Ex (changeable) - PowerPoint Presentation

Slide1

Ex(changeable)Me(sh) Micromegas - transparency and gain dependencies

Fabian Kuger1,2 and Daniel Baur1Julius-Maximilians-Universitä̈t Würzburg (Germany) , 2 CERNSeptember 13th 2016RD51 Collaboration Meeting – Aveiro (Portugal)

sponsored by theSlide2

OutlineEx

changeable Mesh MicromegasConcept, layout and designExperimental setup and methodQuick review of previous resultsResults from the ExMe studies (Simulation + Exp. Data)Update on Transparency measurementsGain change with mesh geometry Pillar distance, mesh sagitta and their impact on the gainResistive anode structure effect on the gas gain(Simulation & description of electron Transparency)2ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Slide3

Ex(changeable) Me(sh) Micromegas

3ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016To study different meshes under comparable conditions we designed and build twoMicromegas with an exchangeable meshes:

-

Independent

mesh

frames

allow mesh exchange

❸

❶

❷

❹

❸ Mesh frame

❶ Drift panel

(stiff-back, internal

gas lines, drift cathode, springs)

❷ O-Ring

❹

Readout

panel

Schematic view of the

ExMe

components

[

first presented during IWAD & 14

th

RD51 Meeting (2014, Kolkata)]

-

ExMe

1 and 2 differ only in their anode material (ExMe1: sputtered; ExMe2: screen-printed)

- Each ExMe chambers’ readout is divided in four sectors, covered by differently spaced pillar patterns. (Pillar-arrangement in triangular lattice with 5-10mm spacing)

Many detector

inherent

parameters

(

amplification- and drift gap thickness, readout

surface, capacitance

etc.)

are kept constant and allow direct

study of mesh parameters and amplification gap geometrySlide4

Experimental Setup & Method4ExMe studies – Transparency & Gain dependencies

- RD51 Collaboration Meeting, Aveiro-September 13th 2016❶❷❸❹

❺

n

e

: statistically distributed number of primary electrons

A

: fraction of electrons lost to gas attachment during drift

T

: fraction of electrons conducted through the mesh (Transparency)

G

: statistically distributed amplification of one electron

c

r

/o

:

read-out constant, converting charge to digital signal

❸

mesh losses

❶

primary ionization

❷

electron drift

❹

amplification

Signal strength S

(= ADC equivalent of charge induced on the Micromegas read-out)

can be described by a

factorized approach

: We measure spectra of a Cu X-Ray and determine the signal strength of the Kα peak under variation of Voltages and Geometry (meshes / chambers)

S = ne ᐧ (1-A) ᐧ T ᐧ G ᐧ

cr/oSlide5

Previous results - Electron Attachment5ExMe studies – Transparency & Gain dependencies

- RD51 Collaboration Meeting, Aveiro-September 13th 2016Cross-sections for attachment in CO2 (top), O2 (middle) and H2O (bottom)

Garfield++

Simulation

(microscopic tracking)

in constant electric fields quantify attachment losses

(A)

e

-

+

CO

2



CO + O-

(B)

e

-

+ O

2

 O

2

-

(C)

e

- + O

2  O- + O (D)e

- + H2O  H + OH- (E)e- +

H2O  H2 + O-

(F)

e

-

+ H

2

O

 HO

+ H

-

Attachment processes in ArCO

2

, Oxygen and H

2

O

E(e-): 0.1-1.0 eV: 3-body attachment

 O

2

dominant

E(e-): >3eV dissociative attachment

 CO

2

dominant + O

2

/ H

2

O add small contribution

(* No attachment in N

2

, Ne, He, Kr. Concentration of H

2

and CF

4

in atmospheric air are negligible.)

S = n

e

ᐧ

(1-A)

ᐧ

T

ᐧ

G

ᐧ

c

r

/o

[

first presented as poster at MPGD 2015 (Trieste)]Slide6

Previous results - Electron Transparency6ExMe studies – Transparency & Gain dependencies

- RD51 Collaboration Meeting, Aveiro-September 13th 2016Garfield++ Simulation (microscopic tracking) combined with ANSYS (FEM) geometry & field map modelingT depends on exact mesh geometry and ED (open area is just a coarse predictor)smaller mesh structures yield reduced T (at comparable open area and E

D)

S = n

e

ᐧ

(1-A)

ᐧ

T

ᐧ

G

ᐧ

c

r

/o

[

first presented as poster at MPGD 2015 (Trieste)]Slide7

Previous results – Experimental data7ExMe studies – Transparency & Gain dependencies

- RD51 Collaboration Meeting, Aveiro-September 13th 2016Transparency measurements presented in 2015 were biased by huge O2 contamination, requiring adjustment of the scaling factor in SIM – EXP comp. Fluctuation in O2 contamination forbade direct gain comparison between different measurementsRepeat measurements with improved gas supply & resealed chambers

[first presented during RD51 Collaboration Meeting Mar2105]Slide8

OutlineExchangeable Mesh Micromegas

Concept, layout and designExperimental setup and methodQuick review of previous resultsResults from the ExMe studies (Simulation + Exp. Data)Update on Transparency measurementsGain change with mesh geometry Pillar distance, mesh sagitta and their impact on the gainResistive anode structure effect on the gas gain(Simulation & description of electron Transparency)8ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Slide9

S = ne ᐧ (1-A) ᐧ

T ᐧ G ᐧ cr/oElectron Transparency – Experimental data I9ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016normalization

Normalizing the signal corrects for different gain per measurement sequence.

Change of gain due to different

U

Amp

 Vanished with normalization

Different

U

Amp

Gain variation by chamber

 Removed with normalization

ExMe1 / ExMe2Slide10

S = n

e ᐧ (1-A) ᐧ T ᐧ G ᐧ cr/oElectron Transparency – Experimental data10ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016normalization

Fraction of signal electrons:

Transparency +

Attachment losses

(independent of

U

Amp

& resistive Anode)

New data with reduced and much more stable contamination – level O

2:

50-200ppm

Experimental data can be directly compared

O

2

fraction still too large to neglect attachment correction for normalization

Most severe effect for

‘low transparency meshes’ (50-30 / 32-18)

30µm wire meshes

18µm wire meshes

E

Drift

[V/cm]

Signal Strength at

T

max

– normalized [ ]

EDrift

[V/cm]Signal Strength at T

max – normalized [ ]Slide11

OutlineExchangeable Mesh Micromegas

Concept, layout and designExperimental setup and methodQuick review of previous resultsResults from the ExMe studies (Simulation + Exp. Data)Update on Transparency measurementsGain change with mesh geometry Pillar distance, mesh sagitta and their impact on the gainResistive anode structure effect on the gas gain(Simulation & description of electron Transparency)11ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Slide12

Gain dependencies - PreludeNeither experiment nor simulation are dedicated to a quantitative gain studyExperiment: - No absolute gain calibration, only relative measurements

- Small fluctuation in O2 concentration effect gain significantlySimulation: - full avalanche simulation in Garfield++ is CPU intense for large avalanches & complex geometries (ANSYS 3D models) - 2D simplification in COMSOL is well suited for qualitative statements 12ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016CPU effective E-Field calculation under variation of :

- mesh wire / aperture- pillar height- anode thickness

- strip widthSlide13

Gain Dependence – Mesh Geometry13ExMe studies – Transparency & Gain dependencies

- RD51 Collaboration Meeting, Aveiro-September 13th 2016The distance of the mesh wires affect the strength of the amplification fieldd=30µm a=50µm d=30µm a=100µm

d=18µm a=32µm

d

=18µm a=60µm

Thinner and more dense wires lead to a stronger + more homogeneous field

Meshes with 18µm wire diameter

Meshes with 30µm wire diameterSlide14

Gain Dependence – Mesh Geometry14

ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Comparing the normalization factors between different measurement sequences, indicates differences in the gas gain

Gain dependence as predicted for all but one mesh: 60-30

(systematically lower gain)

 Strong indication for the effect, but not fully conclusive

S = n

e

ᐧ

(1-A)

ᐧ

T

ᐧ

G

ᐧ

c

r

/o

h

igher gain with 18µm mesh-wires

Decreasing gain with wider aperture

? except 60-30 mesh ?

ExMe1

ExMe2Slide15

OutlineExchangeable Mesh Micromegas

Concept, layout and designExperimental setup and methodQuick review of previous resultsResults from the ExMe studies (Simulation + Exp. Data)Update on Transparency measurementsGain change with mesh geometry Pillar distance, mesh sagitta and their impact on the gainResistive anode structure effect on the gas gain(Simulation & description of electron Transparency)15ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Slide16

Gain Dependence – Pillar DistanceThe sagging of the mesh between two pillars affects the amplification

field and the avalanche development length and their for the gain16ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016y-coordinate [µm] Ey electrical field strength [kV/cm]ExMe - Sectors with different pillar distances A: 5mm, B: 7mm, C: 8,5mm, D: 10mm

Simulated in COMSOL Multiphysics

by variation of the mesh-wire distance to the anode

(‘effective pillar height’ = 120 – 130µm)

Mesh Tension [N/cm]

Sagitta

[mm]Slide17

Gain Dependence – Pillar Distance17

ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Positive correlation between pillar distance & gain is visible (larger distance  more sagging  higher gain), but deviations occur in some data-sets Clear indication for the effect, but no conclusive proof

ExMe

- Sectors with different pillar distances

A: 5mm,

B: 7mm,

C: 8,5mm,

D: 10mm

≈ 25% effect

U

Amp

[V]

Signal Strength at

T

max

[ADC]Slide18

OutlineExchangeable Mesh Micromegas

Concept, layout and designExperimental setup and methodQuick review of previous resultsResults from the ExMe studies (Simulation + Exp. Data)Update on Transparency measurementsGain change with mesh geometry Pillar distance, mesh sagitta and their impact on the gainResistive anode structure effect on the gas gain(Simulation & description of electron Transparency)18ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Slide19

Gain Dependence – Anode Thickness 19ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-

September 13th 2016y-coordinate [µm] Ey electrical field strength [kV/cm]But: in a

Micromegas, the field is not homogeneous along the electron path (as in a parallel plate setup)

stronger field, shorter avalanche

less strong field, longer avalanche

The

amplification field

and the

avalanche development length

are affected

I

n a

parallel plate setup

the loss of amplification due to field strength reduction dominates its increase caused by longer longitudinal extend.

Caution

: Simulation does not take into account an effect on the pillar height by the anode material!Slide20

Gain Dependence – Anode Material20

ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016ExMe 1 (sputtered resistive anode < 1µm) and ExMe 2 (screen-printed anode ≈ 12µm)



The sputtered resistive anode yields a systematically higher gain.



Effect

strength

varies slightly

(due to mesh geometry,

-

tension, gas mixture)

.

U

Amp

[V]

Signal Strength at

T

max

[ADC]Slide21

Gain Dependence – Anode Thickness The two ExMe chambers are designed equally despite the resistive anode structure

21ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016ExMe 1 - sputtered resistive anode ExMe 2 - screen-printed anodeprovided by Masahiro Yamatani, University of Tokyo12µm

<1µm resistive Layer ≈12µm

Pillar (

Coverlay

)

128µm

(2x 64µm)

Wire mesh resting on pillars

?



How does this affect the detectors’ gain?Slide22

ExMe results - SummaryNew new ExMe datasets with much higher gas purity are sufficient to perform (qualitative) gain comparisons:

22ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Reduced wire thickness and smaller mesh aperture yield a larger amplification (due to stronger EAmp)Simulation and data indicate gain increase with wider pillar distance (because of increased mesh sagging)ExMe2 (screen-printed anode) shows systematically lower gain than ExMe1 (sputtered anode)

Surface profile of

ExMe1/2

will

be measured / compared

Mesh tension needs verification

(60-30 mesh gain behavior)

To be done…

①

②

③

Energy resolution

dependanceSlide23

OutlineExchangeable Mesh Micromegas

Concept, layout and designExperimental setup and methodQuick review of previous resultsResults from the ExMe studies (Simulation + Exp. Data)Update on Transparency measurementsGain change with mesh geometry Pillar distance, mesh sagitta and their impact on the gainResistive anode structure effect on the gas gainSimulation & description of electron Transparency23ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Slide24

Electron Transparency

24ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016S = ne ᐧ (1-A) ᐧ T ᐧ G ᐧ cr/oElectron Transparency can be simulated quiet accurately using FEM + Garfield ++, but- Simulations are time consuming (and prone to mistakes, difficult to notice)- Systematic underestimation of T at high UDrift remainsGeometrygas mixture

1.) We know the main predictors for T, but how do they affect it?

Voltages

U

Drift

-

U

Amp

?

2.

) Is

a parameterized mathematical description of T possible / reliable / useful?

Not only a fit, but a formalized description?Slide25

Electron Transparency25ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-

September 13th 2016Different processes contribute to the electron lossduring the mesh transit (=1-T):Ⓐ Field lines (FL) ending on the wiresT = # e-passing trough the mesh# e-approaching the meshⒶ reduces T by <1% in the Micromegas working range. (Eamp

/Edrift

> 10)

analytical solution for wire grids

(

meshes approximated as two crossed wire grids)

2D

and 3D simulation possible

(FEM: COMSOL or

neBEM

)

COMSOL simulation

,

streamline

-

density not prop. to field strengthSlide26

Electron Transparency

26

ExMe studies – Transparency & Gain dependencies

- RD51 Collaboration Meeting, Aveiro-

September 13

th

2016

Different processes contribute to the electron loss

during the mesh transit (=1-T):

Ⓐ Field lines (FL)

ending on the wires

Ⓑ Deviation from FL

d

ue to

electron

i

nertia

T =

#

e

-

passing trough the mesh

#

e

-

approaching the mesh

As

long as

Ⓐ is negligible

& if

T is dominated by

Ⓑ

:

T

Min

= meshes’ open area

(projection along

E

drift

FL)



Higher chance of hitting a wire with increased

v

Drift

The electrons inertia

delays

its

redirection along the FL, the electrons approach closer to wires

analytical solution for wire grids

(

meshes approximated as two crossed wire grids)

2D and 3D simulation possible

(FEM: COMSOL or

neBEM

)

Movement of electrons in gases can be well simulated with the Garfield ++

microscopic tracking - method

Ⓐ reduces T by <1% in the Micromegas working range.

(

E

amp

/

E

drift

> 10)

COMSOL simulation

,

streamline

-

density not prop. to field strength

Overestimation of this effect in Garfield++ due to finite steps

Could explain previously reported discrepancy in T

Sim

. vs. Exp.Slide27

Electron Transparency

27ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016Garfield++ Simulation (microscopic tracking) combined with ANSYS (FEM) geometry & field map modelling yield predictions for electron transparency.

S = n

e

ᐧ

(1-A)

ᐧ

T

ᐧ

G

ᐧ

c

r

/o

Transparency drops far below open areaSlide28

Electron Transparency28ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-

September 13th 2016Different processes contribute to the electron lossduring the mesh transit (=1-T):Ⓐ Field lines (FL) ending on the wiresⒷ Deviation from FLdue to electron inertiaⒸ Deviation from e- pathdue to scatteringT = # e-passing trough the mesh

# e-approaching the mesh

T reduction

due to

Ⓑ limited

T

Min

= meshes’ open area

(projection along

E

drift

FL)



Higher chance of hitting a wire with increased

v

Drift

e

-

inertia

results in delayed redirection and a path closer to the wires

s

cattering results in ‘broad’ e- paths, with probability of deviation in either direction

 probability for a larger deviation increases with higher scattering energy (EDrift)

analytical solution for wire grids

(

meshes approximated as two crossed wire grids)

2D and 3D simulation possible

(FEM: COMSOL or

neBEM

)

Ⓐ reduces T by <1% in the Micromegas working range.

(

E

amp

/

E

drift

> 10)

Ⓒ further reduces T, creating an ‘effective open area’

COMSOL simulation

,

streamline

-

density not prop. to field strengthSlide29

Electron Transparency

29ExMe studies – Transparency & Gain dependencies- RD51 Collaboration Meeting, Aveiro-September 13th 2016S = ne ᐧ (1-A) ᐧ T ᐧ G ᐧ cr/oCan the contributions of these processes to electron losses / Transparency be discriminated? Could this be used for a parameterized T-description ?

Geometry

g

as mixture

Transparency parameters:

Could a universal parameterized description be used for

T prediction / calculation

?

Voltages

U

Drift

-

U

Amp