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

1200

1400

1600

1800

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

400

600

800

1000

1200

1400

1600

1800

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

200

400

600

800

1000

1200

1400

1600

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

200

400

600

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

1600

200

0

50

100

150

200

250

300

350

400

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

20

40

60

80

100

120

140

160

0 800 1600 2400 3200 4000 4800 More

time in pS