Phase 1a Prototype as model for injector

L0 layout + experimental plan + results to date

Ivan Bazarov

X-ray characteristics needed

• For a properly tuned undulator: x-ray phase space is a replica from electron bunch + convolution with the diffraction limit • ideally, one wants the phase space to be diffraction limited (i.e. full transverse coherence), e.g.   ,rms = l /4 p , or 0.1 Å for 8 keV x-rays (Cu K a ), or   n,rms =

0.1

m

m

normalized at 5 GeV Flux Brightness

Brilliance

ph/s/0.1%bw ph/s/mrad 2 /0.1%bw

ph/s/mm 2 /mrad 2 /0.1%bw

I.V. Bazarov, ERL review 03/09/07 April 30, 2020 2

Injector prototype beam goals

• Demonstrate efficacy of achieving thermal emittance at the end of the injector at a bunch charge of 77 pC/bunch or some large fraction thereof • Understand the limitations in the injector (both physics and technology) to allow for improved design in the future

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 3

Experimental plan: areas

I.

II.

Photocathode phenomena Space charge dominated regime III. Longitudinal phase space control IV. Emittance preservation in the merger V.

High average current phenomena VI. Achieving ultimate ‘tuned-up’ performance

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 4

R128 vs. L0

• Simple: gun & diagnostics line • Full phase space characterization capability after the gun • Temporal measurements with the deflecting cavity • Limited diagnostics after the gun (before the module) • Full interceptive diagnostics capabilities at 5-15 MeV

April 30, 2020

• Some full beam power diagnostics

I.V. Bazarov, ERL review 03/09/07 5

L0 layout: near the gun

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 6

L0 layout: 15 MeV straight-thru

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 7

April 30, 2020

L0 layout: merger & chicane

merger diagnostics chicane I.V. Bazarov, ERL review 03/09/07 8

Diagnostics overview

• Beam position resolution: 10 m m (spec) • Energy spread resolution: 10 –4 • Transverse beam profile resolution: 30 m m (viewscreens) 10 m m (slits) 30 m m (flying wire) • Angular spread resolution: 10 m rad • Pulse length (deflecting cavity&slits): 100 fs • RF phase angle: 0.5



Ability to take phase space snapshots of the beam, both transverse planes, and longitudinal phase space I.V. Bazarov, ERL review 03/09/07 April 30, 2020 9

Emittance measurement system

• no moving parts; fast DAQ • 10 mm precision slits • armor slit intercepts most of the beam • kW beam power handling

measured phase space April 30, 2020 I.V. Bazarov, ERL review 03/09/07 10

April 30, 2020

Deflecting cavity

• 100 fs time resolution (with slits) • Used in: – photoemission response meas.

– slice transverse emittance meas.

– longitudinal phase space mapping

I.V. Bazarov, ERL review 03/09/07 11

Flying wire

• 20 m/s flying carbon wire (can go faster) • Applicable with 0.6 MW of beam power • Two units, one in dispersive section to allow studies of long range wake fields

12 April 30, 2020 I.V. Bazarov, ERL review 03/09/07

April 30, 2020 coherently enhanced spectrum

THz radiation

• One of chicane dipole magnets to be used in the analysis of FIR radiation spectrum • Applicable with 0.6 MW of beam power • Provides the autocorrelation of the bunch profile • OTR foils for low beam power measurements

I.V. Bazarov, ERL review 03/09/07 13

Beam experiments

I. Photocathode phenomena – – Exp1. Thermal emittance (R128)

done

Exp2. Photoemission response time (R128)

2 weeks

II. Space charge regime – Exp3. Space charge limited extraction from the cathode (R128)

1 week

– Exp4. Effect of laser pulse shaping on emittance compensation (R128)

2 weeks

– Exp5. Phase space tomography of bunched beam (R128 & L0)

2 weeks R128 + 2 weeks L0

– Exp6. Benchmarking of space charge codes (R128 & L0)

1 week R128

– Exp7. Slice emittance studies (L0)

2 weeks I.V. Bazarov, ERL review 03/09/07 April 30, 2020 14

Beam experiments

III. Longitudinal phase space control – Exp8. Ballistic bunch compression (L0)

2 weeks

– Exp9. Longitudinal phase space mapping (L0)

2 weeks

IV. Emittance preservation in the merger – Exp10. Space charge induced emittance growth in dispersive sections (L0)

2 weeks

– Exp11. CSR effect (L0)

2 weeks April 30, 2020 I.V. Bazarov, ERL review 03/09/07 15

Beam experiments

V. High average current phenomena – Exp12. Ion effect (R128 & L0)

1 week R128 + 2 weeks L0

– Exp13. Long range wakefield effects (L0)

1 week

VI. Achieving ultimate ‘tuned-up’ performance – – Exp14. Orbit stability characterization and feedback (L0)

2 weeks

Exp15. Exploration of ‘multi-knobs’ and online optimization (L0)

3 weeks I.V. Bazarov, ERL review 03/09/07 April 30, 2020 16

Time need estimates

R128 L0 beam running time (everything is working the first try)

9 weeks 20 weeks

 2 (reality factor)

19 weeks 40 weeks

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 17

Physics limit of e-photoguns

Two main limiting mechanisms: • Phase space scrambling due to nonlinear space charge 3D Gaussian initial distribution Optimal initial distribution  n,x = 1.7 m m  n,x = 0.13 m m • • •

V gun kT



= 750 kV = 35 meV Optimum 3D shape

• Photocathode thermal emittance 

n

,

th

 

x

,

y kT



mc

2 Theoretical emittance min:

April 30, 2020 I.V. Bazarov, ERL review 03/09/07



n

[ mm mrad ]  4

E th

[ eV ]

q

[ nC ]

E cath

[ MV/m ]

18

April 30, 2020

GaAs

Exp1. Thermal emittance

GaAsP

• • • kT  =

121



8 meV

at

520 nm

or

0.49 mm-mrad

per

1 mm

rms GaAs still best overall perform.

I.V. Bazarov, ERL review 03/09/07 19

Exp2. Cathode time response

measured temporal response

GaAs

• Measured response time from GaAs and GaAsP at different wavelengths • GaAs response @ 520 nm on the order of a picosecond • Diffusion model correctly describes fast response and a slow tail

response to a 100 fs pulse expected temporal profile

diffusion model: fit to data 50% emission point

800 nm:

15 ps

520 nm:

0.83 ps 50 % 18 %

• Deflecting cavity measurement of temporal profile next month

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 20

Exp4. Laser shaping effect

• Effective means of laser shaping have been devised and tested • Beer-can distribution is the goal for Phase1a (a better shapes exist)

laser shape: where we are today

temporal spatial  x = 0.84 mm,  y = 0.72 mm -2.5

-2 -1.5

-1 -0.5

0 0.5

1 1.5

2 -2 -1 0 x (mm) 1 2 3

gaussian planning to be in few weeks from now flat-top April 30, 2020 I.V. Bazarov, ERL review 03/09/07 21

SOL1 = SOL2 = 3A

First space charge running

E-beam right after the gun (250 kV) and the solenoid

SOL1 = SOL2 = 4.5 A

measured well-defined halo

SOL1

5 4 3 2 1 0 -1 -2 -3 -4 -5 -5 simulated Astra Trace Space at VC1, SOL1 = SOL2 = 4A 0 x (mm) 5 25 0 20 15 10 5

EMS VC1

April 30, 2020

uniform cathode gaussian viewscreen longitudinal tail overfocused

I.V. Bazarov, ERL review 03/09/07

particles folding-over forms well-defined boundary

22

log scale

1 2 3

70 pC/bunch

4 5

1 2 3 4 5 

ny

smallest emittance

= 1.8



0.1 mm-mrad

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 23

Agreement with simulations

Good agreement with Astra prediction: 77 pC/bunch: about 2 mm-mrad

data astra

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 24

Exp6. Codes’ benchmarking

R128: gun & solenoid  L0: 11 MeV • Emittance right after the gun is

within 50%

of the final value • Establish the validity of space charge codes & high degree of emittance compensation in R128

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 25

Exp9. Long. phase space map.

• Combination of slits & deflecting cavity to allow detailed longitudinal phase space mapping • Temporal resolution 0.1 ps, energy resolution 10 –4 • Will be used in a variety of studies, e.g.

– ensuring small energy spread, a prerequisite for successful transport through the merger – optimizing compression scheme

April 30, 2020 I.V. Bazarov, ERL review 03/09/07

Ce:YAG at the end of C2

Energy

26

Exp11. CSR in the merger

D

x

,n,CSR  0.25 elegant m m 0.25

0.2

0.15

0.1

0.05

three 15-deg dipole merger

• EMS systems placed before and after the merger to isolate the CSR emittance growth • Phase space dilution studies as a function of varying charge and bunch length • Longer term possibilities – smaller bends, shielded chamber 0 0.6

0.65

0.7

0.75

0.8

0.85

0.9

bunch length after the injector [mm]

0.95

April 30, 2020

1

I.V. Bazarov, ERL review 03/09/07 27

Exp12. Ions

• Initial calculations show that running 100 mA CW will cause problems with safe beam dump operation • Full beam neutralization over 4 s at 10 –9 Torr • Possible approaches: – develop the average-current dependant optics to account for the full beam neutralization and slowly ramp up the current (test in R128) – introduce the ion gap, e.g. 6 m s every 60 ms (test in R128) – the ion gap will cause large RF transients, it won’t work in L0   Energy stored in the gun:

15.6 J

 Energy stored in a cavity:

0.5-5 J

 1% transient over 1.5 m s 1% transient over 0.1 m s – introduce clearing electrodes (non-trivial changes to the beamline, would rather avoid)

I.V. Bazarov, ERL review 03/09/07 April 30, 2020 28

gun through the dump

Nominal size at the dump 4  = 20 cm

DC beam in R128 (250 kV)

zoomed in Full neutralization assumed

• Ions ‘helping’ to have a small beam • 250 kV  25 kW over 4 cm diameter is probably safe on the dump • 0.6 MW will not be so forgiving!

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 29

L0 dump

• • Two extremely short focal-length quads near the dump blow up the beam by a factor of more than a hundred • Even with the raster, the spot size cannot be less than 8 cm rms at the dump plane. Ions will throw a monkey wrench into the optical setting.

• The optics will have to incorporate the ions to avoid the dump failure mechanism

Challenge

: we are essentially blind at 0.6 MW near the dump as far as the beam profile is concerned.

April 30, 2020 I.V. Bazarov, ERL review 03/09/07 30

Exp15. Multi-knobs & tune-up

• Virtual injector allows absolute control of parameters, real system with a dozen of sensitive parameters will not Pulse duration rms 21.5

Spot size rms Charge Solenoid1 Bmax 0.640 80    1.4 ps 0.057 mm 5.8 pC 0.491  0.010 kG Solenoid2 Bmax Cavity1 phase Cavity2 phase Cavity3-5 phase Buncher Emax Cavity1 Emax Cavity2 Emax Cavity3-5 Emax Q1_grad Q2_grad Q3_grad Q4_grad 0.532  -41.6  -31.9  -25.7  1.73  15.4  26.0  27.0  0.010 kG 1.7 deg 2.0 deg 2.0 deg 0.04 MV/m 0.3 MV/m 0.5 MV/m 0.5 MV/m -0.124  0.002 T/m 0.184  0.023  0.002 T/m -0.100  0.002 T/m 0.002 T/m 100 random seeds (outliers removed) ave( std(   x x ) = 1.04 ) = 0.52 m m m m ave(  y ) = 0.95 m m std(  y ) = 0.62 m m

I.V. Bazarov, ERL review 03/09/07 April 30, 2020 31

Possible strategies

• Should develop ad-hoc means to tune-up the nonlinear system for optimal performance • ‘Manual’ optimization using a calculated Hessian matrix of the beam emittance from the space charge codes:

H ij

  2

C



p i



p j

• Use SVD of the Hessian to form ‘multi-knobs’ that correspond to top few eigenvalues • Other potentials: use

online

direct search method (e.g. simplex) or a stochastic search (e.g. genetic algorithms). Analog computer evaluations will be limited to a few hundred at most.

I.V. Bazarov, ERL review 03/09/07 April 30, 2020 32

Summary

• Experimental plan outlined, both R128 and L0 parts are essential • Move to L0 once  n  0.5 mm-mrad demonstrated at the nominal bunch charge (77 pC) from the gun; premature move is advised-against • There are things we know we don’t know (e.g. ions), and there are things we don’t know we don’t know. We are concentrating on the former.

I.V. Bazarov, ERL review 03/09/07 April 30, 2020 33