3.9 GHz Harmonic Linearizer

3.9 GHz Harmonic Linearizer

Elvin HarmsLCLS-II Accelerator Physics Meeting9 April 2014

Introduction

2

• ACC39 at FLASH• Proposal for LCLS-II LH• Parameters• Assumptions• Summary

Harms, LCLS-II Accelerator Physics 9 Apr 2014

The Linearizer module - What is it, What does it do?

• The 3.9 GHz module, ACC39, is installed in the DESY FLASH injector just after the 1.3GHz ACC1 (first) cryo module.

• It is used in conjunction with ACC1 in order linearize the bunch energy vs. time over the bunch length.

• This in turn makes “bunch compression” to very short bunches with high peak currents more efficient, or a more controlled longer bunch charge distribution.

33 Harms, LCLS-II Accelerator Physics 9 Apr 2014

Bunch Compression with 3.9 GHz Module

4

After acceleration w/o After compression w/o

With 3rd harmonic From P Piot

Short bunch

Flat bunch ~250 fs

From M Dohlus et al


Test Results

5

• Entire cryomodule was first tested at DESY in their CryoModule Test Bench (CMTB) prior to installation in FLASH– Successful assembly– Successful transatlantic

shipment– Some rework at DESY

• Longitudinal realignment• Instrumentation

terminations


ACC39 Performance

6

• Set up for lasing has gone from expert tune up (shifts) to nearly immediate after turn-on


ACC39 Performance

7

• ACC39 routinely operates at 18.9 to 19.7 MV/m• Capable of operation at 22 MV/m

– Limitation set by thermal interlocks – concern about compromising HOM’s on Cavities 3 & 5 (trimmed 2-post style)

• Amplitude stability ≤ 2x10-5 pulse-to-pulse• Phase stability ≤ 0.003° pulse-to-pulse


Experiences & Lessons Learned (and applied)

• New infrastructure had to be built– Commissioning in parallel with qualifying components

• Vertical test stand• Horizontal Test Stand• Coupler Conditioning• Clean rooms• …

• HOM redesign– Not simply a scaled version– Modeling and prototyping important– Robust design is the end result

• HOM coupler antennae– Prototyping and testing is vital before placing order

• Engineering Notes & Operational Readiness Clearance• Transatlantic Transport of a complete cryomodule• Welding certification

– Especially Helium vessels• ‘New’ technologies explored and applied


Looking Ahead – CW Operation

• Heat Load/Cryogenic piping– Limit total cryomodule heat load to <250 watts– Dynamic Heat Load needs to be revisited

• Estimated heat load ~15 watts/cavity @ 15 MV/m• Input coupler power capability/cooling

– Current design should be good to ~2kW cw, but needs to be re-analyzed

• Studies are possible using coupler stand at A0 with a cw amplifier

– Is a variable coupler necessary rather than current ‘fixed’ design


Looking Ahead – CW Operation (2)

• HOM coupler and feedthrough capability– Current (single-post) ‘Formteil’ design

• multi-pacting lower bound ~25 MV/m – Feedthrough – adequate cooling?– Simulation needed to look at modes (THz?) present in the

ends• Absorbing material needed in beam pipe?• Preliminary simulations begun

• Microphonics/Fast tuning– Current tuner is slow (blade) tuner only– Cavities are already fairly stiff – resistant to microphonics/LFD– Consider addition of fast/piezo’s or passive microphonics

compensation


Looking Ahead – CW Operation (3)

• Cavity Global thermal limit is estimated to be of order 25 MV/m

• Alignment– Offset = HOM power considerations– Tilt = transverse emittance implications

• Other issues to consider– Beam diagnostics, especially internal to module– Quads, dipoles– Cryogenic and vacuum interconnectability to adjacent

components– Standalone vs. Integrated module


12 E. Harms APT Seminar 14 January 2014

LCLS-II Layout

P. Emma, L. Wang, C. PapadopoulosM. Ross



Proposed Parameter Set


• Cryomodule– 8 cavities per Cryomodule with 2 Cryomodules– Total heat load ~200 W / cryomodule– No magnet, corrector, or BPM’s in 3.9GHz CM’s– Magnets/Quad/BPM will be downstream in warm section– Cavities spaced for Q0 > 2e9– Beam pipe tapering from 1.3 GHz CM to 3.9 GHz CM– Length of interconnection (distance between CM’s) is free

• no requirements in term of wavelength.– Larger diameter 2-phase pipe/nozzle akin to 1.3 GHz

• will provide additional cooling than existing design– Cryo interface will be CM2/3 upstream and 'standard' feedbox immediately downstream– Design of cryomodule will strive to bring all signal cables out on the aisle side– Cryomodule will be cooled with the proposed 2nd cryoplant capable of ~1kW at 2K

along with the other upstream (non-main Linac) CM’s– Cavity-to cavity distance is short (as in FLASH)– HOM (beam pipe) absorber likely placed between two 3.9 GHz CM’s


3.9 GHz Section Assumptions - 1


• Cavities– Capable of 14.5 MV/m cw– Power dissipation ~17 W / cavity, maximum HOM power ~13 W / CM or 1.6 W / cavity

• (>50% will dissipated at 70K or RT)– Blade tuner design with 3 piezos incorporated.– HOM coupler signals may be used for beam offset measurements– Consider modest Q0 improvement development program (N2 doping), but low priority

and is not project critical• 1- and 9-cell cavities are available for processing & testing




• Couplers– Cavity orientation for placement downstream and every second rotated by 180°– Dig tunnel wall if necessary to make room for coupler clearance

• “not out of the question”– Waveguide from back side of CM will be pre-assembled and extend to top of CM for

connection– Two options for klystron to CM feed: Waveguide or coax

• WG option is preferable• coax is lossier, possible center conductor heating

– Coupler will require modifications for CW regime– Fixed coupling (Q~1.e7) looks OK.





Coupler Orientation Options

Start hereGoal

1.30 GHz

Fermilab Cryomodule Layout:

FLASH Cryomodule Layout:

XFEL Cryomodule Layout:

Arun Saini, N. Solyak



Coupler Orientation Summary

Start hereGoal

1.30 GHz

• Beam optics through harmonic section is studied.• Fermilab configuration• XFEL Configuration• ILC type cryomodule

• Coupler RF kick results in significant emittance growth.• Local compensation of coupler kick is possible and emittance dilution

can be minimized using different orientations of alternative cavity.• XFEL configuration provides best solution in terms of emittance compensation.

Arun Saini, N. Solyak


• Testing– Test cavities in VTS and HTS (subset)

• maybe ANL/Ostroumov team will do horizontal testing– Cryomodules will be cooled down and tested prior to transport to

SLAC– 3 kW cw 3.9 GHz in hand – needs to be made operational – 3.9 GHz Infrastructure exists (or can be modified) to do most testing

• cavities• couplers• cryomodules




• Actions– Will try to start from XFEL CM design --> (Elvin and Tom)– May want to run cavities in pairs to compensate coupler kicks - not

clear that this is necessary (Arun to evaluate)– Andrei is evaluating wakefields but expect them to be small– Need to specify alignment tolerances (Arun to evaluate)– Need to test new 3.9 GHz FPC in CW operation --> (need schedule)– In addition to testing FPC new design, task force needs to be formed

to look at and review design

• Key Dates– 1 June 2014: FRS submitted– 6 October 2014: Design Review


3.9 GHz Section Actions & Dates


• 3.9 GHz (pulsed) operating at FLASH• XFEL building a similar module• Technology can be applied for LCLS-II• Issues related to cw operation identified - manageable• Parameter list compiled – needs to be verified• List of assumptions and actions for LCLS-II LH compiled• Simulation/modeling work begun

– Coupler design and orientation• Effort needs further formalization• Do as much work as possible now

Summary


Thanks


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3.9 GHz Harmonic Linearizer