Accelerating structure and PETS baseline parameters and layouts - selected issues W. Wuensch

1.Accelerating structure assembly 2.Pulsed ΔT in accelerating structure 3.Beam-loading compensation 4.Reacting to breakdown 5.On/off/ramp mechanism 6.Dynamic vacuum 7.Refining design and 10% parameter consistency 8.rf instrumentation Outline

Accelerating structure assembly All the (recent) successful X-band test accelerating structures so far have been °C H 2 bonded pure-copper disks. Validated alternatives will not occur until after the CDR. This fixes our baseline and has important consequences. Influences: 1.Lower tolerable level of pulsed surface heating for structure design. 2.Module layout. Structures are soft so supports, vacuum system etc. become more difficult to design for microns. 3.Expensive manufacturing technique, so increases cost.

Pulsed ΔT in accelerating structure 1 The current value of pulsed surface heating temperature rise in the CLIC nominal structure/parameters is 53 °C (for symmetrically rounded cells, 62 °C for singly rounded) This is above the pure fatigue limit of around 35 °C for soft copper we have considered in the past and which was confirmed in tests by Sami and Samuli. We used to evoke alternative materials to get around the discrepancy but we have a soft copper baseline now. However, the influence of surface damage on Q in SLAC tests less than expected. However, there are indications of a possible influence of ΔT directly on breakdown rate. We begin to see the connection between the work of Flyura, measurements of Antoinne, Valery, Faya, Chris, effects seen in NLC/JLC, S c etc. However, the TD18 at SLAC is now running at 66 °C and (news from Wednesday evening, structure still conditioning).

Pulsed ΔT in accelerating structure 2 We may eventually see stuff happen in the downstream cells of the TD18s which have a peak ΔT of 87 °C at 100 MV/m (equivalent loaded with no loading). T18s had 23 °C at 100 MV/m. TD24’s will have 62 °C. Out of all this we will refine our ΔT specification. The relevant data will flow in during the coming days/weeks/months now that high-power testing of damped structures has begun. Influences: 1.Nothing for now. 2.RF design – Optimum structure would change with ΔT spec. We continue to pursue alternative designs (DDS and choke) to waveguide damping in case the acceptable ΔT turns out to be really low. 3.The overall gradient/efficiency/structure length combination.

Beam-loading compensation We are now developing the computational tools to calculate beam loading with sufficient accuracy. We need beam loading compensation to the per-mil level, including a phase chirp in the transient, so the calculations need to be accurate to at least an order of magnitude better than that. Transients, bandwidths all included. Not so easy… Synthesizing the necessary transient with the drive beam is not obvious. We hope to make significant progress on this subject over the coming year. Influences: 1.Luminosity… 2.But nothing for now because we don’t have solid enough answers yet.

Reacting to breakdowns We do not yet know exactly how the two-beam system will behave following a breakdown somewhere in the system. Do structures (both PETS and accelerating) calm down spontaneously – what is the correlation between successive breakdowns and between structures? When do we need to reduce power and by how much? How quickly, pulses (collateral damage) and time (vacuum), can we ramp the power back up if we need to ? Dedicated tests and measurement campaigns will occur next year in the various high-power test areas. The TBTS would be the ultimate test but will be difficult to run in the correct breakdown rate and power combination regime. Influences: 1.On/off/ramp mechanism. 2.Machine operation and protection scenarios.

On/off/ramp mechanism For the worst case, we target an on/off mechanism which provides variable power generation. A baseline design has been developed and critical components will be tested next year. This includes installing the full mechanism in a two-beam configuration towards the end of the year. One delicate point is the phase of fields in the system when it is not completely on. Phase excursions and the effect on the drive and main beam are under study. One tremendous advantage of the new technique is that it can be set to produce above normal power (the thing works by creating a tuned/de-tuned resonator) thus will allow in- situ re-processing. We never included re-processing overhead (drive beam current) in the drive beam parameters before. Influences 1.Nothing for the moment since we continue to assume we can find a solution. Space reservations are included in the module.

Dynamic vacuum Basic question from beam dynamics: What is the total number of atoms/molecules/ions the main beam sees as it passes through the linac aperture with the rf on? Roughly this translates into the measurement question – How do we determine the effect of a duty cycle driving term on a level vacuum in a cm 3 volume? We do not know if we can make a direct measurement, classical gauges appear to be insufficient, so we complement this with a combined experimental/simulation approach. Dynamical vacuum is probably driven by field emitted electron current induced desorption of atoms absorbed when rf is off. We hope for significant progress over the coming year. Influences: 1.Feasibility. 2.Not the overall layout of the vacuum system. Pumps and pipes only influence static vacuum. 3.Perhaps the preparation requirements of components before installation plus handling and air exposure times. Try to minimize junk stored in the system.

Refining the design and a 10% parameter consistency The current generation of test structures have long mode-launcher couplers. In order to maximize the real-estate gradient, we will need to implement ‘compact- couplers’. We have designs and this does not seem to be a fundamental problem. It is reasonable to maintain the baseline with compact couplers even though a high- power demonstration with them will not made until after the CDR. There are a number of small changes (double rounding vs asymmetrical disks) which have accumulated. Depending on the level of consistency of parameters required, some changes might have to be implemented. Influences: 1.If we are happy with 10% consistency, we don’t need to change anything.

rf instrumentation Wakefield monitors are necessary for some structures, therefore we develop them. New idea for implementation almost ready for consideration for baseline. The fraction of structures which should be equipped with them is a beam dynamics, module assembly and cost issue. Wakefield monitoring in general terms is a crucial tool for understanding intra and inter structure alignment. Drive beam BPMS (DBBPMS?) have not been developed yet. Work is now ongoing ( but we’re short on manpower) to define the other rf instrumentation requirements – breakdown detection, on/off control and monitoring, beam loading and phase monitoring, temperature feedback and stabilization etc. Influences: 1.Module, tunnel layouts 2.Cost