1. LHC beam dump 7.56 TeV operation
W.Bartmann, C.Bracco, E.Carlier, L.Ducimetiere, B.Goddard, N.Magnin, A.Sanz Ull, V.Senaj, N.Voumard, C.Wiesner
3/7/17

2. Layout of subsystems Extraction septa MSD Dump block TDE + shielding
Extraction kickers MKD
Passive protection TCDS
Dilution kickers MKB
Passive protection TCDQ
TCDQ
[m]
Entrance window VDWB
Vacuum line
Exit window
Plus beam position and loss monitors, and dedicated interlocking (not shown)

3. Main subsystem characteristics at 7 TeV
15 magnets of 1.4 m
15 magnets of 4.5 m
6 magnets of 1.3 m
4 magnets of 1.9 m
Fixed extraction angle requires active energy tracking for extraction and dilution kickers, and extraction septum

4. Beam dilution – drift length and sweep
Need to reduce beam density from ~1015 p+/mm2 to ~1011 p+/mm2…
Dilution kicker waveforms
b and Dx functions for dump line (TD62)
~900 m drift  4-5 km bx,y at dump  s ~ 1.5 mm
Dilution  1200 mm sweep length,  ~400 mm
Peak p+ density is ~1.21011 p+/mm2
(cf 6.81014 p+/mm2 for undiluted beam, bx,y ~150 m)

5. Systems affected by higher energy exploitation
Intercepting devices TCDS/Q and beam dump block TDE, plus entrance (VDVB) and exit windows
More energy deposited, smaller beam size
Extraction kickers
Higher operating field (voltage and current)
Dilution kickers
Extraction septum
Higher operating field (current)
Beam Energy Tracking System (BETS)
Larger dynamic range

6. Intercepting devices TCDQ/S, TDE and VSWB
Being upgraded for HL-LHC design of 7.00 TeV, 2.3e11 p+/b in 2.1 um
Operating limits not easy to define (needs FLUKA+ANSYS for each use-case, and highly sensitive to assumed materials properties
In worst-case, any new constraints will mean scaling down peak intensity by some amount (~7.56/7.00) to keep dynamic stresses at ‘HL-LHC’ design limits
These upgrades will take place in LS3
Might place a limit on brightness, but no hard limit on energy
Would require dedicated FLUKA/ANSYS campaign for 7.56 TeV energy to define the intensity limits (as could be done for present devices of course, before LS3…)

7. Extraction kicker MKD Upgrade in progress to improve voltage holding
New switch stack geometry
Increased main capacitance
New trigger transformers
New power trigger modules
Replacement of triggering cables
DC powering of Ross relay
Better protection against dust ingress
Abort gap increase 3.0 us to ~3.1 us
Implementation for LS2
Design operating energy: 7.00 TeV
7.5 TeV for reliability run, tests, …

8. Extraction kicker MKD operating at 7.56 TeV?
Operating/test voltage corresponding to 7.56/8.1 TeV
31.2/33.4 kV without upgrade
Will be 28.7/30.7 kV after the LS2 upgrade
Compare to 26.8/28.9 kV today at 6.5 TeV
Issue with maximum 30 kV rating of proposed additional capacitor to solve
Otherwise MKD should be OK for TeV operation after LS2
Performance at given voltage improved wrt today by new stack geometry
No hard threshold
Operational voltage lower than today’s test voltage

9. Dilution kickers MKBH/V operating at 7.56 TeV?
Current very high (~26 kA), possible implications for contact erosion to be studied for both systems
For MKBH: CONS upgrades planned for LS2 to improve performance
Basically as for MKD generators (capacitance, triggering, geometry, Ross relays)
Nothing planned for MKBV
For MKBV: looks OK without changes
Voltages today at 6.5/7.0 TeV are 13.7/14.8 kV respectively
Voltages will increase to 16.0 kV in operation at 7.56 TeV, and 17.1 kV in tests at 8.1 TeV, which should be OK

10. Dilution kickers MKBH operating at 7.56 TeV?
MKBH will be upgraded in LS2 for improved voltage holdoff
Operating/test voltage corresponding to 7.56/8.1 TeV
28.8/30.8 kV without upgrade
Will be 25.5/27.3 kV after the LS2 upgrade
Compare to 24.7/26.7 kV today at 6.5/7.0 TeV
Otherwise MKBH OK for 7.56 TeV
No hard voltage threshold
Performance at given voltage will be improved wrt today by new stack geometry (after LS2)
Operational voltage lower than today’s test voltage (after LS2)
Installing 2 additional MKBH to reduce failure probability is being studied (for LS3)
New retriggering system to be installed to mitigate erratics (but maybe only in LS3)

11. MSD septa operating at 7.56 TeV?
Assuming nominal septa kicks of:
Total kick of all MSDs: αtot = 2.4 mrad with individual kicks of αMSDA = mrad, αMSDB = 0.16 mrad and αMSDC = mrad
From 7.0 TeV to 7.56 TeV, B-rho increases by 𝐵𝜌 new 𝐵𝜌 nom ≈108.0%.
Increasing MSD current from 880 A to 972 A (by ≈10.5%) gives a field increase of:
MSDA: 0.86T/0.79T = %
MSDB: 1.06T/0.99T = 107.1%
MSDC: 1.24T/1.16T = 106.9%
(Field simulations by A. Sanz – checked for 7.50 TeV values)
Thus, relative weight of the kicks from the 3 septa families changes (saturation effects).
The new total deflection is then given by: 𝛼 tot new =5∙ 𝐵𝜌 nom 𝐵𝜌 new 𝛼 msda nom ∙ 𝐵d𝑙 msda new 𝐵d𝑙 msda nom + 𝛼 msdb nom ∙ 𝐵d𝑙 msdb new 𝐵d𝑙 msdb nom + 𝛼 msdc nom ∙ 𝐵d𝑙 msdc new 𝐵d𝑙 msdc nom and has to be rescaled accordingly.

12. MSD septa operating at 7.56 TeV?
Only 0.35 mm position error at TDE from new trajectory – negligible
Saturation effects might introduce slightly more harmonic content for circulating beam, but small additional effect only, which could anyway be corrected at high energy
MSD power converter maximum current increases from 880 to 980 A
Should also be feasible (1000 A/ 600 V conveter)
No issues expected from cooling (outlet temperature would increase from ~21 to ~24°C)
Other systems should be OK (FMCM, BETS, etc.)
Overall no issues expected from MSD

13. BETS operating at 7.56 TeV?
Currently the tracking system BETS/PLC has a scale of 120 MeV per bit, encoded on 16bits.
This is also what is sent by the BEM cards to Safe Machine Parameters (SMP), and then broadcasted over timing for all LHC systems.
This means that currently the maximum energy that we can measure is TeV
Operation at 7.56 TeV should not be a problem, but testing/reliability runs at 8.1 TeV (above 7.8 TeV) is not straightforward
We might have to renovate the BETS, or to change the energy encoding over 16bits = loss of precision.
Impact on LBDS (BETS, PLC, etc) and other systems (SMP, timing, etc) must be evaluated

14. Conclusions All dump subsystems impacted by operation at 7.56 TeV
For TDE and beam intercepting devices, HL-LHC performance upgrades will be in LS3. Studies needed to define performance limits at 7.56 TeV (post LS2 and post LS3)
After planned LS2 upgrades, extraction and dilution kicker operating voltages will be essentially same as present test voltages
Completion of LS2 upgrades mandatory
Issue of 30 kV additional capacitor voltage rating to solve for MKD
Retriggering for MKBH might only be in LS3
Other improvements for voltage holdoff mean system reliability at 7.56 TeV should be similar to today’s reliability at 6.5 TeV
One unknown is contact erosion with higher currents – difficult to test for. Could propose R&D activity to devise on-line ELQA for pulsed currents – will be challenging
Upgrades should help keep SEB failure probability acceptable at 7.56 TeV – but need more accurate measurement of HEH fluency in UA63/67 & simulations for 7.56 TeV
Impact on BETS, controls and SMP to be evaluated – may need upgrade
MSD septa should not pose a problem – will operate at ~980 kA

15. References
ABT Technical Coordination Meeting, LBDS 7.x TeV operation,
ABT CONS Project Review, CONS for MKB and MKD switches,
A. Sanz Ull, The LHC MSD septa at 7.5 TeV, EDMS