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the alice experiment is dedicated to the studies of the properties of the deconfined qcd medium ( quark - gluon plasma , qgp ) by conducting ultra - relativistic heavy - ion collisions at the lhc  @xcite .
a significant increase of the luminosity for heavy ions is expected in run 3 after long shutdown 2 ( ls2 ) , which implies a collision rate of about 50 khz and @xmath0 = 10 nb@xmath1 .
this luminosity upgrade provides a substantial enhancement of capabilities for measuring observables relevant to the characterization of the qgp at the highest temperatures  @xcite .    in order to exploit the scientific potential of the high - luminosity heavy - ion program in run  3 , alice plans to extend its physics reach by upgrading the alice detector .
the major goals of the upgraded alice detector are as follows ; precision measurements of heavy - quark and quarkonia production at low transverse momentum ( @xmath2 ) to study the mechanisms of heavy - quark thermalization and interactions in the medium , production of low - mass dielectrons to extract information on thermodynamical properties of the medium and to characterize the chiral phase transition , jets and jet correlations to reveal the mechanisms of partonic energy loss in the medium  @xcite .
the time projection chamber ( tpc ) is one of the main tracking and pid devices in the central barrel of the alice detector .
it provides precise charged - particle tracking , momentum measurement , and particle identification in very high multiplicity heavy - ion collisions  @xcite .
the readout rate of the tpc is currently limited by the necessity to prevent ions generated in the the amplification region of the mwpc - based readout chambers from drifting back into the drift volume , which is achieved through active ion gating by operating a dedicated gating grid .
the relevant ion drift times limit the maximum trigger rate of the tpc to about 3.5 khz .
operation of the current tpc with the mpwc - based readout scheme and the current active ion gating scheme at 50 khz pb - pb collisions in run  3 can not be possible .
on the other hand , operation of the current tpc with continuously open gating grid can not be the solution since back - drifting ions from the amplification region will lead to excessive ion charge densities and distortions of the electric field in the drift volume .
the proposed scheme to acquire high rate operational capability and a small number of back - drifting ions is to replace the existing mwpc - based readout chambers and gating grid system by a multi - layer gas electron multiplier ( gem ) system and to run the tpc in an ungated continuous mode .
gems have been developed to cope with the stringent requirements for high - luminosity experiments  @xcite and have proven to provide excellent position resolution , to have very high rate capability , and better ion blocking capability compared to mwpc .
the main considerations for the tpc upgrade and the design requirements are as follows  @xcite :    * the maximum ion backflow ( ibf ) that can be tolerated is about 1% at a gain of 2000 in ne - co@xmath3-n@xmath3 ( 90 - 10 - 5 ) , i.e. 20 back - drifting ions per incoming primary electron ( @xmath4 = 20 ) . * in case of ibf = 1% , space - charge field distortions reach 20  cm and 8  cm in @xmath5 and @xmath6 at small @xmath5 and @xmath7 ( @xmath8 ) in the tpc , respectively . in order to preserve the present momentum resolution , online and offline distortion corrections with a precision better than 500  @xmath9 m , i.e. a few times 10@xmath10 ,
are required . * due to the limited bandwidth of the data acquisition system , reduction of data flow size by a factor of 20 is needed in the online reconstruction by finding the clusters associated to tracks .
* the upgraded tpc must preserve the performance of the existing system in terms of particle identification via d@xmath11/d@xmath12 , implying a local energy resolution better than 12% ( at 5.9 kev ) .
an extensive r&d program has been started in 2012 to study the performance of gem - based detectors ( ibf , gain stability , discharge probability ) , technology choice ( gem stacks including the combination of gems with different pitches , cobra - gem , 2 gem + micromegas system ) , large prototype production by single mask technology , electronics r&d , and simulation studies to establish the strategy for space - charge distortion corrections .
our baseline solution comprises stacks of 4 gem layers , where 1st and 4th gems are standard gems with 140  @xmath9 m pitch , 50  @xmath9 m thickness , and 70 ( 50 )  @xmath9 m outer ( inner ) hole diameter , and 2nd and 3rd gems are large pitch gem foils with 280  @xmath9 m pitch , 50  @xmath9 m thickness , and 70 ( 50 )  @xmath9 m outer ( inner ) hole diameter .
this setup allows to block ions efficiently by employing low / high fields above / below gems and foils with low optical transparency .
figure  [ fig : fig3 ] shows the results of the measured ibf and energy resolution at 5.9 kev at a gain of 2000 for a 4-gem system , where the voltage across gem1 increases from left to right along the x - axis .
it can be seen that an ibf of 0.7% is achieved at an energy resolution of 12% ( at 5.9 kev ) the observed anti - correlation between ibf and resolution is related to the gains of the first two gems : higher electron multiplication at the early stages improves the energy resolution , while it results in larger number of ions escaping into the drift region .
detailed simulations based on garfield++  @xcite were performed to describe the observed ibf performance .
it was found that ibf is very sensitive to the alignment of the gem holes in consecutive layers , which can not be controlled experimentally .
the measured ibf values are best reproduced in simulations , if a random misalignment of the holes is assumed , corresponding to the most probable relative geometrical position of gem foils in a stack .
/d@xmath12 spectrum of 1  gev/@xmath13 electrons and pions recorded at a gain of @xmath14 5000 measured with an iroc prototype employing a triple stack of large - size gem foils .
( y - axis shows the number of counts and x - axis shows d@xmath11/d@xmath12 ( a.u.).,width=330 ]    /d@xmath12 spectrum of 1  gev/@xmath13 electrons and pions recorded at a gain of @xmath14 5000 measured with an iroc prototype employing a triple stack of large - size gem foils .
( y - axis shows the number of counts and x - axis shows d@xmath11/d@xmath12 ( a.u.).,width=253 ]    a prototype of an inner readout chamber of the tpc ( iroc ) was built in 2012 , where triple stacks of gems were produced using the single - mask technology developed by the mpgd workshop at cern .
beam test was carried out at the ps - t10 beamline and the d@xmath11/d@xmath12 resolution was studied as function of the transfer fields and voltages across gems .
figure  [ fig : fig4 ] shows the d@xmath11/d@xmath12 spectra of 1  gev/@xmath13 electrons and pions recorded at a gain of @xmath14 5000 .
the energy resolution is 10.5% for the ibf - optimized field configurations and the resolution is comparable to the d@xmath11/d@xmath12 resolution of the current tpc .    an alternative solution is a system combining 2 gems with a micromegas detector .
micromegas ( mm ) provides low ibf due to the larger ratio of the electric field values in the small amplification gap to the drift field above the mm . if the mm employs a fine mesh ( 400 - 1000 lpi ) , ibf is close to the ratio between two fields itself  @xcite .
the ibf and energy resolution for this hybrid 2-gem + mm system were measured , using a 10 @xmath15 10 @xmath16 prototype detector .
the results are shown in fig .
[ fig : fig5 ] , where an ibf of  0.2% is reached at an energy resolution of 12% at 5.9 kev .
a large - scale solution for the inner and outer tpc readout chambers and the operational stability will be verified in the future .
r73 mm        the new scheme also required the development of new front - end electronics to cope with the reversed polarity , the requirements for continuous readout , and the increased data throughput in high rate pb - pb collisions .
a new front - end asic called sampa has been developed , which integrates the functionality of the present preamp / shaper and altro adc+dsp ( digital signal processing ) and supports continuous or triggered readout  @xcite .
first mpw ( multi - project wafer ) submission was done in april 2013 and further developments are ongoing .
online and offline reconstruction and calibration are very challenging due to the demand of data compression and requirement of the space - charge distortion corrections .
currently a two - stage reconstruction scheme is under consideration . in the first stage of the reconstruction ,
an averaged space - charge distortion map scaled to the averaged multiplicity for certain time intervals is used for the distortion corrections , and cluster finding and cluster - track association are performed , which leads to a data compression by a factor of 20 .
full tracking with the external detectors ( inner tracking system + transition radiation detector ) will be performed in the 2nd stage of the reconstruction , where a high - resolution space - charge map being updated every 5  msec is generated for the full distortion corrections .
figure  [ fig : fig6 ] shows the expected @xmath2 resolution in 50 khz pb - pb collisions without any space - charge distortions ( left ) , with space - charge distortion and distortion corrections at the first stage ( middle ) , and with space - charge distortion and full distortion corrections at 2nd stage of reconstruction ( right ) . in these calculations , space - charge fluctuations mainly due to the number of pileup events and charged particle multiplicities
are taken into account . the obtained @xmath2 resolution after the 2nd reconstruction stage is comparable to that without distortions , if tpc - its global tracks are considered .
to exploit the full potential of the high luminosity of the lhc in run 3 , the alice program for run 3 requires an upgrade of the tpc .
the heart of the tpc upgrade is to replace the mwpc - based readout chambers by detectors employing micro - pattern detectors including gems to allow tpc operation in continuous mode .
extensive detector r&d and simulations have been conducted and a baseline scenario for the detector design has been established .
quadruple stacks of gem layers with different gem pitches provide the required ibf and energy resolution . also a design based on a hybrid configuration of gems and micromegas is studied .
simulations show that the performance of the present tpc can be retained in 50 khz pb - pb collisions after distortion corrections .
further studies of the long - term stability , uniformity of the gain and ibf , and discharge probability are being conducted .
iroc prototypes employing a 4-gem stack and a hybrid 2-gem + mm system are being built and beam tests will be carried out at the ps and the sps in the fall of 2014 .
alice collaboration , `` the alice experiment at the cern lhc '' , jinst 3.08 ( 2008 ) , s08002 .
doi : 10.1088/1748 - 0221/3/08/s08002 alice collaboration .
`` upgrade of the alice experiment : letter of intent '' , cern - lhcc-2012 - 012/lhcc - i-022 , 2012 , http://cds.cern.ch/record/1475243/ alice collaboration .
`` technical design report of the time projection chamber '' , cern / lhcc-2000 - 001 , 2000 , https://edms.cern.ch/document/398930/1 f. sauli . `` gem : a new concept for electron amplification in gas detectors '' , nucl . instr . meth . a 386.2 - 3 ( 1997 ) , pp .
doi : 10.1016/s0168 - 9002(96)01172 - 2 alice collaboration , `` technical design report of upgrade of the alice time projection chamber '' , cern - lhcc-2013 - 020 , 2013 , http://cds.cern.ch/record/1622286 ) r. veenhof .
garfield - simulation of gaseous detectors , 1984 - 2010 , url : http://garfield . web.cern.ch p. colas , i. giomataris , and v. lepeltier .
`` ion backflow in the micromegas tpc for the future linear collide '' , nucl .
instr . meth .
a 535.1 - 2 ( 2004 ) , pp .
doi : 10.1016/j.nima .
alice collaboration , `` technical design report of pgrade of alice electronics & trigger system '' , cern - lhcc-2013 - 019 , 2013 , http://cds.cern.ch/record/1603472
the alice experiment at the large hadron collider ( lhc ) proposes major detector upgrades to fully exploit the increase of the luminosity of the lhc in run  3 and to extend the physics reach for rare probes at low transverse momentum . 
 the time projection chamber ( tpc ) is one of the main tracking and pid devices in the central barrel of alice . 
 the maximum trigger rate of the tpc is currently limited to about 3.5 khz by the operation of a gating grid system . in order to make full use of the luminosity in run 3 
 , the tpc is foreseen to be operated in an ungated mode with continuous readout . 
 the existing mwpc readout will be replaced by a micro - pattern gaseous detector ( mpgd ) based readout , which provides intrinsic ion capture capability without gating . 
 extensive detector r&d employing gas electron multiplier ( gem ) and micro - mesh gaseous detector ( micromegas ) technologies , and simulation studies to advance the techniques for the corrections of space - charge distortions have been performed since 2012 . in this paper , the expected detector performance and the status of the r&d program to achieve this ambitious goal 
 are described .    
 heavy - ion collisions , quark - gluon plasma , alice , time projection chamber , micro - pattern gaseous detector