1. Liquid Argon Calorimeter Performance at High Rates J. Rutherfoord with R.B.Walker University of Arizona 27 February 2014 V1.0

2. What limits the ATLAS Calorimeters? As the Luminosity at CERN’s Large Hadron Collider increases well beyond the design value of ℒ = 10 34 cm -2 s -1, what will limit the performance of the ATLAS liquid argon calorimeters? Here we report a small R&D project which investigates this question. We focus on the Forward Calorimeters where the rates are the highest. 27 February 2014J. Rutherfoord2

3. Liquid Argon Forward Calorimeter 27 February 2014J. Rutherfoord3 15 mm ATLAS Liquid Argon Calorimeter 1000 mm Forward Calorimeter Electrodes

4. 27 February 2014J. Rutherfoord4 Hot 90 Sr  -source deposited on outer surface of the foil. KE   2.28 MeV Replicate HL-LHC conditions in the lab For details on data-taking see B.Toggerson, A. Newcomer, J.R., and R.B.Walker, NIMA 608 (2009) 238

5. 27 February 2014J. Rutherfoord5 External wiring

6. 27 February 2014J. Rutherfoord6 Sampling Calorimeter ~15 TeV/crossing deposited in each FCal   Particles from min-bias collisions At the HL-LHC at ℒ = 5  10 34 cm -2 s -1 at |  |~4.7, ~1 gamma/cm 2 /crossing on face of FCal and at EM shower max ionization rate is ~3  10 11 /mm 3 /s or ~7500 /mm 3 /crossing Scale of the problem IP 5 m Beam Line Worst location EM FCal Module

7. 27 February 2014J. Rutherfoord7 Space-charge effects in LAr gaps  Our measured observable is the current.  Simplifications allow an analytic solution. See J.R., NIMA 482 (2002) 156  r is ionization rate relative to critical value.  As we raise the ionization rate, charge builds up in the gap and distorts the electric field. Field is screened near the anode.  Anode on the left, cathode on the right, E-field to the right. z is distance across the liquid argon gap Anode Cathode

8. The most dramatic effect … 27 February 2014J. Rutherfoord8 When we reach the critical ionization rate the density of electrons near the anode rises by three orders of magnitude.

9. Three 90 Sr beta source activities Strong50 mCi72 mCi Weak 2 mCi 3.4 mCi Control 0 mCi 0 mCi 27 February 2014J. Rutherfoord9 NameSpecifiedReceived and it decays away with half-life of 29 yrs

10. Raw data 27 February 2014J. Rutherfoord10 260 V 60 V 1.9 V Strong source. Each dot is a reading. Two readings per second. 40 readings at potential. Current

11. 0.268 mm gap 27 February 2014J. Rutherfoord11 Operating potential of 260 V Gap Current (nA) Gap Potential (V)

12. 27 February 2014J. Rutherfoord12 0.268 mm gap Gap Current (nA) Gap Potential (V) Operating potential of 260 V

13. 27 February 2014J. Rutherfoord13 Close-up of low potential region Gap Current (nA) Gap Potential (V)

14. 27 February 2014J. Rutherfoord14 ~330 fb -1 equivalent running at worst location in ATLAS Forward Calorimeter 0.268 mm gap

15. Unpredictable currents 27 February 2014J. Rutherfoord15

16. 27 February 2014J. Rutherfoord16 Strategy Ideally we would vary the ionization rate and measure the current But we have only two 90 Sr sources (nominally 50 mCi and 2 mCi) So we vary the critical ionization rate by varying the potential across the gap

17. 27 February 2014J. Rutherfoord17 A A 3/4

18. Three different gaps 27 February 2014J. Rutherfoord18 Gap Potential (V) Ratio of Strong Source to Weak Source Currents

19. Observations We have determined the threshold for entering the space-charge regime There are unpredictable effects in the space-charge regime This R&D project does not tell us how the performance of the calorimeters is degraded. A separate R&D effort, the HiLum project at IHEP/Protvino, should answer this. 27 February 2014J. Rutherfoord19

20. 27 February 2014J. Rutherfoord20 Conclusions Performance of Liquid Argon calorimeters degrades with ℒ but not with  ℒ dt LAr calorimeter is stable at 0.1% level for 330 fb -1 of running In the space-charge regime the currents are somewhat unpredictable The on-set of space-charge effects is well- measured and is determined by  + = 0.08±0.02 mm 2 /Vs

21. Backup Slides

22. 27 February 2014J. Rutherfoord22 At a high ℒ hadron collider … calorimeter showers produced by copious min-bias events … flood the liquid argon gaps with ionization This ionization is …  approximately spatially uniform on the scale of the gap (~ 2 mm)  approximately uniform in time on the scale of the time it takes for charges to drift across the gap

23. 27 February 2014J. Rutherfoord23 Ionization in a liquid argon gap Typical drift velocities  e = 4.3  10 6 mm/s  Ar+ = 1  10 2 mm/s at E = 1000 V/mm We consider the case where the rate of ionization is constant. This produces a DC current through the HV supply.

24. 27 February 2014J. Rutherfoord24 Critical ionization rate Assume Ar + ion drift velocity is proportional to Electric field The critical ionization rate is defined in terms of this mobility is key to space-charge effects Finally define relative ionization rate If, then we are in the space-charge limited regime

25. 27 February 2014J. Rutherfoord25 Value of  + (Ar + mobility in LAr) Measurements of  + reported in the literature vary by about a factor 50 from lowest to highest: 0.02   +  1.0 mm 2 /Vs Most recent and best-looking technique – Norman Gee et al., J.Appl.Phys. 57 (1985) 1097, reading off a plot, gives 0.15   +  0.18 mm 2 /Vs  3% Our preliminary result  + = 0.08  0.02 mm 2 /Vs Temp dependence

26. 27 February 2014J. Rutherfoord26 Critical values for the on-set of space-charge effects Charges on the surface of the electrode plates establish the electric field in the LAr gap. Define critical charge density in the LAr as that which, integrated over the gap, equals the absolute sum of the charges on the plates Assume Ar + ion drift velocity is proportional to Electric field is key to space-charge effects