g. burtin, r.j. colchester, g. ferioli, j.j. gras, r. jung, j.m. vouillot

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16/05/1999DIPAC’99 -- JJ Gras -- CERN SL/BI -- Adaptive Optics for the LEP 2 SR Monitors

G. Burtin, R.J. Colchester, G. Ferioli, J.J. Gras,R. Jung, J.M. Vouillot

Introduction

Conclusion

LEP 1 Layout

LEP 2 Layout

Results

Adaptive Optics for the LEP 2 Synchrotron Light Monitors

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Introduction: The Telescope Layout

_QS18 e

QS12 eßH = 27.3 mßV = 116.6 mD = 0.62 mßH = 24.7 m

ßV = 91.3 mD = 0.02 m

_

IP 8

QS12 e

QS18 e +

+

e e+ _

d = 487 m. d = 300 m.

telescope 869

telescope 863 telescope 837

telescope 831

Layout of the Synchrotron Radiation Telescopes in LEP.

Four Synchrotron Radiation telescopes are installed in LEP around IP8, two for

each particle type, one located at the exit of the experimental straight section where

the Dispersion is very small, the other in the arc where the Dispersion is large.

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Introduction: The LEP 1 era.

With a beam energy limited to 45 GeV we manage to control our telescopes with a relatively standard layout.

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Introduction: The Challenging Problems of LEP 2

But above 80 GeV, side lobes and asymmetric defocusing started to appear, pointing to the expected cylindrical deformation of the Be mirror due to the power deposition of the SR.

To solve this problem, a special set-up had to be made:

LEP 2 The Adaptive Optics

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The Basic Layout

LEP 1 Layout: The Basic Layout

The Imaging Devices

- Light Extraction Mirror:a 10 mm thick, 23x23 mm2 Be mirror.

- Spherical Imaging Mirror:with a focal length of 4 m.

- X - Y Steering Mirror:to align precisely the light with the bench.

The Acquiring Devices

- A Camera:a normal CCD.

- A Linear Density Filter:to match the light intensity to the detector sensitivity.

- Chromatic & Special Filters:to control wavelength and polarisation components.

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LEP 1 Layout: The Major Difficulty

The major difficulty was the precise selection of the light source due to the large bending radius of LEP (3100 m) and its distance from the telescope (21 m).

A slit has been implemented to control precisely the centre of the light origin and the longitudinal acceptance. By moving the slit, the whole acceptance defined by the extraction mirror can be explored.

Telescope Horizontal Phase Space with Beam Trajectories, Extraction Mirror and Slit acceptance bands.

(The axes are the horizontal machine axis x and the trajectory angle

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LEP 1 Layout: The Applied Correction

The correction applied to the measured size is:

REAL-H/V =

MEAS-H/V - LA -

D-H/V

Where w = the slit size and XX represents broadening introduced by:LA = kLA w - the longitudinal acceptance.

DH = kDH /w - diffraction due to the slit uniform window limitation

DV = kDV 2/3 - SR diffraction

This correction currently represented around 10 % of the measured sigma in the horizontal plane and 35 % in the vertical.

The telescopes have been optimised during the LEP 1 runs to provide the beam emittances with best precision (±0.1 nm in the vertical plane).

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LEP 1 Layout: The Tuning Application

The Beuv Scan Tool

LA = kLA w

DH = kDH /w

kDV = 3.9± 0.1kDH = 1.6± 0.2kLA = 40 ± 10

Ability to measure vertical emittances smaller than 1nm to ±0.1nm

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The LEP 2 Layout: The Final Layout

Layout of the Synchrotron Radiation Telescopes in LEP.[LEP 2 Devices in Red]

The Imaging Devices

•Light Extraction Mirror

•Deformable Mirror

•Slit

•Spherical Imaging Mirror

The Acquiring Devices

•Mobile Camera

•Density Filter

•Additional Polarisation Filter

•Filters

mobile CCD polarisationfilter

filters :chromatic andspecial

focalisation mirrorF=4m

SR

slit in focal plane

deformable mirror

x-y steering mirror

Be mirror

linear density filter

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LEP 2 Layout: The Polarisation Filter

When LEP energy exceeded 80 GeV, side lobes appeared on the beam image explained by the combination of the remaining vertical polarisation component of the SR and the cylindrical deformation of the Be Mirror due to the power deposition of the SR.

A dichroic sheet polariser having an extinction ratio of 1/4000 has been added to the original metallic polarisation filters in order to attenuate further these lobes.

LEP 2

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LEP 2 Layout: The Mobile Camera

Above 80 GeV, the beam image is also no longer focussed in the same location in H and V, due to this cylindrical deformation of the extraction mirror.

Slit position scans clearly demonstrate this effect and a mobile camera has been introduced to be able to optimise the telescopes either for H or V measurements.

Result of a Mobile Camera Scan done at 94.5 GeV with 2.4 mA showing the beam image focussed at different location in the horizontal and vertical planes.

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LEP 2 Layout: The Mobile Camera

Other Results of Mobile Camera Scans

E = 94.5 GeV- I = 1.2 mA E = 94.5 GeV - I = 1.8 mA

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LEP 2 Layout: The Deformable Mirror

In order to focus both planes simultaneously and anticipate the LEP beam energy and current increase (towards 100 GeV with currents above 3 mA per beam), an cylindrically deformable mirror has been implemented to compensate the main deformation of the Be mirror.

This outsourced model allows a cylindrical deformation from flat to a radius of curvature of 350 m allowing to correct up to 3 mA at 94.5 GeV.

A home made mirror has been installed this year and should allow us to correct up to 2.8 mA at 100 GeV (with a minimum radius of 270 m).

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LEP 2 Layout: The Deformable Mirror

200

400

600

800

1000

1200

1400

1 1.5 2 2.5 3 3.5 4

Ib [mA]

R [

m]

A series of measurements were made at different energies and it has been estimated that the radius follows a law of the type:

R= 7.7 105 Ib-1.7 Eb

-3

Curvature radius of the Be mirror as a function of beam current measured at 94.5 GeV

with the radius R in [m], the beam intensity Ib in [mA], and the beam energy Eb in [GeV]

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Results

In 1998, the LEP2 layout has been implemented in the electron telescopes and:

• The Be mirror deformation law has been refined.

• A software server has been developed to monitor every minute the beam energy and current and deform accordingly the mirror.

• The polariser has been fine tuned to maximise the vertical polarisation attenuation.

• The slit position has been adjusted to target the SR origin inside the corresponding bending magnet.

•The slit width has been adjusted to 1 mm for a longitudinal acceptance of ± 40 cm.

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Results

• Under these conditions, both telescopes gave the same emittance, within the errors on the beta values.

• The importance of the deformable mirrors is very well demonstrated below, where the same beam is imaged with and without deforming the mirror and filtering the vertical polarisation.

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Conclusion

For the 1999 run, all telescopes have been put in the LEP 2 configuration and a campaign to re-measure the correction coefficients will be undertaken.. It should be then possible to reach a 2.8 mA beam current at 100 GeV before having to move the camera which gives a correction capability of an additional 1 mA, close to the LEP limit.

mobile CCD polarisationfilter

filters :chromatic andspecial

focalisation mirrorF=4m

SR

slit in focal plane

deformable mirror

x-y steering mirror

Be mirror

linear density filter

g. burtin, r.j. colchester, g. ferioli, j.j. gras, r. jung, j.m. vouillot

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