Diode – RF Gun

OBLA LEG Summary

November 15 ^th , 2010

Outline

1) Overview LEG Results

1) Intrinsic Emittance & Quantum Efficiency 2) Emittance at 10 pC / 200 pC (comp. WLHA) 3) Latest Measurement: 200 pC with Duettino 2) FEA tests in LEG

3) Summary and LEG Decommissioning

LEG Installation at OBLA

Diagnostic Line

RF Cavity 1.5 GHz

Diode Voltage Pulser

500 kV, 10 Hz

Cathode Anode

Pulsed Solenoid

166 mm

RF Cavity

Diode – RF Gun

Pulsed Voltage

500 kV in 200ns 0 to 6 mm Gap

10 Hz

100 MV/m

RF Cavity:

2 Cells, 1.5 GHz 45 MV/m

UV Laser Pulses

Field Emitter Array (FEA)

Hollow Cathode Geometry

Inserts:

Cu, Mo, Nb, Y , Al, …

Hollow Geometry:

• Electrostatic Transverse Focusing

• Exchangeable Inserts

Outline

1) Overview LEG Results

1) Intrinsic Emittance & Quantum Efficiency 2) Emittance at 10 pC / 200 pC (comp. WLHA) 3) Latest Measurement: 200 pC with Duettino 2) FEA tests in LEG

3) Summary and LEG Decommissioning

25 30 10

10

Q u a n tu m E ff ic ie n c y

Surface Gradient (MV/m)

Cu St. St.

Ti Mo Nb Al

Bronze TiVAl Mg 'dry' Yttrium Mg 'wet'

Duettino ;6 ps rms; 4-6 muJ on Cathode; 5.2 MeV; gap 6mm DLC Hollow Cathode + Flat Insert

Quantum Efficiency of different materials

Depends on Cathode preparation

Main Activity: Find High QE metallic cathodes !

Intrinsic Emittance versus Cathode Preparation

Q < 1 pC; 5 MeV; F_cathode= 25 MV/m

4.3 4.4 4.5 4.6 4.7 4.8 4.9

0.3 0.4 0.5 0.6 0.7 0.8 0.9

1.0 285 280 275 270 265 260 255

Photon Wavelength (nm)

Cu Mo Nb Al Bronze Yttrium Mg 'dry' Mg 'wet'

ε_Th Formula (Φ₀= 4.4 eV) ε_Th Formula (Φ₀= 4.3 eV) ε_Th Formula (Φ₀= 4.1 eV) ε_Th Formula (Φ₀= 3.9 eV) ε_Th Formula (Φ₀= 3.66 eV)

DLC Hollow Cathode + Flat Insert; Schottky Reduction=0.19 eV

Photon Energy (eV)

ε n/σ x (mm.mrad / mm)

2

2 / 1 0

2 / 1 2 / 3 0

3

) 4 ( mc e F

h ^eff

x Intrinsic

ν πε σ

ε

+ Φ

−

=

Cathode

Preparation has also

consequences on

intrinsic emittance

0.87 2500 grade polished in argon

0.80 2500 grade + Ethanol, polished in air

Abrasive paper

0.47 Al2O3 polished in argon

~ 0 Al2O3 polished in air

Al2O3 + analytical reagent grade, Ethanol C2H5OH

0.27 0-0.25 um

0.41 0.75-1.5 um

0.54 2-4 um

Diamond suspension + analytical reagent grade, Ethanol C2H5OH 0.80 0.1um Luxor + analytical reagent grade, Ethanol

CO2 cleaning

0.82 0.1um orange (polished with pressure)

1.00 0.1um orange (gently polished)

0.82 6.5um brown

0.92 3um green

0.90 1um gray

0.90 1um blue

1.00 0.5um yellow

1.00 0.3um white

1.00 0.1um orange

0.90

< 0.1 um red

relative to 0.1um Luxor Luxor

+ analytical reagent grade, Ethanol C2H5OH

0.87 2500 grade polished in argon

0.80 2500 grade + Ethanol, polished in air

Abrasive paper

0.47 Al2O3 polished in argon

~ 0 Al2O3 polished in air

Al2O3 + analytical reagent grade, Ethanol C2H5OH

0.27 0-0.25 um

0.41 0.75-1.5 um

0.54 2-4 um

Diamond suspension + analytical reagent grade, Ethanol C2H5OH 0.80 0.1um Luxor + analytical reagent grade, Ethanol

CO2 cleaning

0.82 0.1um orange (polished with pressure)

1.00 0.1um orange (gently polished)

0.82 6.5um brown

0.92 3um green

0.90 1um gray

0.90 1um blue

1.00 0.5um yellow

1.00 0.3um white

1.00 0.1um orange

0.90

< 0.1 um red

relative to 0.1um Luxor Luxor

+ analytical reagent grade, Ethanol C2H5OH

Mg Cathode Surface Preparation Recepy

Courtesy of S. Ivkovic, C. Gough Many Cathode Preparation Recepies tested:

Finally for reproducible QE of 7^.10^-5:

1) Mg Cathode from Goodfellow (99.9%) 2) Polishing with Louxor Compound 3) Clean with Ethanol C2H5OH

4) Air dry

Mg Cathode Plugs for CTF3 Gun are in preparation !

Outline

1) Overview LEG Results

1) Intrinsic Emittance & Quantum Efficiency 2) Emittance at 10 pC / 200 pC (comp. WLHA) 3) Latest Measurement: 200 pC with Duettino 2) FEA tests in LEG

3) Summary and LEG Decommissioning

Cathode RF Photogun:

2.6 Cell – S band (2.998 GHz) 10 Hz, Cu Cathode,

F_Extraction= 53 MV/m; F_Peak= 100 MV/m Beam: 7.1 MeV; 140 pC (preliminary)

Two Guns in operation in 2010

Cathode Anode

Pulsed Solenoid 166 mm

RF Cavity

Diode – RF gun Combination:

Diode: 500 kV; 5 mm; < 3 Hz; Mg, Cu, … RF: 2 Cell – 1.499 GHz ; F_Extraction= 50 MV/m, F_Peak= 100 MV/m Beam: 6 MeV; 200 pC

LEG Diode – RF Gun

Low Charge Mode: LEG vs CTF3

n,min

=0.2 +/- 0.05 mm.mrad

SwissFEL Goal: 0.25 mm.mrad

5.1 MeV;

Diode: 300 kV; 6 mm; Cu Cathode

t,laser= 3 ps rms;

x,laser = 125 m (Uniform)

10 pC

Gun 2.6 Cell, S band, 10 Hz

10 pC; 7.1 MeV; 90 % Charge; 189.5 mT

t,laser = 3ps rms (Gaussian)

x,laser = 250 m (Uniform)

n,min

= 0.23 +/- 0.1 mm.mrad

CTF3 Gun

186 188 190 192 194 196 198 0.5

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

ε_n,X - Slit Scan (z=2.662m) ε_n,X - Pepperpot (z=3.013m)

PPT

Projected Emittance (mm.mrad)

Solenoid Peak Field (mT) 28.05.10

High Charge mode: LEG vs CTF3

Gun 2.6 Cell, S band, 10 Hz 140 pC; 7.1 MeV;

t,laser = 3ps rms (Gaussian)

x,laser= 250 m (Uniform)

n,min

= 0.55 +/- 0.1 mm.mrad CTF3 Gun

High Charge Regime : 140 pC LEG

High Charge Regime : 200 pC

n,min

= 1.25 +/- 0.2 mm.mrad

SwissFEL Goal: 0.4 mm.mrad

5.49 MeV

Diode: 500 kV; 5 mm; Nb cathode

t,laser = 3 ps rms (Gaussian)

x,laser= 450 m (Uniform)

Ganter et. al. PRSTAB, 13, 093502 (2010)

Outline

1) Overview LEG Results

1) Intrinsic Emittance & Quantum Efficiency 2) Emittance at 10 pC / 200 pC (comp. WLHA) 3) Latest Measurement: 200 pC with Duettino 2) FEA tests in LEG

3) Summary and LEG Decommissioning

The Jaguar (High Energy Nd: YLF System)

Since Last FLAC meeting:

The Pulsar (High Energy Ti:sa System)

… to WLHA

Only Duettino (low energy system max. 13 J on Cathode )

remained at OBLA for LEG Operation

Typical Laser Profile on cathode …

960 µm

263 nm; 30 µJ;

t,laser

= 3 ps Gaussian JAGUAR

High Laser Energy: Jaguar

Operation with low Laser Energy: the Duettino

Duettino:

Oscillator and 3 Pass Ampl. Stage Gaussian Time Profile

3.5 J; _t,laser= 4.3 ps

Solution for 200 pC Beam:

High QE Cathode

1 mm

5.68 MeV ( = 12.1); 450 kV; 8mm, 2.5Hz D_Yag-PPT= 0.129 m

Laser: Duettino; 266nm; _t,laser=4.3 ps;

r_cathode=3mm; Single Shot;

n,x

=0.55 +/- 0.1 mm.mrad (90 % charge)

High Electron Beam Quality with the Duettino

SwissFEL Goal: 0.4 mm.mrad

180 pC; Mg Cathode

1 2 3 4 5 X (mm)

0.00 111 222 333 444 556 667 778 889

4 5 6 7 8

X (mm)

180 189 198 206 215 224 233 242

Comparison with Simulations

Ideal for comparison with simulations

3 4 5 6 7

3 4 5 6

X (mm)

Y (mm)

180.0 201.0 222.0 243.0 264.0 285.0 306.0 327.0 348.0

4 5 6 7 8

X (mm)

10.00 250.0 490.0 730.0 970.0 1210 1450 1690

2 3 4 5 6

3 4 5 6

X (mm)

Y (mm)

0.00 122 244 367 489 611 733 856 978

2 3 4 5 6

X (mm)

0.00 122 244 367 489 611 733 856 978

Measured Electron Beam Profile :

z = 1.24 m z = 1.93 m z = 2.24 m

z = 2.45 m z = 2.55 m z = 4.39 m

Mg cathode; 45 pC; 5.3 MeV; Laser Gaussian Time Profile; _t,laser=4.3 ps; _r,laser=414 m 330 kV; 6 mm gap; PSL=1240 A; DSL1 = 115 mT; DSL2 = 95 mT

0 1 2 3 4 5 0.0

0.5 1.0 1.5 2.0

B e a m s iz e , rm s σ

( m m )

position (m)

σ_X measure σ_y measure

σ_X ASTRA, PSL= 160 mT

Mg cathode; 45 pC; 5.3 MeV; Laser Gaussian Time Profile; _t,laser=4.3 ps; _r,laser=414 m 330 kV; 6 mm gap; PSL=1240 A; DSL1 = 115 mT; DSL2 = 95 mT

ASTRA / Experiment Comparisons

Measured Emittance:

n,x= 0.4 +/- 0.05 m Simulated Emittance:

n,x= 0.42 m No perfect matching:

Space Charge effect in diode and aperture

To come: OPAL simulations with non-symmetric laser profile

Outline

1) Overview LEG Results

1) Intrinsic Emittance & Quantum Efficiency 2) Emittance at 10 pC / 200 pC (comp. WLHA) 3) Latest Measurement: 200 pC with Duettino 2) FEA tests in LEG

3) Summary and LEG Decommissioning

FEA Tests in the LEG Gun

Summary of Last Results

FEA survived 30 MV/m

10 pC charge accelerated at 300 keV Field emitted pulses of 4 ns

laser illumination tested

Without Laser With Laser

Courtesy of M. Paraliev & S. Tsujino

Sub ns FEA emission

Motivation for short electron pulses:

Compatibility with RF acceleration ( < 666 ps)

Higher Field Emitted Current (less Arcing probability)

NEW RESULTS:

C_FEA: 1.3 nF to 0.6 nF

FID pulser

-1 0 1 2 3 4 5 6

5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0

Time, ns

Voltage, kV

Efanov Pulser

5 kV, 300 ps rise time

Field Emitted Pulses:

460 ps FWHM

Courtesy of M. Paraliev & S. Tsujino

~ 20 pC

w222ch2d

40 k-tip, 10 m-pitch

Higher peak current with sub-ns gating:

20 pC in 460 ps (50 mA) 2 mm diameter FEA

Sub ns FEA emission

FEA Electron Pulses shorter ½ RF Period: 550 ps < 666 ps (L Band)

RF acceleration of Field Emitted Pulses

200 keV beam (no RF) ~3.5 MeV beam ~5.6 MeV beam (+ dark current)

World Premier: First FEA Beam at 5.6 MeV !

Courtesy of M. Paraliev & S. Tsujino

Cathode

Cavity

DS1

FC Screen

FC signal @ 5.6 MeV

RF Phase Scan for Optimum Charge Collection:

Model confirm Bunch length ~ 550 ps No FEA damages by RF

(Tolerance to Backbombardment)

RF Phase Scan with FEA beam

Simulation assuming

0.6 ns FWHM 0.5 ns FWHM

Extreme short FEA pulses of 550 ps duration (World Record) First FEA beam accelerated with an RF cavity (5.6 MeV Beam)

Remaining Issues for SwissFEL application:

• Uniformity of the emission

• Current density and peak current levels

• Low emittance with double gated emission

Pulse length

0.1 1 10 100 1000

Jun.08 Dec.08 Jul.09 Jan.10 Aug.10 Feb.11 Time

Pulse length FWHM, ns

Without RF With RF 100 ns

4 ns 460 ps

550 ps 1 ns

Summary New FEA Results:

Several LEG Publications this year:

C. Hauri et al. Phys. Rev. Lett. 104, 234802 (2010)

R. Ganter et al. Phys. Rev. ST Accel. Beams 13, 093502 (2010) F. Le Pimpec et al. J. Vac. Sci. Technol. A 28, 1191 (2010)

S. Tsujino et al. IEEE Electron Devices Lett., 31, pp 1059 (2010) And more than 10 conference proceedings … .

More to come …

P. Helfenstein et al. Appl. Phys. Lett. , in preparation, (2010) S. Tsujino et al. J. Vac. Sci. Technol. B, submitted (2010) K. Li et al. Phys. Rev. ST Accel. Beams, submitted (2010)

Summary of LEG activities 2009 - 2010

LEG Decommissioning

Diode RF-Gun Operation: January 2009 to January 2011

2 4 6 8 10 12 14 16 18 20 22

10

10

10

10

10

0 50 100 150 200

Duettino, Mg Pulse Stacking

High Gradient

Goal

C h a rg e ( p C )

Hollow Cathode

Shorter Laser Pulse

Tuning Alignment

B e a m B ri g h tn e s s ( A .m m

m ra d

)

Number of month since January 2009

Beam Brightness = 2I_peak/(π²ε²)

Start

Charge FEA: R&D will continue with eventually an installation in RF photoinjector OBLA: Converted in RF Test Stand starting January 2011 (see J. Alex Talk)

Vacuum, Magnets, support, diagnostic parts: Stored for future THz source of SwissFEL Pulser + RF cavity: to be defined … .

The End

Diode - RF Gun Advantages:

- No Dark Current

- Cathode isolated from RF Conditioning, ion backbdt, … . - Electrostatic Focusing possible (Pierce Angle Cathode) - no RF contribution to emittance growth

Diode - RF Gun Difficulties:

- Complexity of Pulser System

- Limited repetion rate for 500 kV (ceramic discharge) - Iris effect (non linear effect near aperture, )

- long distance between diode and RF (emittance growth due to SC)

0.0 0.1 0.2 0.3 0.4 0.00

0.05 0.10 0.15 0.20

1 line; 100% charge; Constant Charge density = 0.42 pC/mm

Laser Spot Size (mm rms)

Emittance (mm.mrad)

Measurements (Pepperpot) Fit = 0.53 +/- 0.05 mm.mrad /mm

28.05.10

Intrinsic Emittance measured in an RF photoinjector

0.42 pC/mm2 at RF Phase = 9 deg;

7.1 MeV/c; =13.9;

Solenoid = 175.9mT No quads; 21 Shots; Pepperpot

28.05.10

0.53 mm.mrad / mm @ 262 nm Diamond Turned Copper

In an RF Photogun :

Hand Polished Copper in diode – RF gun:

0.62 mm.mrad / mm @ 262 nm

~300 ps rise time

~1 ns FWHM pulse width 5 kV amplitude (positive) 0.5 MW peak power

> 30 A injected in the gate

~ 1.5 ns gate pulse Inversion / bipolar pulse DC bias

Monitor signal (jitter)

Sub-ns electrical switching of FEA emission

5 kV pulser

~1ns

Bipolar current bias up to +/-30A

461 ps FWHM

w222ch2d

40 k-tip, 10 m-pitch w222ch2d

40 k-tip, 10 m-pitch

Sub-ns FEA emission: diode + RF acceleration

FEA < 1 ns

RF-cavity, 1.5 GHz

DS1 FC

Screen Diode 200 ns

~3.5 MeV beam ~5.6 MeV beam (+ dark current)

•

FEA beam acclerated to > 4 MeV by diode +RF

•

No FEA damage by RF (e.g. back bombardment)

•

Sub-ns emission (~0.5 ns) shown by RF-phase scan

•

Faster emission down to <333 ps feasible

(lower FEA capacitance, driver optimization etc.)

Pulse length

0.1 1 10 100 1000

Jun.08 Dec.08 Jul.09 Jan.10 Aug.10 Feb.11 Time

P u ls e l e n g th F W H M , n s

Without RF With RF 100 ns

4 ns

460 ps 550 ps 1 ns

Electrically-gated FEA

emission: summary

FEA Pattern evolution with V

Diode: 250kV @ 15mm, 1Hz V

: 116 to 130V

Charge: 2.2 to 7.9pC

Uch = 116V Uch = 118V Uch = 120V Uch = 122V Uch = 124V Uch = 126V Uch = 128V

Uch = 130V Emission pattern changes with V

FEA was destroyed at V

= 130V (spark)

Courtesy of M. Paraliev / S. Tsujino