Review of the rf working group of the ICFA Mini Workshop on Novel Accelerators and Colliders which was associated with the Bob Siemann Memorial Symposium W. Wuensch CLIC meeting

Objective of the workshop Take the advanced acceleration techniques that are out there - plasma wakefield, laser plasma, dielectic wakefield, dielectric laser – and integrate them into an accelerator application, in this case a linear collider. Specifically, the end result was to produce parameter lists for the different concepts. Then have critical subsystem types – injectors and beam delivery – make reality checks. Objective applied to the rf working group We have spent years making linear collider parameters and have regular reality checks (ha ha!) of our concept. So compared to most of the other participants we were boring. The next seven slides are straight from the working group summary that I presented. BUT we had a second, more private, agenda - have our annual structure collaboration meeting. I will show some highlight slides (non CERN speakers only) after the working group summary.

Working Group 1: Microwave Acceleration Summary 10 July 2009

The goal for the first three groups is to: (1)develop self-consistent sets of parameters aimed at a 1 TeV collider with 2e34 total luminosity (an initial version of these should be presented at the beginning of the workshop), (2) list the critical R&D on the acceleration technology and the implied beam generation and focusing systems that are needed for the technology, (3) consider the fundamental limits of the technology and describe the impact of approaching these, (4) consider how new concepts for beam generation and focusing could have a major impact on the designs. Mandate

In addition we had a self-imposed mandate to cover our international X-band structure collaboration since this is the time of year for our annual meeting. We cover our near-term R&D which targets 100 MV/m and which is already very ambitious.

1. Develop self-consistent sets of parameters aimed at a 1 TeV collider with 2e34 total luminosity (an initial version of these should be presented at the beginning of the workshop), NLC/JLC and in CLIC parameter sets are already extremely well developed and studied. NLC/JLC were ‘conservative’. CLIC parameters are an extension made possible by two- beam power generation and are already pushing the limits of existing technology. We believe that the existing parameters are quite close to the optimum – rf frequency, bunch charge, spacing etc. – given the current state of technology.

2. List the critical R&D on the acceleration technology and the implied beam generation and focusing systems that are needed for the technology. A main goal is to show a efficient, HOM damped accelerating structure running at 100 MV/m at a low, around /m, breakdown probability per pulse. We covered this in detail in the ‘structure collaboration’ part of our meeting. Drive beam generation is addressed by CTF3 although a facility with parameters with parameters closer to nominal is being considered. High efficiency power sources need to be developed. Additional critical path items include micron precision instrumentation and alignment and nanometer stabilization of accelerator components. The key advances for the acceleration technology would be an increase in the rf to beam efficiency – this makes a collider more politically palatable and opens the possibility for other power source architectures with more efficient rf power sources.

Consider the fundamental limits of the technology and describe the impact of approaching these. 100 MV/m loaded gradient, roughly 80 MV/m real estate gradient, is still the preferred nominal value. We start to understand rather well the physical limits to gradient. This gradient is also close to the cost and efficiency optimum. The development of an accelerating structure with significantly higher rf to beam efficiency (35% range up over 70%!) would reduce power consumption and cost rather than shift the gradient higher. This could be through rf design or new materials.

4. Consider how new concepts for beam generation and focusing could have a major impact on the designs. The specific proposal we considered with the collimation and final focus group was to reduce the bunch charge by a factor 100. This would direct us towards filling every bucket to regain some of the average beam current. This might allow a solution with only light HOM damping. Still the changes to the rf system which could identify to maintain the rf to beam efficiency did not keep up with the reduction in current. As a consequence power consumption was reduced only about tens of percent rather than an order of magnitude. In addition the subsystems like alignment and stabilization become even more demanding.

Zenghai Li July 8, 2009 Energy of Captured Dark Current vs Location Electrons emitted upstream are accelerated to higher energy (monitored at output end). Electron energy as function of emission location. Eacc=97MV/m. Higher cell number indicates downstream location Cell # 1 20 Simulation

Zenghai Li July 8, 2009 Dark Current Spectrum Comparison “Certain” collimation of beampipe on dark current is considered in simulation data. More detailed analysis Needed. Spectrum from Track3P simulation, 97MV/m gradient. Measured dark current energy spectrum at downstream (need to scale by 1/(pc)

Slightly simplified geometry - GdfidL results courtesy CERN Benchmarking: T3P vs. GdfidL T3P (p=2) GdfidL (Finite Difference) GdfidL results show similar differences as p=1 T3P results

WITH ARC IN LINAC RF Source Power Bridge Load Power Linac Input Power Linac Reflected Power NORMAL LINAC OPERATION “Design Features and Initial RF Performance of a Gradient Hardened 17 GHz TW Linac Structure,” in Advanced Accelerator Concepts, AIP Conf. Proc., No. 1086, pp , H AIMSON R ESEARCH C ORPORATION 13 July 2009 Showing that when an Arc Occurs in the Linac, the Linac Input Power (blue) is Rapidly Truncated and, for the Remaining Portion of the Klystron RF Pulse the Bridge Input Power (red) is Automatically Directed into the Bridge Load (green). Thus, the Linac Power Amplifying Bridge Assists in Automatically Protecting both the RF Source and the High Gradient Linac Structure.

26 MW 32.5 MW 21.4 MW 14 H AIMSON R ESEARCH C ORPORATION Power Distribution to Achieve an Unloaded Accelerating Gradient of 108 MV/m An MHz Dual Resonant Ring System for High Gradient Testing CLIC/KEK/SLAC T18 Structures July 2009

15 Input Power Reflected Power Peak Pulse Heating Breakdown Study with Constant Gradient but Different Pulse Heating from the Pre-Fill ‘Warm-up’ Faya Wang

16 Breakdown Rate for Fixed Gradient Faya Wang

17 Comparison of current BDR rate (blue circle) with the rate curves from the First SLAC T18 structure at different processing times T18_SLAC_2 Faya Wang

Breakdown rate versus Eacc Exponential fit SLAC Workshop Toshi Higo

Valery Dolgashev

Nextef Configuration KT-1 X-band KT-2 C-band Shuji Matsumoto