Generation of intense THz pulses using ultra-short, high-brightness - - PowerPoint PPT Presentation

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Generation of intense THz pulses using ultra-short, high-brightness - - PowerPoint PPT Presentation

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Generation of intense THz pulses using ultra-short, high-brightness electron bunches Jom Luiten 8 Oct 2009 John Adams accelerator institute 1 Coherence & Quantum Technology (CQT) Willem Op t Root PhD student Technical support:

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8 Oct 2009 John Adams accelerator institute 1

Generation of intense THz pulses using ultra-short, high-brightness electron bunches

Jom Luiten

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8 Oct 2009 John Adams accelerator institute 2

Coherence & Quantum Technology (CQT)

Willem Op ‘t Root – PhD student Peter Smorenburg – PhD student Bas van der Geer – Pulsar Physics (GPT) Marieke de Loos – Pulsar Physics (GPT) Marnix van der Wiel – former group leader NL Foundation for Fundamental Research on Matter Technical support: Eddy Rietman Ad Kemper Harry van Doorn

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8 Oct 2009 John Adams accelerator institute 3

Part I: RF photogun

  • technology
  • ultra-short bunches

Outline

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8 Oct 2009 John Adams accelerator institute 4

Part I: RF photogun

  • technology
  • ultra-short bunches

Part II: THz generation

  • free-space CTR THz
  • THz plasmons on a wire

Outline

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8 Oct 2009 John Adams accelerator institute 5

Part I: RF photogun

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8 Oct 2009 John Adams accelerator institute 6

RF photoguns:

the brightest pulsed electron sources...

  • U = 3-5 MeV
  • Q = 0.1-1 nC
  • τ ≤ 1 ps
  • εn ≤ 1 mm·mrad

I = 0.1-1 kA

Injector X-FEL...

SLAC, DESY, ...

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8 Oct 2009 John Adams accelerator institute 7

RF photoguns:

the brightest pulsed electron sources...

...injector LWA...

  • U = 3-5 MeV
  • Q = 0.1-1 nC
  • τ ≤ 1 ps
  • εn ≤ 1 mm·mrad

I = 0.1-1 kA TU/e, Strathclyde, EuroLeap...

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8 Oct 2009 John Adams accelerator institute 8

RF photoguns:

the brightest pulsed electron sources...

...ultrafast electron diffraction...

  • U = 3-5 MeV
  • Q = 0.1-1 nC
  • τ ≤ 1 ps
  • εn ≤ 1 mm·mrad

I = 0.1-1 kA TU/e, UCLA, BNL, ...

200 nm Ti foil, Musumeci et al., UCLA

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8 Oct 2009 John Adams accelerator institute 9

RF photoguns:

the brightest pulsed electron sources...

... intense THz pulses.

  • U = 3-5 MeV
  • Q = 0.1-1 nC
  • τ ≤ 1 ps
  • εn ≤ 1 mm·mrad

I = 0.1-1 kA

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8 Oct 2009 John Adams accelerator institute 10

RF photoguns:

the brightest pulsed electron sources...

3 GHz (λ=10 cm) resonant cavity λ/2 Pulsed laser photoemission...

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8 Oct 2009 John Adams accelerator institute 11

RF photoguns:

the brightest pulsed electron sources...

3 GHz (λ=10 cm) resonant cavity λ/2 ...and RF acceleration. RF field strength ~100 MV/m, limited by vacuum breakdown

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8 Oct 2009 John Adams accelerator institute 12

RF photoguns:

the brightest pulsed electron sources...

3 GHz (λ=10 cm) resonant cavity λ/2 ...and RF acceleration. RF field strength ~100 MV/m, limited by vacuum breakdown

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8 Oct 2009 John Adams accelerator institute 13

RF photoguns:

the brightest pulsed electron sources...

3 GHz (λ=10 cm) resonant cavity λ/2 ...and RF acceleration. RF field strength ~100 MV/m, limited by vacuum breakdown

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8 Oct 2009 John Adams accelerator institute 14

RF photoguns:

the brightest pulsed electron sources...

ATF-BNL-UCLA 1.6 cell photogun

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TU/e approach:

Emittance growth due to non-linear acceleration fields:

  • full cylindrical symmetry
  • no tuning plungers
  • on-axis RF coupling

Emittance growth due to space-charge fields:

  • space-charge blow-out at cathode
  • ideally: ellipsoidal bunches
  • 100 fs photoemission of 100 pC in 100 MV/m

single-diamond turning

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8 Oct 2009 John Adams accelerator institute 16

x y Laser intensity Luiten et al., PRL 93, 094802 (2004)

Shaped fs laser pulse...

1 mm

surface charge density distribution:

( )

2

( ) 1 r r R σ σ = −

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...evolution into uniform ellipsoid.

Luiten et al., PRL 93, 094802 (2004)

→ linear & reversible Coulomb expansion

1 mm

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2nd generation TU/e gun:

  • Elliptical irises

– Highest field strength on cathode;

  • Cavity parts are clamped, not braized

– Easily replaced;

  • Copper cavity inside stainless vacuum can.
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10 20 30 40

R [mm]

10 20 30 40 50 60 70 80 90 100

GPT

z [mm]

  • 100
  • 50

50 100

Ez [MV/m]

Elliptical irises: Highest field strength on cathode

2nd generation TU/e gun:

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cathode plate first (half) cell second cell

2nd generation TU/e gun:

Clamped construction: cavity parts

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2nd generation TU/e gun:

Clamped construction: cavity parts

single-diamond turning

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2nd generation TU/e gun:

Clamped construction: assembled cavity parts

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2nd generation TU/e gun:

Clamped construction: cavity inside stainless steel vacuum can

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2nd generation TU/e gun:

Assembled gun: Solenoid around cavity

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2nd generation TU/e gun:

Entire setup: gun & beamline

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2nd generation TU/e gun:

RF characterization: resonances

f0=2.9980 GHz

π-mode 0-mode

f0=2.9918 GHz

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8 Oct 2009 John Adams accelerator institute 27

2nd generation TU/e gun:

RF characterization: on axis field profile

0,2 0,4 0,6 0,8 1 20 40 60 80 100

z (mm)

E/Emax

Superfish ♦ measured

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8 Oct 2009 John Adams accelerator institute 28

0,2 0,4 0,6 0,8 1 20 40 60 80 100

z (mm)

E/Emax

  • -- Superfish

radius ±5 μm Superfish ♦ measured

2nd generation TU/e gun:

RF characterization: on axis field profile

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2nd generation TU/e gun:

High power RF commissioning:

  • 80 MV/m at cathode (after one month of training)
  • Still occasional breakdown
  • 3 MeV electrons
  • QE ≈ 3·10-5 → bunch charge Qmax ≈ 300 pC

Conclusion: clamping is OK!

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Emittance measurement:

Quadrupole scan:

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Emittance measurement:

Quadrupole scan:

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Emittance measurement:

Quadrupole scan:

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Emittance measurement:

Quadrupole scan: Q = 5 pC

εn = 0.40(5) mm·mrad

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Emittance GPT simulation:

Quadrupole scan: Q = 5 pC, 106 particles

0.2 0.3 0.4 0.5 0.6 0.7

GPT

fx [m]

100 200 300 400 500

stdx [micron]

  • 1.0
  • 0.5

0.0 0.5 1.0

GPT

x [mm]

  • 1.0
  • 0.5

0.0 0.5 1.0

px [keV/c]

  • 1.0
  • 0.5

0.0 0.5 1.0

GPT

x [mm]

20000 40000 60000 80000

Count Rms: 0.15 mm Peak fit: 0.10 mm

Phase-space at focal point

1.7 % 1.7 %

  • very good agreement
  • still space-charge dominated

εn = 0.6 mm·mrad

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Emittance measurement:

Quadrupole scan: Q = 70 pC

εn = 1. 0(1) mm·mrad

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Bunch length measurement:

Coherent Transition Radiation (CTR)

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Bunch length measurement:

Coherent Transition Radiation (CTR)

Q = 70 pC τbunch < 2 ps

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Arrival time jitter:

Coherent Transition Radiation (CTR)

RF phase

20 fs jitter

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Performance TU/e gun:

  • charge Q = 70 pC;
  • measured bunch length τ < 2 ps;
  • at gun exit τ < 0.5 ps (GPT);
  • arrival time jitter < 20 fs;
  • normalized emittance εn = 1 mm·mrad.

peak current 35-140 A

LCLS injector (Akre et al., PRSTAB 11, 030703, 2008)

  • normalized emittance: εn = 1 mm·mrad
  • peak current: 100 A ( 1 nC / 10 ps )
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Part II: THz generation

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Electronics Photonics, optics "THz gap" Frequency

THz radiation

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Medical applications: skin cancer diagnostics Security: body scan Many materials transparent: “T-rays” Science: charge carriers dynamics, molecular physics, imaging of biological tissues, ...

THz radiation

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Single-cycle THz pulses

generated by Coherent Transition Radiation (CTR) Goal:

~ 1 ps BW > 1 THz ETHz = 10-100 MV/m

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Single-cycle THz pulses

generated by Coherent Transition Radiation (CTR)

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Single-cycle THz pulses

generated by Coherent Transition Radiation (CTR)

  • ~0.1 eV per electron
  • Coherent addition → ~N2 → many μJ per bunch
  • bunch length 1 ps → > 1 THz bandwidth
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Single-cycle THz pulses

generated by Coherent Transition Radiation (CTR)

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CTR: radially polarized

Single-cycle THz pulses

CCD A Expected signal CCD B

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Single-cycle THz pulses

generated by Coherent Transition Radiation (CTR)

CCD A

polarizer

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Single-cycle THz pulses

generated by Coherent Transition Radiation (CTR)

CCD B

polarizer

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CCD A

polarizer

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CCD B

polarizer

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Single-cycle THz pulses

THz power & energy in focus

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Single-cycle THz pulses

generated by Coherent Transition Radiation (CTR)

  • Field distribution in focus well understood;
  • 10-100× more energy in focus by shorter bunches

and improved collection efficiency;

  • ~μJ per pulse in focus not possible with RF

photogun;

  • fundamental problem: diffraction-limited focusing
  • f single-cycle pulses!
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‘THz endoscope’ Evanescent EM surface waves = ‘Surface Plasmon Polaritons (SPPs)’ Metal wire is a promising waveguide

Wang and Mittleman, Nature (2004)

Metal wire

E

  • t

~5 ps THz-pulse THz source

THz guiding on a wire

THz surface plasmons

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→ New THz research: Single molecule detection, non-linear optics, etc.

Maier et al., Phys. Rev. Lett. (2006)

λ

localized Intense Metal wire THz source

THz guiding on a wire

THz surface plasmons

Sub-wavelength focusing!

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Polarisation mismatch → mainly reflection! Problem: generation of THz surface plasmons! Radial

E

  • Linear

E

  • Up to now:

E ~ kV/m only

THz guiding on a wire

THz surface plasmons

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Radial

E

  • 300 µm

Radial

E

  • 300 µm

Idea: shoot relativistic bunches into wire tip

THz surface plasmons

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Idea: shoot relativistic bunches into wire tip

THz surface plasmons

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Idea: shoot relativistic bunches into wire tip

THz surface plasmons

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Idea: shoot relativistic bunches into wire tip

THz surface plasmons

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surface plasmon

Idea: shoot relativistic bunches into wire tip

THz surface plasmons

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Radial

E

  • 300 µm

THz plasmon

Idea: shoot relativistic bunches into wire tip Again: coherent addition of fields

THz surface plasmons

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Typical bunch:

1.6 mm 0.4 mm 160 pC 3 MeV

Experimental setup

THz surface plasmons

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Typical bunch:

1.6 mm 0.4 mm 160 pC 3 MeV CCD ZnTe Electro-optic detection

Experimental setup

THz surface plasmons

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ZnTe Bonus: Can plasmon go around the bend?

Typical bunch:

1.6 mm 0.4 mm 160 pC 3 MeV

Experimental setup

THz surface plasmons

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Wire ZnTe 0.35 Vertical electric field (MV/m) 2.5 mm 2 mm

Snapshot

Simulated: Plasmon dispersed by ZnTe Boundary effects As in vacuum

Measured electric field

THz surface plasmons

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ZnTe 0.6 0.3 Electric field (MV/m) 50 75 100 Time (ps) FWHM 6 ps (~ 0.1 THz)

Field in vacuum at wire surface

Wang and Mittleman, Nature (2004)

E

  • t

~5 ps But strong fields!

Measured electric field

THz surface plasmons

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0.4 0.1 0.02 Frequency (THz) 0.5 1.0 Electric field (ps MV/m) Calculated / 5 Measured

  • Radiated spectrum calculated from Maxwell equations
  • Bunch hitting infinite cone
  • Plasmons modeled as skin field of radiation at cone surface

Theory: Smorenburg et al., Phys. Rev. B (2008) Fourier transform

THz surface plasmons

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ZnTe Electric field (kV/m) Time (ps) 50

  • 50

100 40 80 120 150 FWHM 22 ps

Field in vacuum at wire surface

THz plasmons get dispersed & attenuated in bend, but survive! 6 ps 0.5 MV/m

After the bend?

THz surface plasmons

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Frequency 0.1 THz 0.5 MV/m > 100 MV/m Present results Outlook Electric field

Summary

THz surface plasmons

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0.1 THz > 1 THz 0.5 MV/m > 100 MV/m Present results

Intense, broadband THz SPPs can be generated with an RF photogun

Outlook

THz surface plasmons

Summary