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Paper text:
the totem roman pot system  @xcite has been designed for measuring very forward protons originating from elastic or diffractive scattering processes . with different beam optics , four - momentum transfers down to @xmath0
can be reached , corresponding to scattering angles of a few @xmath1rad .
the proton detection is accomplished by stacks of 10 silicon sensors placed in movable beam - pipe insertions  so - called `` roman pots '' ( rps )  that can approach the beam . in some
special runs the distance from the beam centre has been smaller than 1 mm .
the system consists of 24 rps arranged in 4 stations at 147 m and 220 m on either side of lhc s ip5 .
each station is composed of 2 units separated by a distance of 2  5 m .
a unit consists of 3 rps , two approaching the outgoing beam vertically and one horizontally , allowing for a partial overlap between horizontal and vertical detectors , which is useful for the alignment .
the rp movement control system  @xcite has been derived from the one of the lhc collimators .
the top level user interface , a modified version of the central collimation application  @xcite , is operated from the ccc .
it allows the operator to send rp position requests and critical position limits to the middle level control ( see below ) and to read back the measured positions .
a graphical display shows the time evolution of requested and measured rp positions , the position limits , and the dose rates measured by beam loss monitors downstream of the rps ( see example screenshot in fig .
[ fig : interlocktest ] ) .
the middle level is the fesa - based  @xcite collimator supervisor system  @xcite , providing the link between the collimation application and the low level motor control running on a pxi  @xcite crate .
the pxi cpu controls the stepping motors of the individual rps and manages the position read - outs from the stepping motor encoder ( step counter ) and from the independent lvdt system ( see next subsection ) .
the pxi computer also interfaces to fpga cards which independently from the cpu accomplish the interlock - relevant comparison between actual rp positions and their critical limits ( see later subsection ) .
the fpga cards have direct connections from the microswitches and the lvdts and to the beam interlock logic card .
note that from the totem control room no rp movements can be triggered , except emergency retractions via mechanical springs .
this policy enforces centralisation of controls in the ccc and enhances communication between totem and the lhc operators .
however , the totem detector control system monitors passively all operations .      r50 mm     the rp mechanics has a series of microswitches  @xcite ( fig .
[ fig : microswitches ] ) tagging certain positions in the rps movement range and triggering appropriate responses .
the in and out electrical stoppers define the range of motion and stop the pot at its first point of electrical contact with their spring test probes .
these probes have an elastic travel of 3.18 mm beyond the first point of contact .
the out microswitch beyond the electrical stopper acts as a redundant motor stopper .
the out stopper also serves as absolute position reference ; its on - off transition point has been surveyed by laser calibration .
the home switch is relevant for interlock purposes .
the home flag is active in all retracted pot positions beyond the first contact point with the home switch .
the anticollision switch stops the motors if a top and a bottom pot come too close to each other .
the rp system has two redundant position measurements : 1 .
the motor encoders , i.e. step counters , are used for active movement control .
each step of the stepping motor corresponds to about 5@xmath1 m . due to a small position dependence of the step size
, step calibration curves are measured for each rp at least once per year .
the encoder measures the position relative to the out electrical stopper ; it has no absolute position memory . in a fast extraction of a pot with the springs ,
the mechanical coupling between the pot and its motor is removed , leading to a loss of the absolute position information .
hence , to ensure a correct calibration , each rp insertion is preceded by an encoder reset at the out stopper .
2 . the lvdts  @xcite provide an absolute position measurement but are subject to small drifts and need to be periodically recalibrated .
these measurements are used for the position interlock and as an independent cross - check of the encoders .
like all lhc experiments and many machine components , the totem rp system has an input to the lhc beam interlock system to avoid beam operation when a rp is in a position where it would represent a danger .
this logic is implemented in the programmable totem interlock card that receives beam mode information ( smp parameters ) from the general machine timing ( gmt ) , rp position validity information from the fpga cards comparing each rp s lvdt position with its beam - mode dependent critical limits , and simple electrical contact information from the rp home microswitches .
the output signals , injection_permit and user_permit are transmitted by hardware links to the beam user interfaces ( `` cibf '' and `` cibu '' ) ; copies are sent to the detector control system .
the key functions of the interlock logic  @xcite are the following : 1 . withdrawal of injection_permit : prevention of any beam injection when at least one rp does not touch its home microswitch .
withdrawal of user_permit : beam dump and automatic rp extraction if either ( a ) at least one rp is away from its home switch in a beam mode where rp operation is not allowed , or ( b ) at least one rp is outside the position range defined by beam - mode dependent critical limits .    an example sequence of an interlock test performed with the collimation application in the ccc is shown in fig .
[ fig : interlocktest ] .    39.7 mm ) .
the step counter , however , stays at 37 mm because in the emergency extraction with the springs the motor coupling is lost .
a reset at the mechanical reference point ( out stopper ) is needed .
, width=313 ]
the beam - based alignment of the rps is performed with a technique established for the collimators  @xcite : a reference beam edge is produced by closing the horizontal and vertical primary collimators ( tcps ) until they scrape the beam halo at an aperture @xmath2 , where @xmath3 is the beam size at the collimator , calculated for nominal optics functions and for a given normalised emittance .
this beam edge is then used as a reference for the rps : the pot approaches the circulating beam in steps as small as 10@xmath1 m .
when it reaches the edge , a beam loss is measured in a beam loss monitor ( blm ) downstream ( fig .
[ fig : collim ] ) . in this condition ,
the rp and the primary collimator are at the same normalised distance @xmath4 from the beam orbit with an accuracy given by the step size . at the contact position ,
the rp scrapes slightly into the beam . to refine the scraped beam edge ,
the primary collimator in the appropriate plane is moved further towards the beam until it becomes again the aperture bottleneck .
the average of the two normalised reference distances @xmath4 of the primary collimators is used to compute the normalised distance @xmath5 of the rp .      note that this procedure only establishes the distance between the _ thin window _ of the rp and the beam centre .
the alignment of the different _ silicon detector planes _ , both relative to each other and to the beam , is determined by software , using real particle data .        a detailed discussion of the software alignment methods is given in  @xcite .
most degrees of freedom of the relative alignment between the rp sensors can be obtained from the reconstructed tracks .
the underlying idea is that sensor misalignments give raise to residuals , i.e. the distances of the measured hit positions from the fitted tracks .
this technique is sensitive to shifts ( along the sensor read - out direction , i.e. perpendicular to the microstrips ) and rotations of individual detector planes relative to each other , but not to global shifts or rotations .
the transverse overlap between vertical and horizontal detectors ( fig .
[ fig : stationscheme ] ) establishes the alignment between the mechanically independent 3 pots of a rp unit .
[ fig : trackbasedresult ] shows a comparison of the track - based alignment results for an example rp with the optical metrology data .
the global misalignment modes ( e.g. common shifts or rotations of entire stations w.r.t .
the beam ) are inaccessible to the track - based techniques but can be constrained by exploiting known symmetries of certain physics processes .
a prominent example is elastic scattering with its clean hit distribution in the vertical detectors ( fig .
[ fig : elasticprofile ] ) .
the horizontal and vertical symmetries of the distribution lead to an alignment precision of better than 10@xmath1 m horizontally and about 20@xmath1 m vertically .    .
the symmetry line of the vertical distribution ( indicating the position of the beam centre ) is found by inverting the sign of the @xmath6 part ( excluding the region near the acceptance cut ) and shifting it until it coincides with the @xmath7 part.,width=317 ]    9 totem collaboration : technical design report , cern - lhcc-2004 - 002 , addendum cern - lhcc-2004 - 020 .
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calibration of the blm response at 450gev by beam scraping with roman pots , cern - totem - note-2010 - 001 , and references therein .
j. kapar : elastic scattering at the lhc , phd thesis , charles university prague , being submitted .
since the lhc running season 2010 , the totem roman pots ( rps ) are fully operational and serve for collecting elastic and diffractive proton - proton scattering data . 
 like for other moveable devices approaching the high intensity lhc beams , a reliable and precise control of the rp position is critical to machine protection . after a review of the rp movement control and position interlock system , the crucial task of alignment will be discussed .