1. R. Kersevan, TE-VSC-VSM 30/06/2016
FCC-hh Double-Slot Beam Screen: Density Profiles vs Slot Size, and Misalignments
R. Kersevan, TE-VSC-VSM
30/06/2016

2. A first analysis of this kind has already been reported at a previous FCC- hh general meeting, , see ;
In the meantime the cross-section of the double-slot BS has been modified several times, taking profit from the fabrication in house of short prototypes (30 cm), (see C. Garion’s presentation at the FCC Week in Rome, here );

3. From presentation at meeting of 26/11/2015
Case 2: Nominal 3.2 mm slots: Effect of a vertical offsets of 1 or 2 mm
From presentation at meeting of 26/11/2015
A misalignment of 2 mm increases the number of photons reflected towards the opposite side of the beam screen

4. (Black Line: reference profile when photon scattering is not included)
Case 2: Nominal 3.2 mm slots: Effect of a vertical offsets of 1 or 2 mm
From presentation at meeting of 26/11/2015
The average density more than doubles when the BS is vertically misaligned by 2 mm
(Black Line: reference profile when photon scattering is not included)

5. Design Updates Symmetrical design Better impedance
Pumping holes hidden by the screen
Thermal copper coating on the outer side
Bigger pumping holes – no constraint for the distribution
Polygonal shape of the screen

6. Short prototype manufacturing
Assembly and welding
Copper coating

7. The new BS cross-section has a flat polygonal shape, which allows a better tolerance on the machining and positioning of the slot during assembly/welding of the system, which is becoming critical for the design of the 2 m-long test prototype scheduled to be installed on the ANKA light source ring in early 2017;
During a recent EuroCirCol WP4 collaboration meeting (this week, at CERN) it has been suggested to look at the possibility of having a wider (vertically) slot size, in order to better accommodate and intercept the SR photon fan in case of BS misalignment, or intentional orbit bumps (D. Schulte intervention during the meeting, to switch from H to V crossing);
The new BS geometry has been taken, assuming that the beam is off-axis vertically and deposits all of its SR power on the internal side of the BS:

8. Source: C. Garion, personal communication;
Calculated temperature distribution for a vertically displaced beam, orbit parallel to the slot; All of the SR power impinges on the BS wall; Total power is 504 W, ~31 W/m average; The cooling tubes’ temperature is kept constant at 40 K;
The 300 mm-thick internal copper layer manages to efficiently transfer the impinging power towards the area in contact with the cooling tube; The temperature distribution is a-symmetric, but still within acceptable values

9. The new BS geometry has also been modelled with SYNRAD+ and Molflow+, to calculate the SR fan distribution and the related density profiles:

10. BPM and/or bellows
conical tapers
Geometry implemented in the simulations: 14.3 m-long dipole followed by ~1.4 m-long drift with bigger ID to install BPM and bellows, with two conical tapers;

11. Nominal Slot: (2.28 mm) 3.0 mm slot: 4.0 mm slot: 5.0 mm slot:
“Power 2 CB”: power absorbed by cold-bore (leaking through the pumping slots); Units: Watts;
“Flux 2 BS”: photon flux absorbed (directly or scattered) onto the internal part of the beam screen; Units: ph/s
power absorbed by taper at end of drift section; Units: Watts
H2 density profiles vs slot height: fan as the slot height increases, the density diminishes thanks to a higher capture probability of the SR inside the slot; this reduces also the amount of radiation scattered off the BS internal wall (next to the slot);
A qualitatively similar behaviour is expected for lower beam energies (to be done);

12. H2 density profiles vs vertical beam orbit offset (or vertical angle): The hydrogen density increases quite substantially as the beam orbit is displaced vertically, or a 210 mrad vertical angle (corresponding to 3 mm along the 14.3 m-long dipole) is simulated on the orbit; Note that this refers to the older BS cross-section (rounded walls, not polygonal);

13. Conclusions:
A re-design of the double-slot beam screen has been carried out: it now has a polygonal shape (octagonal with rounded corners), and optimized pumping slots and reinforcing ribs positions; the profile of the tip of the “deflector” has also been modified (not discussed here);
A vertically-displaced beam at full power sending its SR fan directly onto the internal wall of the BS does not create an anomalous temperature distribution (C. Garion);
Increasing the vertical height of the two symmetric longitudinal slots results in a better trapping of the SR fan and the related SR-induced desorption (~20% improvement between 2.28 mm (nominal) and 5.0 mm); (“nominal”== as designed so far);
A vertically-displaced beam (for the 2.28 mm slot, old BS
cross-section) results in higher SR-induced gas densities,
~ 2.8x higher for a 5 mm V offset with respect to a
perfectly aligned orbit;
The effect on the impedance due to an increased slot size
must be calculated/estimated;