200 400 600 800 900 950 980 1005
Dynode Voltage
Hit
s o
Gai
n %
Anode Dynode
Anode fixed at 1000V Anode at Dynode +200V
Simulated Gain of VPT (low magnetic field)
Hit contributions with Anode at 1000 V and a Magnetic fiel of 0.1T at 15 degrees
0
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200 400 600 800 1000 1005
Dynode Voltge
Hit
s o
r G
ain
%
Anode Dynode
Hit contributions with Anode at 1000 V and a Magnetic field of 0.01T at 15 degrees
0
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800
1000
1200
1400
1600
200 400 600 800 900 1000 1005 1006
Dynode Voltge
Hit
s o
r G
ain
%
Anode Dynode
0.01T with Anode fixed at 1000V 0.1T with Anode fixed at 1000V
Time Response of the VPT
0200
400600
8001000
12001400
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200
0
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Time in pS Dynode Voltage
Time Response of VPT
200
400
800
By increasing the Dynode Voltage, the gain increases and the pulse width narrows
IMMEDIATE FUTURE
• Prepare poster for the Beaune Conference (we will circulate a draft around Wednesday 15th).
• Setup equipment for testing model
• Start experimental data collection
Conclusions
• The Simulation system consist of 2 different software packages: – A commercially available package called SIMION for data
generation and Ion tracking in static electric and magnetic fields.– Specially written program for simulating the dynode properties of
the VPT as well as data handling.
• The combination of the above has been essential for understanding the behaviour of VPTs.
• By comparing the results from these simulation with data acquired from real VPTs at RAL and Brunel, it will be possible to trust results from simulation which are impractical to test in the lab. i.e orientation of VPT in full 4T field etc…
Appendices
K
A
D
Dynode at 200V Dynode at 1005V
Dynode at 800V Dynode at 1015V
Secondary electron absorption with anode at 1kV and at a magnetic field of 0.01T at 15o
K
A
D
Dynode at 200V Dynode at 1005V
Dynode at 800V Dynode at 1015V
Secondary electron absorption with anode at 1kV and at a magnetic field of 0.1T at 15o
Time Response of VPT with VA=1000V, VD=1005V
Time Response of VPT with VD=1005V
0
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0 800 1600 2400 3200 4000 4800 More
time in pS