Amsterdam, April 2,2003 P. Colas - Micromegas TPC 1

Micromegas TPC: New Micromegas TPC: New Results and Prospects Results and Prospects Micromegas TPC: New Micromegas TPC: New Results and Prospects Results and Prospects

• New tests in magnetic fieldNew tests in magnetic field

• FeedbackFeedback

• Cosmic setupCosmic setup

F. Bieser1, R. Cizeron2, P. Colas3, C. Coquelet3, E. Delagnes3, P. Desaunnais2

A. Giganon3, Y. Giomataris3, G. Guilhem2, J. Jeanjean2, V. Lepeltier2, V. Puill2, Ph. Rebourgeard3, J.-P Robert3, M. Ronan1

(and many others, not mentionned)1) LBL, 2) LAL Orsay, 3) DAPNIA Saclay

ECFA-DESY workshopworkshop

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 2

Measurements in a magnetic fieldMeasurements in a magnetic fieldMeasurements in a magnetic fieldMeasurements in a magnetic field

2T magnet in operation since March 2002 in Saclay.

2 data taking periods until now:

June 2002: ion feedback measurements in a 1cm-drift MWPC TPC and a Micromegas TPC

February 2003: measurement of the gain and of the ion feedback with a 15cm-drift Micromegas TPC

Future: take cosmic tracks in a 50 cm, 1000 channel Micromegas TPC

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 3

Measurements in a magnetic fieldMeasurements in a magnetic fieldMeasurements in a magnetic fieldMeasurements in a magnetic field

The 2T, 530mm bore magnet is now in its final location.

It provides a 1% homogeneous field over +-25cm.(An 8 T, 150mm bore magnet could be used in the future)

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 4

The 15 cm drift TPCThe 15 cm drift TPCThe 15 cm drift TPCThe 15 cm drift TPC

Gains up to

7000 for Ar+3%CF4 at 75 kV/cm over 50 m

64000 for Ar+10% isobutane at 84 kV/cm over 50 m

were measured in Orsay (january 2003).

In the following measurements, the gain was about 1300 and the gas was Ar+10%CH4

THE RUNNING CONDITIONS ARE REALISTIC:

Long drift : 15cm

drift field : 240 V/cm

Moderate gain

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 5

Measurements in a magnetic fieldMeasurements in a magnetic fieldMeasurements in a magnetic fieldMeasurements in a magnetic field

Measure the iron 55 peak position and width as a function of the magnetic field

B=1 tesla

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 6

Measurements Measurements in a magnetic in a magnetic

fieldfield

Measurements Measurements in a magnetic in a magnetic

fieldfield

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 7

Ion feed-back: the micromegas funnel Ion feed-back: the micromegas funnel effecteffect

Ion feed-back: the micromegas funnel Ion feed-back: the micromegas funnel effecteffect

S1/S2 ~ Eamplif / Edrift ~80 000 / 200

Due to electron diffusion, most ions are created outside the funnel tube : when S1/S2 is large and comparable to the pitch (S2 small wrt avalanche cloud size) , the ions areunlikely to follow back the field lines to the drift space.

The ions return to the grid: ion feed-back and

related space charge

effects are suppressed

S1

S2 L

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 8

l

Calculations : add the contribution of each hole to the electron yield -> get the fraction in the funnel tube

2D

3D

Ion feedback theoryIon feedback theoryIon feedback theoryIon feedback theory

The two important quantities are : the dispersion after 50 microns and l, the grid pitch

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 9

Results

1500 lpi (sigma/l=0.75)1000 lpi (sigma/l=0.5)500 lpi (sigma/l=0.25)

5.2_

_ ratiofield

feedbackion 03.1_

_ ratiofield

feedbackion 1_

_ ratiofield

feedbackion

Ion feedback theoryIon feedback theoryIon feedback theoryIon feedback theory

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 10

ion feedback

0

0,05

0,1

0,15

0,2

0,25

0,3

0,2 0,3 0,4 0,5 0,6 0,7 0,8

sigma/l

ion

fee

db

ack

Field ratio

Feedback 2D

Feedback 3D

Conclusion: the ion feedback is close to its optimum of 1/field ratio if /l > 0.5 (i.e. >1000 l.p.i. meshes for usual gases)

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 11

ION FEEDBACK MEASUREMENTS

Vmesh

Vdrift

I2 (mesh)

I1 (pA-meter)

X-ray source

Primaries + feedback

I1+I2 ~ G x primaries

Obtain primaries from G=1 (Vmesh small)

Eliminate G between the 2 equations to obtain the feedback fraction

Typically 25 pA

Typically 350 pA

Typically 35 nA

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 12

The feedback does not depend significantly on the magnetic field

It is in agreement with prediction for Ar-CH4 (90:10) and for a 500 lpi grid (data from June 2002)

Optimal case

(reached for a 1000 lpi grid)

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 13

In the February 2003 data taking, the field ratio was 334, corresponding to an optimal feedback of 0.3%. The observed feedback, 0.9%, is what is expected for a 500 lpi grid.

-> We have a good understanding of the feedback in a micromegas TPC. It is insensitive to magnetic fields up to at least 2T.

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 14

TPC for the cosmic test

Field cage

Detector

Front end electronics

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 15

2x10 mm2 pads

1024 pads

1x10 mm2 pads

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 16

Front-end electronicsFront-end electronicsFront-end electronicsFront-end electronics

STAR readout.

32 channels per card, 10-bit ADC, up to 512 50ns time buckets.

16 cards have been modified to raise the pedestal, to gain sensitivity to negative signals, and protection diodes have been added

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 17

Tests of front-end electronicsTests of front-end electronicsTests of front-end electronicsTests of front-end electronics

1000 channels being tested.

Noise <4counts per channel3cards to be repared2% of the channels have pbs

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 18

Status of the cosmic testStatus of the cosmic testStatus of the cosmic testStatus of the cosmic test

Magnet operational, in final position

Endplate fully wired

Mesh manufactured last week, to be tested

Cosmic trigger being installed

New high purity gas system being ordered

Field cage under construction

Amsterdam, April 2,2003 P. Colas - Micromegas TPC 19

CONCLUSIONCONCLUSIONCONCLUSIONCONCLUSION

• New measurements have shown that the behaviour of a micromegas TPC in a magnetic field is not deteriorated. Gain, transparency, ion feedback have been shown not to be affected by a 2T magnetic field.

• Collaboration with Carleton showed charge spreading on a micromegas with the anode coated with a resistive foil

• These results, together with aging tests and other measurements and simulations already presented, show the adequation of micromegas technology to the readout of the LC TPC

• A cosmic test is in preparation, with 1000 pads.