Discussion on the TDI impedance specifications

1. Discussion on the TDI impedance specifications Grudiev 25/06/2012 Impedance meeting

2. Part on the thermal loading • Resistive losses. • With the present design the resistive heat load for the jaws in the injection position is assumed to be about 100 W per jaw [10]. This needs careful checking in simulation, including also sensitivity analysis e.g. for surface coating degradation. • Trapped RF modes. • The heat load from trapped RF modes in the TDI tank must be taken into account. The heat load will depend on the mechanical design and beam structure. This load is always present, even when the jaws are retracted. Detailed simulations must be made to estimate the loads, and in addition adequate margins based on experience to date taken into account in the cooling and mechanical design.

3. Part on the impedance • IMPEDANCE • The vacuum elements must have a low electrical resistivity in order to have low transverse machine impedance. The surface resistance Rs of the material used for the beam shielding and absorber materials surrounding the LHC circulating beam should be smaller than 3.9 10-11●d3 Ohm, where d (mm) is the chamber inner diameter. A good electrical contact between the absorber blocks and the tank should be envisaged. Rs[Ohm] = 3.9e-11 * d[mm]^3; Take an example: small gap = 10mm => Rs = 1 /(sigma * thickness) = 4e-8 Ohm For Cu: sigma = 6e+7 1/Ohmm , thickness = 0.4 m ! Is not reasonable for small gaps ! Should we keep it ??? Take an example: big gap = 200mm => Rs = 1 /(sigma * thickness) = 5e-4 Ohm For Cu: sigma = 6e+7 1/Ohmm , thickness = 0.03 mm Is becomes to be reasonable for big gaps

4. Part on the beam screen • Beam shielding • The vacuum chamber or beam-shield geometry for the LHC circulating beam should be smooth to minimise beam–induced wake-fields. The surface roughness Rt, or any topology on the inner surface of the vacuum chambers, such as a saw-tooth pattern to lower both the photo desorption yield and photoelectron yield, must be < 200 um. All cavities, however short, should be shielded whenever feasible and the angle of transition between different chamber cross-sections should not exceed 15º. • Pumping slots with a surface area of up to 20% of the pumping shields are acceptable in terms of longitudinal and transverse impedance provided the slots have rounded corners and their major axis is in the beam direction. • Bellows, required for thermal expansion compensation and alignment, must be shielded with an impedance of < 0.1 mOhm. The contact resistance between chambers should be < 100 uOhm. • Beam shields [13] should not interfere with the aperture requirements. • Add something about the shielding being able to evacuate heat where needed. [13] I. Collins et al., Beam screens for LHC arc magnets, Functional Specification, EDMS LHC-VSS-ES-0001.00 rev 0.2, 2002-02-01. https://edms.cern.ch/document/107716/1.0 LHC DR 5.2.2 long. Resistance of shielded bellow: R <100uOhm 5.2.3 geom. BB imp. of shielded below: Z/n <10uOhm