CW injector design studies at PITZ for European X-FEL Shankar Lal, - - PowerPoint PPT Presentation

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CW injector design studies at PITZ for European X-FEL Shankar Lal, - - PowerPoint PPT Presentation

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CW injector design studies at PITZ for European X-FEL Shankar Lal, Guan Shu, Hamed Shaker, Houjun Qian and Frank Stephan Photo Injector Test facility at DESY in Zeuthen (PITZ) Outline FLASH overview European X-FEL overview Photo

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CW injector design studies at PITZ for European X-FEL

Shankar Lal, Guan Shu, Hamed Shaker, Houjun Qian and Frank Stephan Photo Injector Test facility at DESY in Zeuthen (PITZ)

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Outline

  • FLASH overview
  • European X-FEL overview
  • Photo Injector Test facility at DESY in Zeuthen (PITZ) overview
  • CW injector for European X-FEL upgrade
  • CW injector beam dynamics simulations
  • CW - NC- VHF gun RF design
  • CW buncher RF design
  • Summery
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Free-Electron LASer in Hamburg (FLASH) overview

  • Constructed in the early 2000s: UVV-FEL @ TTF2
  • Prototype for European X-FEL
  • User operation of FLASH 1: Summer 2005 (world 1st X-FEL)
  • User operation of FLASH 2: April 2016

Ref: (1) https://flash.desy.de/, (2) http://accelconf.web.cern.ch/AccelConf/fel2017/papers/mod02.pdf

DESY Hamburg

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European X-FEL overview

  • SCRF accelerator :1.7 km
  • Undulators : ~ 200 m
  • Beam transport & instruments ~ 1km
  • Soft to hard x-ray: 0.5-0.05 nm (0.25-25 keV)
  • European XFEL TDR : 2002
  • Construction work started :2009
  • First lasing SASE1: May 2017 ( 0.9nm)
  • First user run started : Sep 2017

Ref : (1) http://xfel.desy.de, (2) Winfied Decking, IPAC2017, (3) Matthias Scholz, IPAC2018

DESY Hamburg

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PITZ: Photocathode gun test facility for FLASH/European XFEL

The Photo Injector Test Facility at DESY, Zeuthen (PITZ) focuses on development, testing and

  • ptimization of high brightness electron sources for SASE FELs (FLASH & European XFEL)
  • Test-bed for injectors for FLASH and European XFEL
  • Fundamental research in photo injector physics: cavities, cathode, photoemission etc.
  • Application of high brightness beam: plasma acceleration, THz and UED etc.

Ref : (1) http://pitz.desy.de/, (2) M. Krasilnikov et al. PRSTAB15, 100701 (2012)

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Electron source for FLASH/European X-FEL

  • L-band (1.3 GHz) Photocathode RF gun
  • 1.6 cell, normal conducting
  • Electric field at cathode : ~60 MV/m
  • Beam energy : ~6.5 MeV/c
  • Pulse length : 650 us (micro bunch spacing 220 ns)
  • Bunch length : up to 20ps (variable using laser)
  • Cathode : Cs2Te (QE~5-10%)
  • Peak charge: up to 5nC/bunch
  • Average power :6.5 MW x 650us x 10Hz :~42 kW

Ref : (1) http://pitz.desy.de/, (2) M. Krasilnikov et al. PRSTAB15, 100701 (2012) Laser Electron RF Gun RF Feed Main Solenoid Bucking Solenoid

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European X-FEL : Present and Future operation mode

Electron source Present operation Possible future upgrade: Need CW gun for operation

NC pulse gun developed at PITZ

Electron source

NC CW RF gun is under design study at PITZ as a backup solution SCRF gun R&D is ongoing at DESY in Hamburg

Up to 10 µA Up to 20 µA Up to 27 µA

Time [sec]

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European X-FEL : CW gun for future upgrade

Design parameters CW SRF Gun for EXFEL Pulsed NC Gun for EXFEL APEX-1 CW Gun for LCLS II (NC) Duty cycle [%] 100 ~0.65 100 Operation frequency [MHz] 1300 1300 186 RF input power [kW] 0.75 ~ 42 ~ 100 Cathode gradient [MV/m] 40 60 19.5 Beam energy at gun exit [MeV] 3 6.1 0.75 Advantages:

  • Advanced technology, SRF→ intrinsic CW operation
  • Potential for high gradients & high beam energy,

better beam performance Challenges:

  • Integration of cathode in a SC cavity, i.e. cathode

exchange, cathode lifetime, multipacting, cavity contamination etc. SRF GUN Advantages:

  • Mature RF and mechanical technology
  • Easy cathode exchange
  • APEX experiments indicate high beam performance for

XFEL, adopted by LCLS-II. Challenges:

  • Cathode gradient and beam energy limitation
  • Operation stability with significant power loss

NC CW RF GUN

Ref: (1) Elmar Vogel, and (2) Guan Shu, Meeting of Hamburg Alliance New Beams and Accelerators , DESY Hamburg, September 2018

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DESY VHF gun : Goals and Constraints

  • 1. Gun resonant frequency
  • 187 MHz (APEX) (1300/7) is not compatible with XFEL timing system.
  • 217 MHz (1300/6) and 162 MHz (1300/8) are candidates.
  • 2. Cathode gradient: up to 30 MV/m
  • Higher gradient improves beam brightness
  • Breakdown limits: APEX gun tested ~1.9 Kilp w/o breakdown in CW mode

 2*Kilp @217 MHz is ~30.4 MV/m

  • 3. Gun power :<100 kW (demonstrated in APEX gun test)
  • 4. Higher beam voltage: >750 kV
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Inspired by ERL injector design: DC gun  VHF RF gun

  • Quasi DC gun beam dynamics:
  • Beam bunch length: ~60ps, <4 deg
  • Photoemission phase: ~90 degree
  • Low power dissipation:
  • Lower thermal power density
  • 90 kW, <30 W/cm2 , <70 deg C
  • Ultra high vacuum
  • Vacuum slots around cavity wall
  • Vacuum ~10-10 -10-11 torr
  • Size compared to Pillbox cavity
  • Compact cavity size
  • Enhanced cathode field

VHF gun concept: LBNL

Ref : F. Sannibale et al. Phys. Rev. ST Accel. Beams 15, 103501 (2012)

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DESY VHF gun: RF simulation results

Parameter APEX EXFEL Unit Mode 1 Mode 2 Operation mode CW CW CW Frequency 186 217 217 MHz Voltage 750 864 690 kV Cathode gradient 19.47 30.0 24.0 MV/m Intrinsic quality factor, Q0 30900 32160 32160 Shunt impedance 6.5 7.5 7.5 MΩ Nominal RF power for Q0 87.5 100 64 kW Stored energy 2.3 2.4 1.5 J Maximum surface field 24.1 (1.7 Kilp.) 38.5 (2.5 kIilp) 30.0 (2.0 Kilp.) MV/m Maximum wall power density 25.0 35.2 22.5 W/cm2

Peak E Peak H E field H field

29 cm 68 cm

APEX gun

  • peration~750 kV

EXFEL gun

  • peration 690

kV~860 kV

No MP near operating point

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DESY VHF gun based injector: Beam dynamics simulations

  • Validating gun performance using LCLS-II injector 100 pC case

 Gun: Resonance frequency = 217 MHz, Ecathode= 30 MV/m, phase variable  Thermal emittance : 1 ~ 0.5 mm.mrad/mm  Laser temporal profile: flattop 60 ps with 2 ps edges, radially Gaussian truncation at 1-sigma, both variable

  • Injector layout

 1st solenoid position changed according to new gun geometry  Other elements position stay the same as LCLS-II Solenoids: focusing variable

  • Buncher: voltage increase from 240 kV to ~400 kV
  • Accelerating structure: TESLA cavity

 Cav 1 amplitude and phase variable

  • Injector optimization by ASTRA simulations, driven by genetic optimizer

Optimize emittance and high order energy spread of 10 A solutions Genetic optimizer setup

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Beam dynamics simulations: Preliminary results

  • DESY VHF gun (mode 1) vs APEX gun, PITZ gun
  • Emittance
  • 30% reduction vs APEX gun, close to PITZ gun.
  • ~0.1 um emittance achieved with low thermal emittance.
  • H.O. energy spread
  • ~3 keV (1/3 of APEX gun) and close to PITZ gun.
  • DESY VHF gun (mode 2) vs APEX gun
  • Both transverse and longitudinal beam quality is similar

100 pC APEX DESY VHF PITZ gun Unit

Thermal 1 1 0.5 0.85 μm.rad/mm Ecath 20 30 30 60 MV/m Ipeak 15 10 11 4 A 100% ε (projected) 0.29 0.20 0.12 0.17 μm.rad 95% ε (projected) 0.21 0.15 0.09 0.11 μm.rad H.O. energy spread 9.6 2.4 2.7 3.7 keV

PITZ gun LBNL gun DESY VHF gun DESY VHF gun

Longitudinal phase space with 1st and 2nd order chirp correction

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Buncher design: Goals and Constraints

1300 MHz 1300 MHz 217/162 MHz

  • Frequency : 1.3 GHz (SCRF accelerating structure frequency)
  • Accelerating voltage : 400 kV ( beam dynamics optimization underway)
  • No of cells 2 or 3: Availability of space
  • Power dissipation < 5 kW/cell (demonstrated in APEX )
  • Configuration : one - 2 cell / two- 2 cell / one -3 cell (beam dynamics optimization underway)
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1300 MHz buncher : Literature survey

Ref: (1). V. Veshcherevich and S. Belomestnykh, “Buncher cavity for ERL”, PAC 2003; (2) T. Takahashi et al., “Development of a 1.3 GHz buncher cavity for the compact ERL”, IPAC 2014; (3). H. Qian et al., “Design of a 1.3 GHz two-cell buncher for APEX”, IPAC 2014

Parameters\ Laboratory Cornell/ Jlab :ERL KEK:ERL LBNL:APEX Geometry

  • No. of cells

1 1 2 𝑆𝑡ℎ = 𝑊2 𝑄

𝑑

4.2 5.33 7.8 Nominal Acc. Voltage (kV) 120 130 240 Power dissipation (kW) 3.42 3.17 7.4

Proposed PITZ/DESY design: KEK design (highest shunt impedance/cell ) with multiple cells

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1300 MHz pre-buncher: RF Design

Alternative designs

  • Higher mode separation increased to ~3 MHz
  • Shunt impedance: ~9 M (15% higher compare to LBNL)
  • Peak magnetic field shifted near beam pipes: easy for cooling

Option 1: TESLA shape with re-entrant at end

Electric field array plot Magnetic energy distribution

Option2 :Geometry similar to TESLA (SCRF) cavities

Electric field array plot Magnetic energy distribution

  • Higher mode separation increased to ~3 MHz
  • Shunt impedance: 7.7 M (similar to LBNL design)
  • Peak magnetic field shifted near beam pipes: easy for cooling
  • Shunt impedance: 9.9 M (25% higher compare to LBNL)
  • Power dissipation: 16 kW (8 kW/cell, higher compare to LBNL)

Practical issues

  • Small Mode separation (~ 1 MHz)
  • Maximum heat near inter-cell coupling iris: difficult to remove

Electric field array plot Magnetic energy distribution

Two-cell re-entrant type

Multipacting simulations results On- axis Electric field profile

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Three cell buncher design

TESLA (SCRF) cavities with re-entrant at end

Electric field array plot On- axis Electric field profile Magnetic field distribution

  • RF power ~13 kW@ 400 kV
  • Power dissipation <5 kW/cell (smaller compare to LBNL)
  • Multipacting study underway
  • Shunt impedance ~12 M
  • Mode separation (f-/2 ~ 3MHz)

Geometry similar to TESLA (SCRF) cavities

Electric field array plot On- axis Electric field profile Magnetic field distribution

  • RF power ~15 kW@ 400 kV
  • Power ~5 kW/cell (similar to LBNL design)
  • MP remedies are available
  • Shunt impedance ~10.7 M
  • Mode separation (f-/2 ~ 3MHz)
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Summary

  • A normal conducting CW injector system for Europen X-FEL upgrade is being

studied at PITZ, DESY.

  • Preliminary beam dynamics simulations predict beam parameters better then

APEX injector

  • RF design of 217 MHz VHF gun is carried-out.
  • RF design of 1300 MHz CW buncher with different geometries and number of

cells carried-out.

  • Beam dynamics simulation studies are underway to optimize the beam

parameters with lower RF requirements.

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Acknowledgments

I would like to thanks PITZ/DESY team for useful discussions and critical feedback. I would also like to thanks Dr. Valentin Paramonov from Institute for Nuclear Research of Russian Academy of Sciences, Moscow, Russia for his feedback on buncher design optimization.

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Thank you for your attention

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Backup slides

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Photo Injector Test Facility at DESY, Zeuthen (PITZ)

Ref : (1) http://pitz.desy.de/, (2) Frank Stephan, PITZ Collaboration Meeting, Zeuthen, December 2018

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DESY VHF gun design at PITZ DESY

Based upon APEX gun profile, nose area shows significant influence on cavity characteristics

Parameterized 2D gun cavity model

 Increase cathode gradient (20→30 MV/m)

  • A ↓, B ↓, H ↓

 Improve shunt impendence

  • A ↑, E ↑, H ↓, R8 ↓ ,θ1 ↑, θ2 ↑, θ3 ↑

 Decrease surface peak Electric field

  • A ↑, R8 ↑, θ1 ↓, θ3 ↓

Parameters scan → correlations

 Fix cavity frequency at 216.7 MHz, cathode gradient at 30 MV/m, trying to find a chain of dimensions to maximize shunt impendence and minimize surface E  CST MWS built-in optimizer, genetic algorithm

Optimization strategy:

LBNL VHF gun

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VHF gun : 3D RF simulations

Loop cooling channel

Vacuum chamber

  • 104 vacuum pumping slots
  • ~20 NEG modules (enable 10-9~10-11 Torr)
  • Stored energy ~ 9.7 × 10-8 J

RF input coupler

  • Symmetrically placed
  • Magnetic coupling
  • Coupling factor ∝ loop angle

(nominal 0.5 per coupler)

  • Loop require cooling

RF pickup

  • Qe = 5.15×1013
  • RF leakage ~ 0.06 mW

Cavity

  • Stored energy ~ 2.4 J
  • Power loss ~ 100 kW

Viewpoint

  • Inspection on cathode
  • Spare

port for laser incident

  • Qe = 1.33×1013
  • RF leakage ~ 0.25 mW

Rear plate view Front plate view

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Buncher: alternative designs

  • Geometry similar to TESLA (SCRF) cavities
  • Mode separation ~ 3MHz
  • Shunt impedance ~ 7.7 M

Electric field array plot for  mode On- axis Electric field profile Magnetic field distribution

  • Shunt impedance lower
  • MP remedies are available

Electric field array plot for  mode On- axis Electric field profile Magnetic energy distribution

  • Central geometry similar to TESLA (SCRF) cavities

with re-entrant at end

  • Peak magnetic field shifted near end pipes
  • Mode separation increased to ~3 MHz
  • Shunt impedance ~9.2 M

GR~ 0 at 3.5 MV/m