1. Observations with different bunch spacings
LHC Heat Loads:
Observations with different bunch spacings
P. Dijkstal and G. Iadarola
Many thanks to:
B. Bradu, L. Mether, E. Metral, G. Rumolo, B. Salvant

2. Heat load evolution: scrubbing curve
The different sectors are not following the same scrubbing curve
Slope observed for the same heat load value are very different

3. Heat load evolution at 6.5 TeV
When normalizing heat loads to intensity we find out that the curves are strongly correlated and they practically differ only for a constant offset
Easiest interpretation:
There is a source of heat load (most likely e-cloud in the dipoles) that is very similar for all arcs, and conditions in the same way in all sectors
Plus there is another source of heat load which is different from sector to sector, scales ~linearly with intensity, and does not condition at all
2015
2016
Data are post processed to use the same calibration for all fills (thanks Philipp and Benjamin!)

4. Each color corresponds to cells with similar heat loads (within ~15 W)
Heat load evolution at 6.5 TeV
When normalizing heat loads to intensity we find out that the curves are strongly correlated and they practically differ only for a constant offset
Easiest interpretation:
There is a source of heat load (most likely e-cloud in the dipoles) that is very similar for all arcs, and conditions in the same way in all sectors
Plus there is another source of heat load which is different from sector to sector, scales ~linearly with intensity, and does not condition at all
This is true also at a cell by cell level
Each color corresponds to cells with similar heat loads (within ~15 W)
Data are post processed to use the same calibration for all fills (thanks Philipp and Benjamin!)

5. Heat load evolution: effect of energy ramp
The heat load increase between injection and high-energy is practically the same for all sectors
 mechanism responsible for difference between sectors is independent on energy
HL6.5TeV - HL450GeV

6. Each color corresponds to cells with similar heat loads (within ~15 W)
Heat load evolution: effect of energy ramp
The heat load increase between injection and high-energy is practically the same for all sectors
 mechanism responsible for difference between sectors is independent on energy
Each color corresponds to cells with similar heat loads (within ~15 W)
HL6.5TeV - HL450GeV

7. Heat load evolution: differences between sectors
Heat load observations are compatible with the following scenario:
There is a source of heat load (most likely e-cloud in the dipoles) that is very similar for all arcs, and conditions in the same way in all sectors
On top there is another source of heat load which is different from sector to sector, scales ~linearly with intensity, is independent on energy and does not condition at all
Can this second heat load source be an impedance?
Important information to answer this question can be obtained comparing observations with different filling patterns
Here we recollect the main observations, to be compared against detailed impedance estimates

8. Heat loads with different filling patterns
We recollected analyzed the following fills performed with different kinds of filling patterns. The comparison is performed for beam parameters and heat load values measured after 2 h in stable beam
For the sake of brevity, in these slides we focus on only on two sectors:
Sector 23: example of high load sector
Sector 34: example of low load sector
The complete set of plots is available here

9. Heat loads with different filling patterns
Measured data vs expectations from synchrotron radiation and resistive wall impedance
Arc 23
Arc 34
Synchrotron Radiation
Impedance (R.W.)
Measured

10. Heat loads with different filling patterns
Measured data vs expectations from synchrotron radiation and resistive wall impedance
Arc 23
Arc 34
Synchrotron Radiation
Impedance (R.W.)
Measured
Normalize to the number of bunches…

11. Heat loads with different filling patterns
Measured data vs expectations from synchrotron radiation and resistive wall impedance
Arc 23
Arc 34
Impedance (R.W.)
Measured
Normalize to the number of bunches and subtract contribution from synchrotron radiation

12. Heat loads with different filling patterns
Measured data vs expectations from synchrotron radiation and resistive wall impedance
Arc 23
Arc 34
+10%
+20%
Normalize to the number of bunches and subtract contribution from synchrotron radiation
Then normalize all measurements w.r.t. the 100 ns case
x14
Is it possible to construct an impedance that matches this quite peculiar behavior?
(see consideration from Benoit HSC meeting 20 Feb)

13. Cell-by-cell plots at 6.5 TeV
(only for S23 in these slides, complete set for all sectors can be found here)

14. Much larger than impedance and synchrotron radiation estimates
25 ns, 48 b/train, 601 b.
Much larger than impedance and synchrotron radiation estimates
Synchrotron Radiation
Impedance (R.W.)
Measured
100 ns, 685 b.
Compatible with impedance and synchrotron radiation estimates

15. Much larger than impedance and synchrotron radiation estimates
25 ns, 48 b/train, 601 b.
Much larger than impedance and synchrotron radiation estimates
Synchrotron Radiation
Impedance (R.W.)
Measured
Synchrotron Radiation
Impedance (R.W.)
Measured
100 ns, 685 b.
Compatible with impedance and synchrotron radiation estimates
Zoom: cell-by-cell structure is different

16. Much larger than impedance and synchrotron radiation estimates
25 ns, 48 b/train, 601 b.
Much larger than impedance and synchrotron radiation estimates
Synchrotron Radiation
Impedance (R.W.)
Measured
Synchrotron Radiation
Impedance (R.W.)
Measured
50 ns, 296 b.
Compatible with impedance and synchrotron radiation estimates

17. Much larger than impedance and synchrotron radiation estimates
25 ns, 48 b/train, 601 b.
Much larger than impedance and synchrotron radiation estimates
Synchrotron Radiation
Impedance (R.W.)
Measured
8b+4e, 529 b.
Larger than impedance and synchrotron radiation estimates
Cell-by-cell structure similar to 25 ns case

18. Cell-by-cell structure similar to 601b case
25 ns, 48 b/train, 601 b.
Synchrotron Radiation
Impedance (R.W.)
Measured
25 ns, 48 b/train, 2200 b.
Cell-by-cell structure similar to 601b case

19. Cell-by-cell structure extremely similar to 48b/train case
25 ns, 72 b/train, 2040 b.
Cell-by-cell structure extremely similar to 48b/train case
Synchrotron Radiation
Impedance (R.W.)
Measured
25 ns, 48 b/train, 2200 b.

20. Summary
Heat load observations are compatible with the presence of a source of heat load which is different from sector to sector, scales ~linearly with intensity, is independent on energy and does not condition
Information to identify this source can come from fills performed with different filling patterns
In the high heat load sectors, the heat load increases by a large factor when moving from 100 ns or 50 ns to 25 ns beams.
The increase is much smaller when moving from 600b (25 ns) to full machine and from trains of 48b to trains of 72b
At a cell-by-cell level the 100 ns and 50 ns fills show loads that are compatible with impedance and synchrotron radiation (apart from a few outliers, measurement?)
The cell-by-cell pattern observed with this schemes is different from the one observed with 25 ns
The case of the 8b+4e beam is intermediate. Heat loads are significantly smaller than in the 25 ns case but still larger than impedance and syn. radiation, especially for the high load sectors (similarities are visible also in the cell-by-cell pattern)