Graham Beck, QMUL. Oxford Tracker Forum, June 2013. 1 Thermal Design of the ATLAS Upgrade Strip Staves 2007 (start of my involvement in thermal study):

Graham Beck, QMUL. Oxford Tracker Forum, June 2013. 1

Thermal Design of the ATLAS Upgrade Strip Staves

2007 (start of my involvement in thermal study):

Q. What are the thermal limitations on upgrade performance?

A. Probably Thermal Runaway and Shot Noise (both T-dependent) . . . Not sure which is the more critical!

--------I will concentrate on thermal runaway:

- Brief look at existing ATLAS Barrel module design (late 1990s).

- How understanding evolved during Upgrade design (2007-)

Thermal structure and performance of the proposed Staves.


After radiation damage, leakage current I is dominated by bulk generation.

Shot Noise (strip) ~ √I

Sensor heat, Q ~ I × Vbias

Assuming mid-gap carrier generation, expect:

(non-linear behaviour responsible for runaway …)

The silicon band gap (~ 1.11 eV at 300K) falls with T (independently of radiation damage) contributing appreciably to T dependence of intrinsic carrier concentration.

Fit to T range of interest: Eg [eV] ≈ 1.21– m .T [K] =>

- close to measured T dependence for ATLAS irradiated sensors.

- explained + investigated to greater accuracy by Alex Chilingarov (RD50 2011) [ “np ~ exp(-1.21/kT)”, Morin&Maita, Bell Labs, 1954 !!]. (backup:Biard)

Temperature Dependence

kTTETI g 2exp~ 2

kTTI 221.1exp~ 2


Double-sided module, bridged readout hybrid.Evaporative C3F8 cooling at nominal -25C.

4 Si wafers glued to TPG baseboard (dashed outline). High thermal conductivity, but prone to delamination => laser perforated + encapsulated in epoxy.

Baseboard design (FEA) aimed to:- minimise sensor Tmax (was thought to dominate runaway)- minimise mass (%X0) => relatively small T (~1.5C) across sensor

Existing SCT Barrel Module - thermal features

However: ~75% of thermal resistance (sensor heat => fluid) contributed by cooling path outside the sensor area: small area (narrow, ½ sensor length) resistive layers (grease, glue, kapton, fluid film). Common path for heat from both sides of module.

TPGEpoxy BeO

Thermal GreaseCu/Kapton shunt shield

Epoxy+BNAl block

Cooling pipe C3F8 evap.film.

64 mm


Sensor Corner Temperature (Tmax) vs Sensor Power Density at 0C "As built" FEA (2007)

-15

-10

-5

0

5

10

0 40 80 120 160 200 240 280

Sensor Power at 0C [mW/mm2]

Sen

sor

Tem

per

atu

re [

C]

max.power (10y)

Coolant

-15C

-20C

-25C

Existing SCT Barrel Module - thermal FEA features (1)

Concerns about uncertainty in radiation damage motivated plots of Sensor maximum Temperature vs. Sensor Power (at 0C).

Generally: difficult to find end point of these curves accurately

Note: a rise in coolant temperature of about 7C is enough to lose “headroom” wrt expected power.

Above plot is from an FEA model built after assembly, so input fairly well known …


Existing SCT Barrel Module - thermal FEA features (2)

Some “Quirks” mainly during design phase ~

1. Off-module cooling structure not well known / not rigorously included in FEA.

Eventually: FEA consistent with study of an irradiated module => power headroom factor slightly > 2. Deemed Adequate, but (hindsight) => only a 7C headroom in coolant temperature – a bit tight??

. . . We did not have a very sound understanding of runaway dynamics.

3. Some strange-looking plots - ending in mysterious vertical lines – defensible?

2. There was a perception that thermal runaway happens “at” the location of maximum T


UPGRADE for sLHC: increased instantaneous & integrated Luminosity.

Improve Track Resolution =>Shorter strips at low radius (120 mm => ~25 mm: 4 rows on a 100mm square sensor)

Bigger strip detector (replace straw tubes) => multi-module STAVES (end insertion)

~5-fold increase in ultimate Sensor power density (at same T) =>CO2 evaporative cooling => Lower temperature (easily gains a factor > 2).

Improve cooling geometry: ~ whole length of wafer, improved cooling pipe contact.

Variety of design concepts: d/s module (development of existing) – asymmetric cooling (hybrid L, sensor R) – etc.

Baseline (since 2008) is the Integrated Stave: modules (sensor+hybrids) mounted on a CFRP “plank” Plank incorporates a U-shaped cooling pipe (evenly distributed across width).

(S.Diez Cornell, 2011)


Runaway Power vs Slope of Runaway Curve at Zero Power

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Initial Slope [ C/ [mW/mm2 at 0C]

Ru

naw

ay P

ow

er [

mW

/mm

2 at

0C ]

Int. Hybrid/inboard pipes-no bus

S-SCT max power density

Thin DS-DC340

Int.H+Bus

Thin DS-2WmK

Thin DS-DC340

Int. Hybrid - no bus

- 9 design variations. Bridged DS, SS, integrated hybrid. - Runaw ay curves generated in 2-d Excel. Thermal conductances ALL assumed constant, including HTC).- Plot is of Pow er at Runaw ay (mW/mm2@0C) vs slope of the characteristic at zero sensor pow er (hybrid heat only). Black: -30C fluid Red: -25C fluid Curve: prediction of point-like sensor model.

UK-SS No Hybrid

3.3 /Slope

Int.H+Bus:

Runaway Performance – crude comparison of the various designs

FEA not practical for the many, evolving designs! Approximate as 2-d thermal R network (~30 nodes). Iterate for sensor max.T vs Q(0C)

Wrapping up this study resulted in a compelling plot -

Sensor Max Temperature vs Sensor Power (discrete resistor approx),Assumes 3kW/m^2-k (~C3F8), two (def. 4mm dia.) pipes.

-25.00

-20.00

-15.00

-10.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00Sensor Power (mW / mm2 at 0C)

Se

ns

or

Ma

xT

(C

)

Thin DS-DC

Thin DS-2WmK

Int.Hyb./inboardpipes(2.8dia)

Int. Hybrid/inboard pipes-CGL NO BUS(Runaw ay at 7.9)

Design-Grease

UKmcmd-DC

T(fluid) = -30C-Runaway

UKSS-DC→

ThickDS-2WmK

←UKSS-2WmK

Int.H+Bus-CGL

Runaway Power is inversely proportional to Initial Slope ??

WHY?

- and is it easy to predict runaway?

(max sensor T was used here: expect a better fit for mean T).


“Analytic model of thermal runaway in silicon detectors” G.Beck, G.Viehhauser NIMA 618 (2010) 131.

“Minimal Model” approximation, assumes:

- T variation across the sensor can be neglected (sensor effectively point-like)

- Negligible T dependence of all thermal conductivities (and fluid htc).

Q(TS )

R

TS

T0

Remainder of thermal circuit is

equivalent (Thevenin’s theorem) to a

single thermal resistor R connected to a

sink at T0 .

T0 = Sensor temperature at zero Q (but

with readout electronics powered).

Temperature variation of power dissipated by sensor:

SAS TTTQ exp~ 2

where TA = 1.2eV/2k ≈ 7000K (carrier activation temperature) The two parameters R and T0 are

found from FEA.

Sensor mean temperature rise and critical values for runaway are then given by a set of ~simple formulae.


Steady State Continuity – balance of generated and conducted sensor heat.

The heat generated by the sensor at TSis given by:

RTT

TT

TTTQ

refSA

S

refSref /

11exp

2

0

Two solutions for TS : stable (e.g. operating point O above) and unstable, at higher temperature.

R

TTQ S

cond0

where Qref is its value at some reference temperature Tref (usually 0C). This must be balanced by conduction:

So that:

Sensor Temperature vs Reference Power

-30

-20

-10

0

10

0 1 2 3 4 5Qref

TS

T0

TS,crit

Qref,crit

Unstable Balance

Stable Balance

T(Sensor)

Power

T0

O

Sensor Heat conduction to sink. (~1/R )

Heat generated by the sensor

TC

Sensor Corner Temperature (Tmax) vs Sensor Power Density at 0C "As built" FEA (2007)

-15

-10

-5

0

5

10

0 40 80 120 160 200 240 280

Sensor Power at 0C [mW/mm2]

Sen

sor

Tem

per

atu

re [

C]

max.power (10y)

Coolant

-15C

-20C

-25C

FEA normally explores the stable region,remember -

s

As

ref

Arefrefgen T

TT

T

TTQQ expexp 22


Comparison with “First Observation of Thermal Runaway in the Radiation Damaged Silicon Detector”Kohriki et al, IEEE Trans.Nucl.Sci, 43. 1200 (1996).

Analytic Continuity Curve: parameters adjusted to Kohriki 1996.

0

1

2

3

4

5

6

0 50 100 150 200Vbias [V ]

I[mA]

I crit

V crit

Runaway Latched by PS Current Limit.(Mean T =45C)

Stable Balance

(below FD)(Mean T ~33C)

The above continuity curve, expressed in terms of current and voltage has the same (double valued) form.

A rough fit of R to the Kohriki data shows how the higher T solution is stabilised by a current limiting power supply.

CDF Sensor, 4 1013 p/cm2


Thermal FEA of Upgrade Stave Module

Each stave (half barrel length) will be 13 modules long.

Thermal FEA model is of a single module

(now with proposed 256 channel r/o chips cf 128 in photo!)

Sensor FEA elements are thermoelectric, with T-dependent resistivity (as Andreas/CMS).

We (usually) neglect convection & radiation – so conduction only – and assume coolant TC

= -30C with htc 8000W/m2K.

Graphitised foam +Epoxy/BN joints CFRP facings

Hybrids placed asymmetrically (bcs!) - expect eventually symmetric.Hybrid heat ~ 5.6W/ face.


T distributions (Serial Powering – zero sensor power)

Sensor max-min = 3.5C

Mean Sensor T = -25.2C = T0 in the analytic model

Next: run with a small amount of sensor heat defined at 0C, and find the resultant mean temperature. The slope gives the other parameter for the analytic model, here: R = 0.90 C/W


RESULTS from FULL FEA and ANALYTIC MODEL

-30

-25

-20

-15

-10

0 1 2 3 4 5 6 7q(0C) (mW/mm2]

T S [C]

0

5

10

15

20

-30 -25 -20 -15 -10 -5

Tc [C]

T S- T c [C]

Tc crit

Red dots: full FEA results. Blue line: analytic continuity curve (T0 = -25.2C, R = 0.9 C/W).

After 3000 fb-1 expect max. fluence (r = 38cm, z=1.2m) of 1.2 x 1015 neq/cm2 (includes a safety factor x2) => sensor power = 0.65mW/mm2 (at 0C, 500V bias).

=> Runaway headroom of ~ factor ×9.4 in terms of sensor power OR 21 degrees in coolant temperature.

Sensor mean T vs q (at 0C) (at TC -30C) Sensor (T – TC) vs TC at q=0.65mW/mm2


0

1

2

-35 -30 -25 -20 -15 -10 -5 0 5 10

TS [°C]

q [mW/mm2]

Operating Point

conduction

(slope ~1/Rt ) dashedcurve: generation x 9.4

line: conduction + 21o

Tc

0.65

Sensor HeatGeneration and Conduction

vs Sensor T

generation

T rise due to hybrid

So: we appear to have a lot of headroom against thermal runaway.

Quick FEA runs + the analytic model allow us to explore sensitivity to reducing component thicknesses or conductivities. And there are a few uncertainties of course…


Update to hybrid power (Vddd=1.2V) and more realistic (75% ) power conversion efficiency: 5.6W => 6.7W => 1C reduction in headroom (short arrow)

IFF double ASIC power (track trigger) => Direct 5C reduction in headroom (long arrow)

Headroom at 0.65mW/mm2(0C) and -30C coolant for various changes in thermal K

Sensor Glue IRS2125, 0.44W/mK

Halve *ALL* Ks together

Fluid htc (=>4000)

Foam Glue (BN/Hysol)

CFRP Kx

Cable Bus Ky

CFRP Ky

Foam Kxyz

Baseline Design

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Coolant Temperature Headroom [C]

4 8 126 1021 Power Headroom Factor (approx)

x = across stave, y: vertical, z: along beam

Green bars are results of halving conductivity

Headroom for some changes wrt the Baseline


We should be careful that performance isn’t degraded at the ends of the stave:

End of Stave electronics – GBT + optolink + power conversion: will reduce temperature headroom by only ~ 1.5 degrees.

But the main problem here is to cool the GBT etc effectively – to be investigated …

-26-25

-24-23 -22 -21C Z=0

Sensor surface

We have already extended thermal coupling around the pipe U-bend to avoid this potential 6 degree loss of headroom.


. . . And what about that shot noise question? Is it more important than runaway?

The analytic model tells us that that at the critical Qref for runaway the actual sensor power is given by:

Qcrit = T02 / R.TA ≈ 9W

Taking rms enc = √ ( 6 * I[nA] * 25 [ns] ) (ABCN shaper) ,

5120 strips and 500V bias gives: 3.8mA/strip , rms shot noise: 760 e .

- slightly worse (830 e) if the coolant temperature rises instead.

Maybe not much to choose between them, after all !


BACKUP


BACKUP SLIDES


FEA Input Data


Plot from “Band Gap Regulator Analysis” - J. R. Biard, Honeywell, 9/10/2002 presentation

Note that when the linear fit is substituted in the Boltzmann factor exp(-Eg/2kT) it is the intercept that gives Eef !

1.06

1.08

1.10

1.12

1.14

1.16

1.18

1.20

1.22

0 100 200 300 400 500

Temperature (oK)

Ban

d G

ap (

eV)

-1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

E

G (

meV

)

-40oC 25oC 200oC

EG = -2.74471E-4T + 1.205852

EG = 1.17-(4.73E-4*T2)/(T + 636)

Y.P.Varshniform (1967)(Shift of CB wrt VB)


Time-dependent FEA. Thermal Capacitance?

Barrel Module (LHC) Transients (from initial 0C): 6W hybrid/-15C fluid/sensor power 0, 20, 40, 60, 80, 100 uW/mm^2(0C)

-5

0

5

10

15

20

0 400 800 1200 1600 2000 2400

Time (s)

Tem

per

atu

re (

C)

Left Sensor Top Left Corner

Left Sensor Top Right Corner

Right Sensor Top Right Corner


(Existing Barrel Module) T-distribution is well behaved even during runaway


-25C (fluid)

-15

-9.4

-11

-6.8

QMUL Abaqus (2007, 2009). FLUID -25C. Leakage Power 120 mW/mm2 at 0C (target value) Temperature Contours at 1C intervals from -21C to +3C.

(1C agreement with ATLAS thermistor data)

T rise across sensor is rather small (below runaway). Mean TS ~ 9.8 C. At Zero Leakage Power: ~11.9 C (2.1C lower). Deduce 70% of thermal resistance is “off-sensor” (~ as end-cap, upgrade stave designs…).

Documents

Graham Beck, QMUL. Oxford Tracker Forum, June 2013. 1 Thermal Design of the ATLAS Upgrade Strip Staves 2007 (start of my involvement in thermal study):