Tertiary Collimators

AG, Follow-up of TCTV Collimator Impedance, RLC 17.03.2006

AG, Origin of TCLIA/TCTV Broadband impedance, RLC 10.03.2006

EM and AG, Tertiary Collimator Chamber, RLC 28.10.2005

EM and AG, Tertiary Collimator Chamber 2, RLC 04.11.2005

AG, TCTV Transverse Impedance, RLC 10.02.2005

EM, Effect of TCTV Transverse Impedance, RLC 10.02.2005

Preliminary conclusion about the minimum gap of the LHC tertiary collimators that we can accept from the beam coupling impedance point of view (Francesco Ruggiero's email to Ralph Assmann, after RLC meeting on 17.03.2005): assuming 4 TCTV + 4 TCTH collimators per beam => minimum full gap ~30 mm.
This conclusion is based on simulations by Alexej Grudiev for a single TCTV collimator at IR2 with 180 mm x 70 mm elliptical beam pipe: trapped modes are a concern and have to be damped by removing the RF contacts and locating appropriate ferrites in the external vacuum tank. However the transverse broad-band impedance for a 3 mm collimator gap is about 600 kOhm/m, i.e. more than 20% of the total LHC impedance budget! The impedance budget would increase by almost 200% with 8 such collimators, while it is reasonable to expect an increase below 10% from a single type of well-engineered impedance source. Increasing the full gap to 12 mm would reduce the broad-band impedance by a factor 10, while a full gap of 30 mm gives a reduction by a factor 100: at this stage, the impedance corresponding to a 30 mm gap gives us a sufficient safety margin and we think it should be your design target.
If one insists with the TCT concept as it is proposed now, we will need to compute the impedance for each TCTV and TCTH collimator, and properly weight it with the local betatron function, which can be ~10 times larger than the ring average: I expect that the final conclusion will be the same. As a general comment, it is a bad idea to install tertiary collimators at locations where the vacuum chamber has large transverse dimensions.