1. Results from 2016 COLDEX runs and future experimental plans
R. Salemme
on behalf of the TE-VSC group
Injector MD days 2017
CERN, Geneva, Switzerland
March 24th 2017

2. Context: HL-LHC Inner Triplets (IT)
New HL-LHC insertions:
intolerable increase of heat load due to electron cloud
Increase of background to the experiments
HL-LHC mitigation:
Cannot rely anymore on beam conditioning!
Baseline: amorphous carbon (a-C) coating (δmax < 1.0±0.1)
Ex-situ on new beam screens for IR 1/5
In-situ on existing beam screens in IR 2/8
Alternative to baseline: Laser Engineered Surface Structures (LESS)
a-C coating is qualified at cryogenic temperature with LHC type beams in COLDEX
G. Arduini and G. Iadarola, 5th Joint HL-LHC/LARP Annual Meeting, October 2015
2.2∙1011 ppb
1.15∙1011 ppb
~200 W
Copper OFE
(as received)
(conditioned)
a-C coating
(as received)
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

3. COLDEX MDs in 2016
3 x 24 hrs blocks allocated in 2016 during (3) LHC TS weeks
One MD (June) lost due to water cut (loss of cryogenic conditions)
Two blocks (September and November) moved at last minute, but carried successfully
MD1/2016: “bare” surface conditions
MD2/2016: ~3.0e16 CO2/cm2 pre-adsorbed on the BS
HL-LHC temperature configuration (BS 60 K, CB 3 K) and standard configuration (BS 10 K, CB 3 K)
Improved cryogenic stability and better electron sensitivity
LHC beams, 25 ns in trains of 72, at flat bottom only (26 GeV/c)
No SPS hardware limitation touched (MKP temperature and pressure rise, low and high energy dumps), down-time limited to few hours per run
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

4. Predictions
Heat load and electron activity predictions are made simulating the electron build-up with the pyECLOUD code
“Furman-Pivi-Cimino”-like a-C coating SEY model developed
The multipacting threshold is situated in the SEY window
and depends on the bunch intensity (field-free, geometry)
For SEY<1.1:
the HL is << 1mW/m → not detectable
the chimney electron current is in the A → detectable in 2016!
100 mW/m
SEY
<1.1
Heat load [W/m]
Chimney current [A]
SEY ≤ 1.1
2015 detection limit
2016 detection limit
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

5. Results of MD1: bare surface
No pressure rise correlated to electron stimulated desorption
Transmission of H2 desorbed at RT extremities readily observable at 60 K, absent at 10 K (physisorbed on the BS)
Confirmed by reduction of transmitted gas applying a solenoidal field at extremities
No detectable dynamic heat load
within the measurements detection limits (~100 mW/m)
insensitive to nr. of batch (72 bunches, 25 ns) or bunch intensity
No multipacting electron activity measured
Ion/electron current measured for the first time with a detection limit of A (bias: 20 V)
The source of electron cloud (seeds electrons by residual gas ionization) was finally measured for the first time.
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

6. Results of MD1: bare surface
Measured ionization current dependently with beam current and vacuum density, the cross section being defined by the beam energy (constant)
The non-zero SEY of the surface plays a little role
Example: Ionization current during BS warm-up with beam
Beam circulating intensity
extremities
a-C
Electron current
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

7. Results of MD2: ~3.0∙1016 CO2/cm2 Measurement carried:
First, at 60 K;
Then, at 60 K after flushing about half of the coverage to the CB via thermal desorption at 100 K;
Finally at 20 K.
No pressure rise correlated to electron stimulated desorption
No partial pressure rise due to stimulated desorption of CO2
No CO2 recycling and/or flushing!
No detectable dynamic heat load
within the measurements detection limits (~100 mW/m)
insensitive to nr. of batch (72 bunches, 25 ns) or bunch intensity
No multipacting electron activity measured
Additional characterization (vs. nr. of batches, bunch intensity, energy spectrum) of ion/electron current carried
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

8. Results of MD2: ~3.0∙1016 CO2/cm2
Example: absence of e-cloud vacuum/cryo signatures
beam circulation
Steady pressure: see difference with extremities! (small H2 transmission yet visible)
No ΔT without/with beam
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

9. Conclusions
No sign imputable to electron cloud was (yet) measured in the COLDEX setup with an a-C coated beam screen
The electron cloud source (seed electrons) was measured and characterized in 2016
Large quantities (~1016 molecules/cm2) of physisorbed gases (CO2 in 2016, H2 and CO in 2015) seem not to modify the surface behaviour
no recycling and flushing observed
Along to MDs, the cryogenic vacuum characterization carried off-beam with COLDEX suggests a new operating temperature window for the HL-LHC Inner Triplets beam screens
~60-80 K is under discussion, see 24th HL-LHC TCC
a-C coating of the BS surface has successfully mitigated electron cloud in the COLDEX setup
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

10. LESS treated segmented OFE Cu BS, ID67, installed in 2017
Perspectives for 2017
LESS treated segmented OFE Cu BS, ID67, installed in 2017
Recent interest in Laser Engineered Surface Structures (LESS)
→ alternative choice for HL-LHC as of (EDMS: )
A COLDEX LESS treated BS was fabricated and installed in 2017
Installation completed last Wed 23rd March
A real challenge tackled thanks to the superb collaboration of all the stakeholders involved (TE-VSC, ASTeC and Dundee University UK collaboration, EN-MME)
Two WAMPAC calorimeters installed as well!
One LESS treated, the other not (Cu OFHC)
Pilot installation of LHC-type BLMs along the COLDEX experiment (ECR ) in collaboration with BE-BI
LESS treated WAMPAC 5.3
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

11. Requests for 2017 2017 beam time (Injector Schedule v1.0):
No SPS scrubbing run time out of LHC operation available for long term studies → more than 7 Ah were accumulated with a-C just in parasitic to the SPS scrubbing runs (15-16)!
3 x 24 hours dedicated MDs assigned, but one not aligned with the LHC TS (June)
switch to 12 hours MDs in the same week (beginning/end) after first measurements desirable to enable differentiated studies?
In parallel: mandatory cryogenic vacuum characterization for HL-LHC temperature window with all gases of interest (COLDEX GIS)
Outgassing at RT (pump-down)
Sticking probability (transmission method)
Gas molecular capacity (adsorption isotherms)
Gas release (TDS)
etc.
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

12. R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

13. a-C coating gas adsorption and release
40-60K
60-80K
> 80K CO H2 H2 N2
Limited pumping speed
CB > 150 K TMP pumping at extremities
Adsorption and release of the gas species is studied with Thermal Desorption Spectroscopy
Gases are physisorbed and desorbed by a-C as a function of the molecular coverage
explained either by a Redhead’s first order desorption model - with variable activation energy - or a second order model with a two-phased desorption event
In the K window, H2 is always released, whereas the other species (CH4, N2, CO, CO2) are released above 80 K no matter the coverage
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

14. H2 adsorption isotherms, at different T
1 Cu monolayer = 3∙1015 H2/cm2
as expected
BS T
equilibrium pressures after 24 hrs
a-C coated surface becomes an efficient cryosorber at cryogenic temperatures
H2 capacity > 2∙1017 molecules/cm2 at 10 K, ~100 times larger than Cu
Higher temperatures allow to store less gas
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

15. The COLDEX setup in the SPS LSS4
Mimics the LHC cold bore (CB) and beam screen (BS) cryogenic beam vacuum system, in presence of LHC-type beams
a-C coated OFHC made BS
5 – 150 K
316LN stainless steel CB K
RT chimney
ID 67 / OD 70 mm
Transparency = 1%
ID 113 mm
316LN SS SPS transitions RT
316LN SS SPS transitions RT
ID 100 mm
ID 100 mm
Solenoids
Solenoids
LBS = m
SPS sector 430 = 8 m
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

16. The COLDEX parameters and measurements
Goals
Surfaces properties
Beam intensity, energy and time structure
BS temperature
CB temperature
Gas molecular coverages
Cryogenic vacuum properties
Sticking (transmission meas.)
Thermal Desorption
Adsorption (isotherms, isosteres) ….
Operation of the BS in a new temperature window (above 40 K)
Electron cloud performance with proton beams in LHC-like system
Pressure increase (VGIs) and gas composition (RGAs)
at centre (chimney) → beam screen
at RT extremities
Heat load
cryogenic calorimetric method
Electron flux
electrodes on BS and inside the chimney
Performance qualification of a-C coatings at cryogenic temperature with LHC type beams
R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva