Hans gave an introduction to animate the discussion:
The present nominal CLIC parameters set is no longer compatible with our present knowledge of limitations. Fortunately, it seems that solutions can be found which are compatible with these limitations and do not compromise the performance.
An important ingredient is the new accelerating structure design (HDS type). It allows for a reduction of the bunch spacing by a factor 2. This requires modifications in the injector chain to deal with the higher bunch repetition rate. A list of possible combinations of RF frequencies in the CLIC complex for this new bunch spacing was shown.

Hans proposed the approach of two parameter lists for 30 GHz and for a lower frequency, followed by checks of subsystem compatibility and a comparison of the lists.

This triggered a general discussion on fixed or flexible assumptions on the target parameters. The result was that the parameters should be chosen for the best performance with a subsequent sensitivity analysis to assure a safe design.

The following points were discussed:

• The present structure design leads to the bunch charge q_b=3*10⁹ and defines the bunch length σ_z.
• The final focus (FF) design can be maintained. The crab crossing cavity frequency has to be changed which could increase the kink instability.
• It is favourable to have shorter pulse length to reduce pulsed surface heating (√t dependence).
  The bunch spacing can be reduced to 1/2 the present one. The pulse length should be reduced to 1/2 the present value but not less in order to avoid problems with the short length of the first combiner ring.
• From the possible structure designs proposed by Alexej a structure can be chosen which is compatible with an RF pulse length of 65 ns.
• The damping ring needs more realistic parameters. The horizontal emittance ε_H is 20% larger than the target value, the vertical one ε_V is 8 nm (target 3 nm), the longitudinal one slightly shorter than target. This should be taken as the new parameters. Possible problems with the presence of a 3 GHz RF cavity in the ring have to be checked.
• Using a set of more realistic damping ring parameters, the new bunch charge and bunch length values, simulations from the damping ring to the IP have to be performed to determine the luminosity per bunch crossing.
• Using the results from these simulations, the repetition rate has to be chosen as the multiple of 50 Hz that gives the luminosity closest to the present design.
• A draft parameter list will be discussed in a CLIC meeting by mid February, assuming a first iteration of the exercise described above is completed then.

A draft parameter list will be set up on the web, accessible from the CLIC Meeting pages.