TPC Laser system

1
TPC Laser system
Design Review, CERN, 27 January 2003 Børge S.
Nielsen, J.J. Gaardhøje, N. Lindegaard and Jørn
Westergaard Niels Bohr Institute A. Lebedev,
Brookhaven National Laboratory

• Functions of the system
• Basics of the design
• Construction tolerances and alignment
• Lab tests at NBI
• Status of system components
• Production status and installation

2
Laser system objectives

• Electronics testing
• Sector alignment
• Drift velocity monitoring
• Pressure, temperature
• Temperature gradients (stratification?)
• ExB effects, space charge
• Two possible approaches
• Relative measurements, rely only on time
  stability of laser ray position
• Absolute measurements, requires knowledge of
  absolute position of laser ray. More ambitious

3
TPC Laser principle
20-40 µJ/pulse, ? 1 mm
266 nm, 100 mJ/pulse, 5 ns pulse, ? 25 mm
4
Beam pattern inside TPC
Radial beams Stratetic sector boundary
crossings Avoid laser beam crossings 8 layers
of rays along z
336 laser tracks in full TPC
?
5
UV laser
Spectron Laser Systems (UK) model
SL805-10-UPG Pulsed UV laser (NdYAG) 100 mJ / 5
ns pulse _at_ 266 nm, max 10 Hz
expanded beam ? 25
mm
divergence lt 0.35 mrad, pointing stability lt 0.1
mrad
remote controllable (RS-232)
6
Laser hut and laser beam transport (1)
7
Laser hut and laser beam transport (2)
8
Beam transport on TPC end plates (1)
Shaft side
Beam monitor
Beam splitter 99/1
Prism 30º bend
Beam splitter 50/50
Prism 30º bend
Beam splitter 50/50
Beam splitter 33/67
Beam entrance 90º mirror
Muon side beam entrance
9
Beam transport on TPC end plates (2)
Muon side
Beam monitor
Beam splitter 99/1
Prism 30º bend
Beam splitter 50/50
Beam splitter 50/50
Beam entrance 90º mirror
Beam splitter 33/67
10
Muon arm side beam transport
limited space between TPC and space frame ?
move beam transport 10º from vertical plane
adds 2 mirrors on shaft side modifies beam
transport on muon side
Shaft side
standard prism
knee in beam transport on shaft side
Muon side
special prism
attach 50 mm pipe on outside of TPC permanently
We are currently considering to move back into
the vertical plane
11
Optics on TPC end plates
Market survey on optical components for
end-plates ongoing
Design in progress on opto-mechanical supports on
end-plates
(copy/modify STAR systems)
Prism holder
Laser beam
Construction foreseen in NBI workshop
Piezo-electric adjustment system from New Focus
on order ?
12
Mirror adjusters
Based on commercial piezo solution
? 3 fully adjustable mirrors per half-TPC
Ethernet interface
13
Laser rod with mirrors
14
Mirror support rings
All rings have been produced at NBI
15
Micro-mirror z positions

• 4 micro-mirrors per rod, at about (0, 1/3, 2/3,
  1) length
• vary z positions slightly between odd (a) and
  even (b) rods

(a)
(b)
(a)
(a)
(b)
(b)
16
Alignment by Poisson spot
Screen
Wide laser beam
Ball or disc
d
Camera
? 266 nm
17
Operational aspects

• Sensors and remote controls
• Remote setup and monitoring of laser
• CCD cameras for beam positioning entrance
  mirrors on end plates
• 
  end points on end plates
• 
  end of laser rods
• Remote beam manipulation ? 4 mirrors in laser
  hut
• 
  1 entrance mirror on each end plate

• Data taking
• Test special calibration runs trigger from
  laser
• 
  trigger laser (? several µs _at_ 10 Hz)
• Normal physics runs low rate
  trigger from laser

18
Stability of laser and beams
Design with micro-mirrors ? laser ray
positions determined by the mirror positions and
angles, not by the main laser beam or
movable optics.
Mechanical stability of the TPC is good enough
for precise (?100 ?m) relative measurements once
the TPC is installed.
During construction and installation, the TPC
will undergo stresses due to handling (rotation)
and change of loads (ROCs, cables etc).
Absolute positions must refer to TPC end
plates, ROCs and Central Electrode.
19
Construction tolerances and alignment accuracy (1)

• What is known precisely and absolutely during
  construction?
• (?100-150 ?m)
• pad plane z and wire z and x/y position
• central electrode z position

• Well measured relative to each other (?100-150
  ?m, 0.05 mrad)
• internal dimensions and angles in micro-mirror
  bundles
• micro-mirror bundles in support rings
• bundle support rings in uninstalled rods

• Less well measured or prone to move during
  handling
• (?500 ?m, 0.2 mrad)
• rod positions relative to ROCs, central
  electrode and ALICE x,y,z

20
Construction tolerances and alignment accuracy (2)

• Additional alignment relative to end plates
• with horizontal and loaded TPC
• (?100-200 ?m, 0.05 mrad)
• measure rod / micro-mirror bundle positions by
  survey
• through rods (fiducial marks useful)
• measure some beams near inner cylinder with HeNe
  
• laser after rod installation

• Internal alignment and iterations (offline
  analysis)
• electrons from central electrode ? absolute
  z
• electrons from ROC pad plane and wires ?
  absolute z, x/y
• laser tracks close to outer rods ? good relative
  alignment
• laser tracks are straight lines
• iterate to best absolute positions of laser
  rays
• track time variations

21
Status and Tests

• Laser lab at NBI
• Micro-mirror production in Moscow
• Tests of micro-mirrors at NBI
• Status of other components

22
Laser lab at NBI
rod with micro-mirrors
CCD camera
power supply
1064 nm laser
amplifier
expandingtelescope
? quadrupler ?266 nm
? doubler ?532 nm
23
Reflected 1 mm beam
z31cm
z200cm
FWHM0.93mm
FWHM0.95mm
pure Fresnel diffraction
24
Reflected 1 mm beams (2)
16 cm
19 cm
23 cm
Fresnel diffraction
FWHM1.00mm
1.01mm
1.10mm
47 cm
31 cm
100cm
Measured
1.17mm
0.93mm
0.93mm
150cm
200cm
z250 cm
beam divergence 0.35 mrad
1.14mm
0.79mm
0.95mm
25
Micro-mirror production
All 60 bundles produced and delivered in
September 2002, but problems with surface
quality on some mirrors
and mechanical precision on some cups
preliminary 46 accepted based on surface
quality most of these will
be accepted after being
mechanically improved at NBI
additional 30 bundles almost
ready in Moscow

mechanical reference surface
brass cup
reflecting surfaces
1 mm ? quartz fibres cut at 45º, polished,
coated 7 micro-mirrors/bundle
Micromirror bundle
26
Angles measurements (1)
? and ? angles of all micromirror faces measured
by goniometer in Moscow
??? ?? ? 0.0014? (5 arc sec)
27
Angles measurement (2)

• Re-calculation of angles to take into
• account offset centres of 7 mirrors
• systematics in re-calculation still
• to be understood

28
Angles measurement (3)

• However
• reference surface not good
• bundle not fixed to support
• in reproducible way
• only relative angles any
• good from this measurement

Reference surface has been turned off in NBI
workshop to provide good surface
29
Mirror surfaces (1)
Looked at all mirror reflections after 2.5
m. Example bundle 1819
30
Mirror surfaces (2)
Analysed all mirror reflection profiles after 2.5
m. Example bundle 1819
Rejected bad reflectivities and image
shapes. Good mirrors generally have quite
uniform reflectivity
31
Mirror reflectances
Amplitudes of profiles on previous slide
Micro-mirror number
Acceptance level 150 (arbitrary units)
Preliminary accepted bundles (based on
reflectivity and image quality)
46 bundles out of 61 received
32
Angles measurements _at_ NBI (1)
All mirror angles have been re-measured at NBI,
using laser beam setup.
Estimated precision ?? ? 0.5 mrad (2 arc
min) ?? ? 0.5 mrad (2 arc min)
33
Angles measurements _at_ NBI (2)
34
Angles measurements _at_ NBI (3)
Mean -0.08? Sigma 0.51?
Mean -0.29? Sigma 0.94?
35
Linear measurements _at_ NBI
Mean -0.23 mm Sigma 0.57 mm
L
36
Rod gluing

• Drill holes in short rods
• Mount mirror bundles in support rings.
• Glue mirror support rings onto short rods.
• Theta alignment guaranteed by jig
  machined surface
• 4. Glue short rods into full length rods.
• Phi alignment adjusted during gluing
• Measurements on final rod possible before
  installation.
• Foreseen for February-March at CERN.

37
Production status and installation schedule
Rod system Micro-mirror bundles ready 1
Feb 2003 Mirror support rings produced
Mirror testing ongoing, fall 2002 beginning
2003 Rod production at CERN spring 2003
Optics system Principle design done
Detailed design spring 2003 Production and
installation mid 2003 - spring 2004
Integration Principle design done
Install laser in UX25 2005 2006
Commissioning Together with TPC chambers
2nd half 2004 2005
Notes and presentations http//www.nbi.dk/borge
/tpclaser/