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

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

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

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 1.25-1.50 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 10-10 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

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 10-10 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

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

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

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

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

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 2016 (EDMS: 1600527) 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 1740232) in collaboration with BE-BI LESS treated WAMPAC 5.3 R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

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

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

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 40-60 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

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

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 2.5 - 4.5 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 = 2.232 m SPS sector 430 = 8 m R. Salemme – Results from 2016 COLDEX runs and future experimental plans – CERN, Geneva

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