1. How does a cryogenic system cope with e-cloud induced heat load
How does a cryogenic system cope with e-cloud induced heat load? …and what can we learn from heat load measurements ?
Benjamin Bradu
CERN
Technology Dpt.
Cryogenic Group
Operation Section
4th June Isola d’Elba

2. Contents Introduction LHC Cryogenics LHC cryogenic beam screens
E-cloud heat load measurements
How to cope with e-cloud transients ?
Conclusion
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load? 2

3. Introduction
The e-cloud and the cryogenics… Is the same fable than… The hare and the turtle
&
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

4. LHC cryogenics 8 cryoplants for 8 sectors (3.3 km each)
8 refrigerators at 4.5 K (4 old + 4 new)
Produce supercritical helium at 3 bar and 4.5 K
8 refrigeration units at 1.8 K
Pump superfluid helium at 16 mbar to reach 1.8 K
The 2 main cryogenic characteristics
It is slow and…
It does not like the fast events… A + B
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

5. LHC cryogenics LHC cold mass: M = 37 000 000 kg !
Why is it slow ?
Transient time is proportional to M, Cp, dT, m, S, L
M is the mass of what you want to cooldown (Metal and fluids).
Heat capacity Cp depends on temperature (faster when cold)
Above 80 K: dT is limited above 80 K by mechanical constraints (just for cooldown/warmup)
Below 80 K: dT is limited by liquid temperature (helium = bar)
The massflow m is mainly limited by rotating machines and pipe diameters
S is the surface of exchange limited by the space we have.
L is the length between cryoplants and equipment to cooldown: time delay.
LHC cold mass: M = kg !
LHC helium mass: M = kg !
LHC is in a confined area: limited space
LHC is long: Cryolines = 3.5 km / sector
Time of flight for Helium 5 K = 8 hours
Time of flight for Helium 3 K = 20 min
Time of flight for Helium 20 K = 1 hour
(e-cloud heat loads return)
Conclusion: it is very slow and delayed !
NB: Cp of He II ~ 4000 J/kg-K
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

6. LHC cryogenics Fortunately, most of heat loads are static
Cryo must provide cryogenic conditions for many equipment
Superconducting magnets at 1.9 K +/ K
Stand Alone magnets filled with Liquid helium at 4.5 K
RF cavities filled with Liquid Helium at 1.35 bar +/- 20mbar
Distribution Feed Boxes levels filled with liquid helium at 4.5 K
Current leads cold ends at 50 K +/- 4 K
Current leads warm ends < 320 K
Inner triplet thermal shield +/- 2 K (!)
Beam screens outlet temperatures (e-cloud) < 30 K …
Fortunately, most of heat loads are static
About 70% of total heat load
Unfortunately there are dynamics heat loads…
… and some of them are “fast” for cryo
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

7. LHC cryogenics dynamic heat loads
(main ones)
All power are expressed as equivalent 4.5 K
1 1.8 K ~ K
1 4.5 K-20 K ~ K
1.8 K dynamic heat loads*
Resistive heating due to magnet splices: 0.75 kW /sector
Rise in 20 min with magnet ramping
Beam induced heat load: 1.2 kW/sector
Instantaneous with beams and collisions
Eddy current during magnet ramping: 2.5 kW/sector
20 min transient during the magnet ramping only (disappears once stable)
4.5 K - 20 K dynamic heat loads*
Beam induced effects on beam screens: 2.6 kW/sector
Synchrotron radiation: 0.5 kW/sector
Image current: 0.6 kW/sector
E-cloud: 1.5 kW/sector
Instantaneous with the beam
*as defined in LHC design report (2004) for nominal operation in S12
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

8. LHC cryogenic Beam Screens
LHC Beam Screens
Ensure a good beam vacuum
Limit heat loads on the 1.9 K cold mass
Independent He cooling loop per half-cell (53 meters length in arc)
485 BS cooling loops over the 27 km of the LHC
Cooled by conduction with supercritical helium inlet at 3 bar and 4.6 K
Helium outlet at 1.3 bar and ~20 K
4 parallel cooling pipes of 3.7 mm each
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

9. E-Cloud heat load measurement
Total heat load on beam screens (QBS) can be measured in each cryogenic loop.
In theory: QBS = Qsr + Qic + Qec
Qsr = Synchrotron radiations
Qic = Image current
Qsr & Qic nominal values and scaling laws validated at 50 ns bunch spacing (no e-cloud)
Qec = QBS - Qsr - Qic
nb = bunch number
Nb = protons per bunch
E = energy
σ = bunch length mean
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

10. Beam Screen heat load measurement
PT961
3 bar
TT961
5 K
Not reliable
TT84x
6 K
BEAM
TT94x
20 K
PT991
1.2 bar
QRL
Line D
1.25 bar m
QRL
Line C
Line C’
EH84x
-Qcm
CV94x
Support
Posts (53m)
Beam Screens
(53 meters)
+Qs
+EH
+QBS
Cold Mass
Simple energy balance on the beam screen circuit
QBS = m*(h961 – h94x) – Qs – EH (+Qcm)
m = massflow = f (CV94x, PT961,PT991,TT947)
h961 = enthalpy of line C = f(PT961,TT961)
h94x = enthalpy of BS outlet = f(PT961,TT94x)
Qs = constant static heat load
EH = electrical heater
Qcm = heat transfer to cold mass (~ 1 20 K)
Each cooling loop is then calibrated with the electrical heater
We manage to reach about 5% of relative error
VALID ONLY FOR
STEADY-STATE !!!
(~20 min)
B. Bradu. Beam Screen heat loads

11. Quite homogeneous around the machine (~ 8 W)
50 ns P1
485 beam screen heat load measurements
Fill #5980
22nd July 2017
nb = ns
E = 6.5 TeV
I = 1.4*1014 / beam P2 P8 P7 P3
No e-cloud
Quite homogeneous around the machine (~ 8 W) P4 P6 P5

12. Very variable between 50 W and 200 W
25 ns P1
485 beam screen heat load measurements
Fill #6675
12th May 2018
nb = ns
E = 6.5 TeV
I = 3*1014 / beam P8 P2 P3 P7
Very variable between 50 W and 200 W
still not understood…
e-cloud or not e-cloud ?
That is the question… P4 P6 P5

13. How to cope with e-cloud transients ?
Large heat loads
Fast transients
Very variable along the ring
(but reproducible!)
Cryogenics
Slow
Two main ideas as in the La Fontaine tales:
Be prepared
Start on time
“Going slow was how she made haste”
« Elle se hâte avec lenteur»
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

14. Be prepared: Pre-loading
NO BEAM
BEAM
4.5 K Refrigerator
~1.5 kW of
electrical
4.5 K
4.5 K Refrigerator
No electrical
heater
~ 60 BS loops over 3 km
~ 60 BS loops over 3 km
~ 50 W of electrical heating at 5 K-20 K per loop
 ~ K in total
Beam induced heat loads
 Between 1.5 kW and K depending on sectors
No electrical heater at the peak
 This transition must be done synchronously with the beam induced heat loads (see next slide)
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

15. Start on time: Feed-Forward control
Feedback CV 94 x TT 84
Line C’
Beam Screens ( L = 53
meters )
PID
KFF 1 . 18
in arc +
uFF
Qdbs
Qsr
Qic
Qec
PLC
Line C 3
bar 4 5 K D 2 20 SP / 6
no beam
beam S P 22 -
BEAM EH
Beam parameters
Energy ,
intensities
bunch numbers
bunch length mean
B. Bradu. Individual Feed-Forward control for LHC beam screens

16. Beam Screen heat load estimation
(1) Synchrotron radiations
(2) Image current
(3) e-cloud
+ Others
SR (1) and IC (2) comes from LHC DR and are OK with measurements
EC (3): Variable along the LHC ring
Allow to model EC + unknown other phenomena if any
Equation scaled according to measurements (qeci/qecr/Nb0)
Equation scaled according to machine cleaning and injection scheme (Keci and Kecr)
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

17. Heat load measured Vs estimation
Heat load measurements
Heat load estimations from beam information for the Feed-Forward
Low heat load half-cell
(Peak ~ 50 W)
Medium heat load half-cell
(Peak ~ 100 W)
Very high heat load half-cell
(Peak ~ 230 W)
High heat load half-cell
(Peak ~ 150 W)
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

18. Result on a high-load half-cell
Fill #6675 on 12th May 2018: ns and 6.5 TeV
Temperature
well controlled
during the whole fill
High Load
= 210 W
Cancelling electrical
heating power
B. Bradu. Individual Feed-Forward control for LHC beam screens

19. Result on a low-load half-cell
Fill #6675 on 12th May 2018: ns and 6.5 TeV
Low Load
= 50 W
Power consumed
stable during the fill
Temperature
well controlled
during the whole fill
Cancelling electrical
heating power
at the peak
B. Bradu. Individual Feed-Forward control for LHC beam screens

20. Cryogenic refrigeration power profile
LHC dynamic heat loads (HL) in reality:
1.8 K : much lower than expected
Better splice resistance and less beam-gas scattering
4.5 K  20 K: much higher than expected
Probably due to e-cloud ? (only visible at 25 ns)
As consequence  Cryo re-configured the cryoplants differently
Transfer the 1.8 K refrigeration power to the 4.5 K20 K refrigeration power
Allow bigger dynamic heat loads at 4.5 K  20 K for e-cloud
Cryogenic hardware capacity limit
Total
Dynamic
Heat Loads
4.5 K20 K HL
4.5 K20 K HL
1.8 K HL
1.8 K HL
STATIC HEAT LOADS
Expected dynamic
heat load sharing
Measured dynamic
heat load sharing
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

21. Cryogenic refrigeration power profile
If no pre-loading neither Feed-Forward:
These transients cannot be managed by the cryo turtle !
NB: The dynamic heat 1.8 K profit of the damping effect in
the superfluid helium and this is not the case between 4.5 K 20 K.
Cryogenic hardware capacity limit
Dynamic
Heat Loads
Dynamic
Heat Loads
Dynamic
Heat Loads
Dynamic
Heat Loads
STATIC HEAT LOADS
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

22. Cryogenic refrigeration power profile
If pre-loading and Feed-Forward:
These transients can be managed by the cryo turtle !
Cryogenic hardware capacity limit
Dynamic HL
Dynamic HL
Dynamic HL
Dynamic HL
PRE-LOADING
STATIC HEAT LOADS
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

23. Conclusion
Today the Feed-Forward is deployed and tuned individually in the 485 beam screen loops; cryo is close to the optimal operation.
Cryo is not limiting the LHC operation.
But… we are now approaching the hardware cryogenic capacity limit
Beam induced heat loads cannot increase significantly in the coming years…
Accelerator physicists must be creative to help us 
“Rushing is useless; one has to leave on time. “To such truth witness is given by the Tortoise and the Hare.”
B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?

24. B. Bradu. How does a cryogenic system cope with e-cloud induced heat load?