Design Note on the ALICE TPC laser calibration system

Transcription

1 Design Note on the ALICE TPC laser calibration system B. S. Nielsen, J. Westergaard and J. J. Gaardhøje Niels Bohr Institute, Copenhagen, Denmark A. Lebedev Brookhaven National Laboratory, USA ALICE TPC Collaboration 10 June 2002

2 Contents 1 Introduction 3 2 System performance goals 3 3 Principle of generation and distribution of laser rays 5 4 Laser beams generated by reflection from micro-mirrors 6 5 The Poisson spot 8 6 System design: from laser to narrow beams Laser hut and beam transport to the TPC end-plates Beam transport on TPC end-plates Laser rods Beam pattern Spacial precision and stability Construction and surveying Alignment by offline analysis Operational aspects and data analysis Setup and remote control Data taking Online checks and offline analysis

3 1 Introduction This note describes the current design of the laser calibration system for the ALICE Time Projection Chamber (TPC) [1]. Narrow laser beams inside the drift volume of the TPC simulate ionizing tracks at predefined locations. The system can be used for tests and calibration either outside or during normal data taking with the aim of understanding the chamber performance. Of particular interest is the testing of electronics, alignment of the read-out chambers, and measurements of variations of the drift velocity due to mechanical imperfections and non-uniformities in the gas, temperature and the electric and magnetic fields. In addition to the laser calibration, the TPC will implement an electronics pulser system for gain and time equilibration of the read-out channels and a gas amplification calibration using radioactive Krypton. Many features of the ALICE laser calibration system follow the system successfully built and operated by the STAR experiment at RHIC [2]. 2 System performance goals Narrow beams of pulsed UV laser light can be used to simulate ionizing tracks in the active volume of the TPC [3]. We use pulsed monochromatic laser beams of 266 nm wavelength ( ev) and 5 ns pulse duration with approximately gaussian cross section with m. The ionization in the gas volume along the laser path occurs via twophoton absorption by organic impurities with ionization potentials in the range 5 8 ev. The molecules of the pure Ne CO drift gas have ionization potentials above 10 ev and will not ionize at the beam intensities considered. Most metallic surfaces have work functions below ev and will emit electrons by photoelectric effect when hit by UV light of this energy. Being a first order effect in the light intensity, we expect a considerable amount of low energy electrons from the diffusely scattered UV light produced by reflections. Metallic surfaces are primarily the central electrode, the mylar strips of the electric field degrader and the wires and pad planes of the read-out chambers. As the ionization process is due mostly to unknown gas impurities, it is difficult to determine the necessary beam intensity a priori. Experience from other experiments and preliminary results from laser tracks in the final TPC read-out chambers show, however, that energy densities of approximately J/mm per 5 ns pulse at 266 nm wavelength is sufficient to obtain an ionization corresponding to several minimum ionizing particles. We have designed our system to have up to J/mm per pulse. The aim is to measure the response of the TPC to several hundred simulated tracks generated simultaneously throughout the TPC drift volume at predefined positions. Such laser events can be generated in special calibration runs or interspersed between physics events, or they may even be overlaid physics events for testing purposes. 3

4 The system is planned to be used first for testing of the readout electronics and associated software with tracks at known positions and varying ionization levels. Alignment errors between sectors can be measured and corrected using those simulated tracks that cross from one sector to another. Drift velocities can be measured by subtraction of the drift times corresponding to simulated tracks at different z positions. Finally, spatial distortions originating from mechanical imperfections, temperature variations in space and time, space charge and effects can be addressed and monitored. The goal is to obtain a uniformity of the TPC drift field within a relative error of * * corresponding to a spacial resolution of #%$&')( m and +*, m, depending on drift length and track orientation. In order to obtain matching calibration results, the individual laser tracks must have transverse dimensions comparable to these numbers and the stability of their position must be assured at the same level. The nature of the laser tracks assures that the tracks are always straight lines to within a fraction of their transverse size. Testing of the electronics performance and relative alignment of the sectors will be important in the initial phase of the experiment. For this purpose, we supply hundreds of simulated tracks distributed throughout the active TPC volume. All sectors will be crossed by laser tracks as will many of the sector boundaries. Using the fact that the laser tracks are continuous straight lines, sectors may be aligned relative to each other to a precision given by a fraction of the transverse track dimension. The continuous variation of the ionization along the tracks will likewise check the relative gain settings between chambers. When using the laser tracks to map the uniformity of the drift velocity, the best geometry is one where the tracks have constant drift times and are perpendicular to the wires. For this configuration, clusters are smallest and the electronis and reconstrution programs give the best possible single point resolution. This has led us to provide tracks in planes at constant -, many of which radiate with approximately constant.. Tracks at different - throughout the drift volume allow easy determination of drift velocities from single laser events. In order to obtain a relative drift velocity measurement precision of 10 "! in a single event, the relative 3 - position of the tracks must be known to /0-012 m for a 2 m span in drift lengths. As the drift velocity can be measured on a single event basis, time variations and effects due to space charge induced by large multiplicity events can be measured by interspersing laser calibration events throughout normal data taking. For monitoring of more detailed drift velocity variations, many laser events or special laser runs may be necessary. For this purpose, reproducibility of the laser beam positions is important. The aim is to have spacial 3 reproducibility at the m level or better. A complete mapping of the drift field with the aim of studying permanent non-uniformities, will require knowledge of the absolute positions of the laser beams. We aim at surveying a number of the beams during the construction of the TPC to a precision of about m at the origin of the laser tracks and slightly worse at the end of the tracks. For absolute measurements, we always refer to the TPC end plates. Mechanical deformations of the TPC field cage after the surveys may deteriorate the knowledge of the track angles, while the precision of the origin of the tracks should remain close to the above number. "! 4

5 3 Principle of generation and distribution of laser rays The basic principle of generating hundreds of narrow laser beams simultaneously in the TPC volume was developed for the STAR experiment and has been modified appropriately for ALICE. A commercial laser generates an energetic pulsed beam of UV light with very low divergence, expanded to a 25 mm diameter. Through an optics system of beam splitters, mirrors and bending prisms this wide beam is split and guided into the TPC through windows at several entry points. The wide beams travel along the inside of the hollow outer rods of the TPC, used for holding the mylar strips that define the electric field. The wide beams hit bundles of very small mirrors (1 mm diameter) that deflect small parts of the wide laser beam into the TPC drift volume. The dimensions and directions of the narrow beams are given by the size, positions and angles of the micro-mirrors and only to a very minor degree by the parameters of the wide beam. The micro-mirrors along the length of the rod are grouped in small bundles and placed such that they do not shadow each other. The unused part of the wide beam can be used for position and intensity monitoring and is dumped at the end of the rod. All elements of the optical guidance and splitting system are static, except for a few remote controllable mirrors to fine-tune the beam path. Figure 1 shows a sketch of the principle. Figure 1: Principle of generating laser rays in the TPC volume. In ALICE, we plan to equip six rods with each four micro-mirror bundles in each half of the TPC. Each mirror bundle will contain seven mirrors, giving a total of 336 simultaneous narrow laser beams in the TPC volume. The wide beam originates from a single laser for each TPC half and is split and guided into the six rods. 5

6 4 Laser beams generated by reflection from micro-mirrors Due to the quadratic nature of the ionization process, a laser beam with transverse size ; ; 46587:9 m will result in a simulated straight track with <=#%7?>A@ 46587:9*B"C ED m. In the - direction, a small additional smearing of the simulated track happens due to the 5 ns H pulse length. Assuming a square time distribution ( (EBFC G ns) and drift velocity IKJ IQJ R m. ED m/ns, one gets the small additional term L+NM <=#O7P>A@ (BC ) The generation of narrow beams happens inside the TPC by reflecting a wide laser beam off small mirrors at a 45S incidence angle as described above. In practice, the micro-mirrors are made from 1 mm diameter quartz fibres, cut at a 45S angle at one end and the resulting eliptical surface polished and coated for total reflectivity for 266 nm light. To increase the number of laser tracks inside the TPC, bundles of micro-mirrors are assembled from 7 short fibres in a unit which will generate 7 narrow beams roughly in a plane perpendicular to the wide laser beams at different predefined azimuthal angles, as given in Figure 2. The reflection angles of each of the 7 mirrors in a bundle are constructed with a tolerance of GS in both azimuth and dip angle, and the angles of each bundle will be measured to a precision of 0.05 mrad. Figure 2: Design of the micro-mirror bundle as an assembly of seven quartz fibres cut at and azimuthal reflection angles of each mirror bundle. (TS The profile and total energy of narrow beams generated by reflection from a micro-mirror bundle used by STAR have been measured in the lab. A uniform laser beam of the final characteristics for ALICE intersected the mirror bundle and the reflected beams were measured by a calibrated energy meter and imaged with a CCD camera as a function of the distance, -, from the mirror bundle. The measured energies did not vary substancially for different micro-mirrors and were stable as function of time, reflecting the quality of the coated surfaces and the laser. Figure 3 shows the measured profiles at different distances, -. The patterns match qualitative what one would expect from Fresnel diffraction of a plane wave 6

7 passing through a circular aperture (purple line) and the measured FWHM remain at or below 1 mm up to - cm. At the largest distances the width is bigger than expected from Fresnel diffraction, but the increase is consistent with the expected beam divergence of 0.35 mrad. Figure 3: Observed beam profiles of a laser ray generated by reflection by a 1 mm diameter micro-mirror at distance -, compared to the expected pattern from pure Fresnel diffraction of an infinite planar wave. The enlarged beam size at large - is consistent with the expected beam divergence of 0.35 mrad. 7

8 5 The Poisson spot It was pointed out by Poisson that the diffraction pattern behind a black sphere or disk inserted in a plane wave displays a bright line in the middle of the geometrical shadow behind the object. When intersected by a plane, this Poisson line shows up as a bright Poisson spot in the middle of the shadow region. Similarly, a ball placed near the centre of a wide laser beam produces a Poisson line in the shadow. As the divergence of this line is small compared to that of the undisturbed laser beam, the phenomenon was proposed for long distance alignment [4]. The validity of the mathematical formulation was verified by a red He-Ne laser in the lab [5]. Figure 4 shows the beam profiles we expect to see in the centre of a wide beam with a 4 mm or 6 mm sphere inserted 10 m or 20 m upstream for U nm. The diffraction patterns near the edges of the geometrical shadow wash out the sharpness of the edges, but the central Poisson spot stands out clearly. Figure 4: Calculated beam profiles when introducing a spherical ball in a wide laser beam. One sees the shadow region of the ball, with the sharp Poisson spot in the center. We plan to be able to introduce a ball in the wide beam right after the laser and use the Poisson line to center optics along the full laser path, for beam alignment purposes. The advantage of using this technique is that the edges of the wide beam are not very sharply defined and may be modified by apertures in the beam path. The Poisson line, on the other hand, is a sharp reference of the middle of the beam. This approach was also used in STAR. 8

9 ( 6 System design: from laser to narrow beams In this section, we describe the elements of the laser system in some detail. First we go through the steps leading to the generation of the 336 laser rays in the TPC volume, and then we will address some systems and controls aspects. 6.1 Laser hut and beam transport to the TPC end-plates Energetic pulsed laser light in the UV region can be obtained from a Nd:YAG laser (1064 nm) equipped with two frequency doublers, generating pulses of UV light at 266 nm wavelength. The same kind of laser was used at STAR, and also in NA49 and CERES/NA45 at the SPS. The beam from the laser has a typical diameter of 9 10 mm and will be expanded to mm by a telescope. The power density of the narrow beams is given by that of the wide beam inside each rod. A J/mm density in each of the six mm beams translates into a total energy per pulse of 100 mj out of the laser. We have already bought a laser from Spectron Laser Systems Ltd, model SL805-UPG, which is specified to provide 100 mj/pulse of 3 5 ns duration and 266 nm wavelength at a repetition rate of 10 Hz. Close to the laser, the beam has a square profile that develops smoothly into a gaussian profile after m. This model currently runs up to 130 mj/pulse in the lab and is delivered with an upgrade option to raise the energy to 180 mj/pulse or more. Built into the laser is also a beam expanding telescope to enlarge the beam diameter to 25 mm and reduce the beam divergence to approximately 0.3 mrad. The lasers will be placed in a hut outside the L3 magnet at -VXW ( m under the LHC beam-line. Together with the actual laser heads and their power supplies, the hut will contain any beam optics necessary before the wide beam is guided towards the TPC. The hut will provide the personnel safety against UV light in the underground hall. The rest of the beam guidance system will be enclosed in pipes. Figure 5 shows a sketch of the hut on a platform under the compensation magnet with two lasers inside. The laser beams exit horizontally in the vertical plane under the LHC beamline. Figure 6 shows the further beam path towards the TPC end plates. Both beams pass through a vertical slit in the L3 magnet and has a direct path towards the TPC protected by light tubes. H One beam will hit the nearest end plate at Y WZ mm where a mirror will reflect it by [ S into the vertical plane of the TPC end plate. The other beam passes slightly lower, and after a knee of two [ S reflections it enters a tube mounted on the outer skin of the TPC field cage at. ED S ( S off the vertical plane). It continues in a straight line to the far (muon side) end plate of the TPC where another [ S mirror bends it into the vertical plane parallel to the plate. Some of the mirrors in this beam optics system are remotely adjustable, as are the points and directions of the beam exit from the laser hut. Together these mirrors define the beam vector at the entry on the two TPC end plates. T 9

10 Figure 5: Laser platform housing the two lasers, optics elements and power supplies. The laser beams exit horizontally directed towards the TPC. The platform will be enclosed with a hut. 6.2 Beam transport on TPC end-plates Figures 7 and 8 show the beam path in the vertical planes parallel to the TPC end plates, at a distance of about 120 mm from the plates. After the [ S mirror that turns the beam into the vertical plane at the bottom of the TPC, a 50% beam splitter directs half of the beam in each direction around the periphery of the end plate. Prisms deflect the beams by \ S such that each half of the beam passes over the prolongation of three of the outer TPC rods. At these points, beam splitters at (]S angles direct equal intensity beams into each rod along the - axis by deflecting 33%, 50% of the remaining and 100% of the then remaining beam by a [ S angle. Any small remaining beam can be monitored and dumped after the last mirror. The beam paths at the two TPC ends are virtually identical, except that one of the prisms on the muon side has a smaller bending angle to compensate for the beam entrance shift of S in.. It should be noted that the position of the six entrance points are chosen such that the prolongation of each laser rod beyond the central electrode is an empty gas rod. The optical elements on the TPC end plates (beam splitters, prisms and mirrors) are placed in small boxes with removable lids as indicated. Each box is firmly attached to the TPC end plate and the angles of the optics can be fine adjusted manually inside the box after 10

11 ( Figure 6: Laser beam path from laser hut to TPC end plates. Laser 1 hits a mirror on the Shaft side TPC endplate at Y WZ mm; laser 2 is bent through a knee and continues to the Muon side TPC endplate along a path at. ED S. installation. Figure 9 shows an example of the mechanics in one such box designed for STAR. Similar designs are planned for ALICE. Between boxes, the laser beam travels in removable hollow tubes for safety reasons. Once installed, the system is expected not to need additional adjustments, with the exception of the entrance [ S mirror which will be equipped with a remote position sensor (CCD camera) and remotely controlled adjustments of the two angles. This is because the TPC end plate is a good optical table and the position of the wide laser beam is not critical, so long as it is guaranteed to hit the micro-mirrors. 6.3 Laser rods Wide laser beams enter the six laser rods at each TPC end through sealed quartz windows. They travel down the inside of the rods as illustrated in Figure 1 and are intersected by four ^ micro-mirror bundles before arriving at the TPC central electrode at -. At the central electrode, the beam passes through another sealed quartz window to the hollow gas rod in the 11

12 Figure 7: Laser beam path and optical elements on the Shaft side TPC end plate. other half of the TPC and exits at the far end through a third sealed quartz window. On the far side, the beam position can be monitored by a camera, using the Poisson Spot technique. Figure 10 shows the principle of mounting of the micro-mirrors in the TPC rods. The holders (Figure 11) are integrated into aluminium rings glued between the macrolon tube pieces that build up the 2.5 m long rod, and holes are drilled in the macrolon tube to allow the narrow beams to exit the rod and enter the drift volume. As the position and in particular the mounting angle of the micro-mirrors define the narrow beam positions inside the drift volume, care is being taken to assure the mechanical stability of the mirror holders. The gluing procedure for these rings will be specially adapted to control the position of the mirror holders and the angles of reflected beams measured after the assembly of the 2.5 m long rod. Also, special care is taken to place the mirror holders close to the rod supports to the TPC which will be adjusted to the ideal position before the rod installation. 12

13 Figure 8: Laser beam path and optical elements on the Muon side TPC end plate. Even if small movements of the rods cannot be excluded during the assembly of the TPC, the stability of the finished and installed TPC is believed to be well below the requirements E of approximately m reproducibility (angle reproducibility of 0.05 mrad). Figure 12 shows the position of the micro-mirror support rings in full length laser rods. The four bundles are spaced roughly equidistantly along the 2.5 m long rod. Slightly different - positions in every second rod in. assure that beams from neighbouring rods will not cross (see next section). 6.4 Beam pattern The pattern of the narrow beams in the TPC volume follows from the micro-mirror angles shown in Figure 2 and the - positions in Figure 12. When defining the angles and - posi- 13

14 Z \ Z Figure 9: Example of the design of the interior of an optics box to be installed on the TPC end plate. The shown box contains a \ S bending prism. tions, we have aimed at generating beams radiating at constant - that cross sector boundaries strategically, i.e. at points where alignment between sectors would benefit the most. We have also tried to avoid having too many tracks with small angles relative to the wires of the read-out chambers. Figure 13 shows the resulting pattern for the beams at one position in -. Beams from neighbouring laser rods in. are shifted slightly in - relative to each other to avoid most of the apparent beam crossings. The central ray of the seven from each bundle radiates in R. in a way very similar to ionizing tracks from the interaction point while the other angles were chosen in order to flood all sectors and assure beams that cross over all sector boundaries. In this way, we have tried to define a pattern optimized for testing and sector alignment as well as for drift distortion measurements. One should note that although the position of the beams should be known to a high precision, the production tolerances of ES on the angles result in deviations of the shown paths of up to 40 mm near the inner cylinder. Fd Fd [ Fd In the - direction, the planes of laser tracks are situated at -_a`bc\ D( mm EFd [ Td Td for odd laser rods (in. ) and -e,`b mm for even rods. 7 Spacial precision and stability The TPC calibration would ideally require absolute knowledge with infinite precision of the spacial position of all laser tracks in absolute ALICE coordinates. Given the mechanical tolerances, the best absolute coordinate frame for each half TPC is defined by the plane of the end plate. All read-out chambers and the plane of the central electrode defining the 14

15 g g d d d d Y d d Figure 10: Laser rod with micro-mirror bundle and its support. Also shown is the end view of the tube with the position of the four mirror bundles. high voltage surface will be aligned and adjusted relative to the end plates. With the aim of F"! 3 obtaining a relative electric drift field error below and a spacial resolution of m or better, these surfaces must be defined relative to each other during construction to a precision E of approximately m. The laser tracks flood the large volume between the central electrode and the read-out chambers. This section outlines the current ideas of how to obtain a spacial precision of the laser tracks that match the relative precision of the read-out chambers and the central electrode. In doing so, the reference frame must necessarily be that of the end plates and wire chambers. A final precision goal of Wf m for space points translate to the following requirements on spacial coordinates and angles of the laser systems: R /ih /Y /b-j*1, Wk m F F /bl /0.mjn1 (ow mrad By far the most important issue g in the definition of the laser track positions is the placement of the micro-mirrors, both in h -j g and in particular in the angles l.mj. The only other error that enters is the incidence angle of the wide laser beam on the micro-mirrors which is relatively easy to measure and keep constant due to the long lever arms in the optics system. 15

16 d Y d Figure 11: Alumininum support ring with micro-mirror bundle, to be glued into the laser rods. 7.1 Construction and surveying The mechanical construction errors of the micro-mirror bundles are specified to be less than ; m in the spacial measures and less than QS in all reflection angles. The critical surfaces n are, however, expected to stay within a ( m tolerance and the angles of the reflected beams from the mirror bundles are subsequently measured to a precision of Fp ( mrad. After assembly of the rods with the micro-mirrors, a second measurement of the angles will be performed, relative to fiducial marks placed at the ends of each rod. A precision of F mrad should be possible. The assembled rods are very stiff and relative shifts between micro-mirrors within a rod during the installation into the TPC and mounting of the mylar strips are not envisaged within the measurement errors mentioned. However, the absolute position of the rods relative to the end plates can probably not be guaranteed to better than ( m. The construction of the TPC involves assembly of the field cage while it is standing on first one and then the other end, before it is finally turned into its final horizontal position. It is unavoidable that mechanical stresses will influence the absolute position of the rods at b the W m level. However, the stiffness of the rods should guarantee a continued good relative alignment of the micro-mirrors in the same rod. We plan to make a survey of the micro-mirrors with the TPC resting on its final support feet and with most of the loads mounted on the end plates. This should be possible with the help of the fiducial marks by looking through the installed rods with a g telescope that refers mechanically to the outside of the end plate. In this way, the absolute h -j of the micro-mirrors should be known to ER m and the mirror angles given by the measurements of the uninstalled rods. Wq 16

17 Figure 12: Full length laser rod with micro-mirror bundles and their support with the foreseen position of the four mirror bundles in - (odd rods top, even rods bottom). A final measurement procedure of at least some of the laser tracks is under discussion but needs further mechanical design work. On Figure 13 it is noticed that almost all laser tracks cross in front of an inner read-out chamber close to the end-point of the tracks. It seems feasible to construct a measurement device to determine the track end-points relative to the inner read-out chambers. The device should attach to the end plate in place of an inner read-out chamber before installation of the chamber, using the same reference as for the final chamber. A measurement arm would go into the TPC volume and intersect the laser beams of at least the first and possibly the second plane of beams. Further away from the end plate, the mechanical stability of such a system is presumably not sufficient to obtain a good measurement. 7.2 Alignment by offline analysis Given that the read-out chambers and the central electrode constitute the best measured surfaces during the TPC construction, it will be an advantage to be able to use these surfaces as references also for the laser tracks. As mentioned, all metallic surfaces inside the TPC are 17

18 Figure 13: Laser tracks in R-. inside the TPC drift volume. likely to emit electrons by photoelectric effect synchronously with the laser pulse. This is in particular true for the whole central electrode, but also for the pad plane of the read-out chambers and possibly some of the gating and cathode wires. We plan to use this information to define the two end-points of the drift in an iterative alignment procedure where also all the above mentioned knowledge of relative and absolute laser beam positions result in the best possible definition of the laser track positions in the TPC coordinate system. Even without the ultimate absolute precision of all laser tracks, much can be learned from a few well determined tracks, in particular if they span a large lever arm in -. Close to the outer TPC radii, the angular uncertainties play only a minor role, and it should be possible to obtain a very good drift velocity measurement near the six rod positions in each half of the TPC. Furthermore, time variations can be tracked to a very good accuracy throughout the TPC. The main uncertainty in the time variation of the laser beam positions presumably comes from slight torsions in the field cage due to variations in the mechanical loads and the external temperature. In stable running conditions, these effects should be minimal. 18

19 8 Operational aspects and data analysis 8.1 Setup and remote control The initial setup of the optics will need careful manual adjustment of all the mirrors and prisms in the system. Special measurement tools can be installed temporarily in the beam path. Experience from STAR shows that this rather lengthy procedure should only need to be done once, at least for the optical components fixed to the two end plates of the TPC. Adjustments inside the laser huts and along the beam transport to the end plates may have to be re-adjusted after a shutdown. The laser heads with their power supplies are equipped with remote control facilities through an RS232 connection. A few essential beam optics elements will be remotely controllable by pico-motors. This includes the entrance mirror on each TPC end plate and the two mirrors in the knee of the beam transport to the muon side, in addition to a few beam control elements in the laser huts. Simple CCD cameras can be placed at the entrance points on the end plates to guide the steering of the beam from the hut to the end plate in question. Similarly, cameras at the two ends of the split beams on each end plate will guide the steering of the entrance mirrors, as will cameras at the exit of the gas rods downstream from each laser rod. Experience will show how many cameras need to be installed permanently. Experience from the first years at STAR shows that a trained operator can manipulate the few parameters easily while observing the images on the cameras, and that the system is then stable over the length of typical calibration runs. In ALICE, we plan a more automatized version of the beam steering by computer readings of the camera images and automatic adjustments, with the aim of making the whole system self-adjusting. 8.2 Data taking Data taking with the laser tracks must run in various configurations. Special calibration runs can be more or less automatic. The laser can deliver a trigger pulse to start the TPC read-out or it can, with some restrictions, receive a trigger from outside. For stability reasons, the flash lamp of the laser should be run at a fixed frequency, but for each flash the exact time of the laser pulse can be triggered from outside within a range of about a microseconds. During normal data taking runs, laser calibration events can be interspersed as a special lowrate trigger. The trigger frequency can be set to any down-scaled value of the laser frequency which is currently 10 Hz. 19

20 8.3 Online checks and offline analysis It is envisaged that simple, well understood parameters extracted from the laser events in special calibration runs or during normal data taking can be extracted online. This is in particular the case for parameters that monitor position stability, such as drift velocity measurements and timing stability of the TPC readout. The database of measured beam positions should be available to the online programs. More complicated measurements, such as maps of the drift distortions in space and time, will require dedicated offline analysis. References [1] ALICE Technical Design Report of the Time Projection Chamber TDR, CERN/LHCC , 7 January 2000 [2] A. Lebedev, A laser calibration system for the STAR TPC, Nucl. Instr. Meth. A478 (2002) [3] H.J. Hilke, Detector Calibration with Lasers A Review, Nucl. Instr. Meth. A252 (1986) [4] L.V. Griffith et al., Magnetic alignment and the Poisson alignment reference system, Rev.Sci.Instrum. 61 (1990) 8 [5] B. Melholt Nielsen, Poissons Plet: Laser Kalibrering af ALICE Time Projection Chamber, Bachelor thesis, University of Copenhagen, June