Nietubyć - thin film pb photocathodes prepared with the cathodic arc

Thin film Pb photocathodes prepared with the Thin film Pb photocathodes prepared with the cathodic arc cathodic arc

R. Nietubyć, J. Sekutowicz, P. Kneisel, J Smedley

The Goal

The goal is to build a Nb injector with theThe goal is to build a Nb injector with the superconducting superconducting cathode made of lead for use in CW or near-CW operated high cathode made of lead for use in CW or near-CW operated high average current acceleratorsaverage current accelerators

Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010

Capabilities I, promissing properties of Pb

Superconductor of 1 st type

Critical H 8 mT@ 1.3 GHz at 2 K

Critical T 7.9 K

Work function 3.95 eV

Pb can be used as a superconducting photocathode and deliver 105 1 nC bunches per second when flashed with 4μJ pulses of 213 nm light

The goal was to build an ultra high vacuum cathodic arc deposition facility dedicated to niobium coating with lead and to establish the Pb/Nb coating technology. This effort should lead us to the repetitive and reproducible lead deposition in the photo-injector cavities.


Calculations of J. Smedley et al

The phenomena critical for superconductivity

• ohmic centres (defects, impurities [oxides])

• electron emission (roughness)

are closely related to cavity wall morphology

Capabilities II, preparation method

Deposited material arrives in a form of highly energetic ions emitted from the cathodic spot, which can be additionally accelerated by external electric field

Interaction with substrate and already deposited layer:

•sub-plantation

•enhanced diffusion in outer layer

•permanent exceed of material

•gradual cooling and condensation

50x10-3

40

30

20

10

0

ions

[1/Å

]

50403020100depth [Å]

primaries


100 eV

Capabilities III, cathodic arc experience at SINS


A laboratory has been established in the Plasma Physics Department at SINS in 2004. It participating in thin film SRF cavity programme performed in the frame of CARE. It resulted in an established procedure of three-cell Tesla-like Cu cavity coating with 2 µm Nb layer. A disadvantage was poor adhesion (< 80 bar HPWR)

3-cell cavities structure was coated For samples Nb/sapphire RRR=43

http://care.lal.in2p3.fr/

Proof of principle experiment


Various Pb samples were prepared:

Lead layer has been deposited onto Nb in the simplest geometrical arrangement. Substrate was placed in front of arcing cathode.

Next it was cleaned with KrF 10 ns pulses 248 nm 0.3 mJ/mm laser pulses

QE was measured

magnetron sputteredelectro-platedvacuum depositedarc depositedbulk lead

Arc deposited layer showed the highest QE

Lead QE vs Photon energy

1.0E-04

1.0E-03

1.0E-02

4.00 4.50 5.00 5.50 6.00 6.50 7.00

Photon energy (eV)

QE

Theory

Measurement

Experimental results confirm the calculations

Second approach - cavities

Those trial depositions showed the capability of cavity treatment. In particular to match cleanliness requirements and to solve mechanical problems

Pbpure


Regular devlopement of the method

Optimization of deposition system• transmission and deposition rate• micro-droplets filtering• temperature control• cleanliness and vacuum

Photocathodes preparation• deposition processes • after deposition treatment

• chemistry• laser flashing

Measurements• surface diagnostics• QE

(M18)Lead deposition on half cells and 1.6 cell cavitiesM10.4.2

(M12) Lead deposition on samples for photocathode developmentM10.4.1

Milestones

(Report, M36)Cold test results for the test cavities with and without the deposited lead photo cathodeD.10.4.3

(Report, M12)QE data for Pb/Nb deposited photo cathode samplesD.10.4.1

Deliverables

WP 10.4


http://eucard.web.cern.ch/eucard/index.html

Deposition system with knee-shaped microdroplets filter

polarisation = -110 V base pressure <10-7 mbar

arc current = 25 Acoil current = 120 Aarc voltage = 17-18 Vion current = 2.5 mAdeposition rate ≈ 0.5 nm/s deposition time < 80 minwall temperature <32 °C

2.0x10-12

1.5

1.0

0.5

0.0

curre

nt

50403020100mass [at. units]

Rest gases


Optimisation of depsotion, microdroplets removal

Microdroplets removal – knee-shaped filter

dryBent1.6 cellpoly60008

dryBent0.5 cellmono27007

dryBent0.5 cellmono27006



oilStraight1.6 cellmono18003

oilStraight1.6 cellpoly18002

oilStraight1.6 cellpoly18001

PumpSetupDistanceNb typeTime [s]No

SEM pictures of as-delivered sample No 1 (left) and sample No 5 (right). Note different scale.

Location of the heater with lead stub for plasma formation

M.10.4.1 Lead deposition on samples for photocathode development


Micro-droplets were efficiently removed. Unfortunately in cost of very low transmission

Modeling a filter shape:The best was case a when applied:200 A which gave

12 - 16 mT

Verification with sample plugs

Optimisation of depsotion process, QE measurementsD. 10.4.1 QE data for Pb/Nb deposited photo cathode samples

QE with lamp sourceBefore Cleaning

-0,00005

0

0,00005

0,0001

0,00015

0,0002

0,00025

4 4,5 5 5,5 6 6,5 7

Photon Energy (eV)

QE

Laser: 213 nm 1 min 25 Hz 0.2 mJ/mm2 per pulse

QE with lamp source0.21 mJ/sq mm Cleaning

0

0,0005

0,001

0,0015

0,002

0,0025

4 4,5 5 5,5 6 6,5 7

Photon Energy (eV)

QE

1 MV/m


Pb

Nb

QE=2.110-4 QE=2.010-3

Laser cleaning improved the QE, mostly by oxide removal. Unfortunately, it caused a lead re-crystallisation. QE measure for cleaned sample. QE would be at least twice higher if the coverage was complete.

(i.e. 20 times higher then for Nb)

(i.e. still worse than measured in the proof of principle

experiment

How to avoid a layer destruction?

• by making the layer thicker.

• by making the laser cleaning smoother


Increased transmission to enable thicker filmsTo improve the coating efficiency, the system has been modified by replacing the rectangular knee by the 30º bent tube. That solution ‑enabled to increase 3 times the ion current saving the lead flux free of macro-particles. Chosen angle provides the minimal bend angle for which the lead droplets, which all move rectilinearly, cannot reach the target

30°coil current = 70 Aion current = 8 up to 14 mAdeposition time < 80 min

no micro-droplets

Optimisation of depsotion process, transmission

Optimisation, final resuts for plug samples


Gentle laser treatment : 190 nm, 30 min, 300 Hz 0.01 mJ/mm2 per pulse

as compared to:213 nm, 1 min, 25 Hz, 0.2 mJ/mm2 per pulse

QE with lamp sourceCenter, No Cleaning

-0,0001

0

0,0001

0,0002

0,0003

0,0004

0,0005

4 4,5 5 5,5 6 6,5 7

Photon Energy (eV)

QE

1 MV/m

QE with lamp sourceCleaned, 0.75 mJ/sq mm

0

0,0005

0,001

0,0015

0,002

0,0025

0,003

0,0035

4 4,5 5 5,5 6 6,5 7

Photon Energy (eV)

QE

1 MV/m

QE=3.310-3

Improved deposition geometry and gentle laser cleaning procedure enabled the layer to survive in much better shape. As a result the highest QE was reached. It is however still lower than that measured in the proof of principle experiment. Still a thicker layer is needed.

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0keV

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

cps/eV

Pb Pb Pb C O

Carbon and oxygen contamination

As deposited Pb 0.904, C 0.063 O 0.033

Cleaned Pb 0.930, C 0.045. O 0.025

Pb deposition onto cavities back wall of, a setup


Mask:

a stainless steel tube (1) with a bellow (2) and niobium cap (3)

1 2 3

1

23

DESY cavity 1, March 2010

1.E+09

1.E+10

1.E+11

0 10 20 30 40Epeak [MV/m]

Q Cold RF test performed for the coated cavities showed quality factors (Q) close to 3109 at gradients on the

cathodes of 29 MeVm-1 and 39 MeVm-1.

M.10.4.2 Lead deposition on half cells and 1.6 cell cavities

effective deposition time = 80 min (16×5 min arc + 30 min cooling)base pressure < 2⋅10-7 mbarcoil current = 75 Aion current = 10 mAdeposition time = 80 minwall temperature < 34 °Cthickness < 200 nm (roughly from EDS measurements for samples)

Typical BCP acid treatment and HPWR were applied. The photocathode was chielded with the mask

That promissing test has been interrupted by a helium leakage and could not been continued after it was mend. The cavity was sent to Świerk to prepare another Pb layer. Typical BCP acids mixture is capable to remove the old layer

A final test before coating the cavity to be used in Hobicat


HZB cavity for Hobicat, May 2010

Back wall consist of three single crystalline domains.

A heluim tank is assembled.

Beam quality test are planned for this cavity in the end of 2010

Exactly the same deposition procedure was repeated for this cavity

A uniform spot of 1 cm in diameter was obtained. Next to a deposition the cavity was filled with N 2 and sent to JLab

20 days later, when it was opened

Film got orangeAfter HPWR it got blackNext it disappeared during the standard treatment

Lead nitride Pb2N3 or Pb3N4

In contrary to PbO does not passivate the surface but penetrate in depth

As the layer was successfully deposited and spoilt only due to the reaction with N 2 We considered that the experiment is

worth of severe work and schedule rearrangement in order to try again. The cavity was sent to Świerk and coated again.


DESY cavity 2, last week

coil current = 75 A, enhanced coil wrapping around the back wallion current = 11 - 14 mAdeposition time = 80 min


4,9589610×60

4,961445×120

4,96714120

4,9642360

4,9644915

4.958993

;lattice constant

Dep. time

8

6

4

2

0

log(

coun

ts)

12010080604020

2θ [deg]

log

(cou

nts)

10080604020

2θ [deg]

iiolon703 iiolon711 iiolon715 iiolon719 iiolon723 iiolon727 iiolon731 iiolon735

Lattice constant does not depends on thickness

Orientation distribution does not change neither

Amorphous or nanocrystalline phase in the interface

Observable amount of PbO appears in roughly 10 hours of the exposition to air

XRD studies of Pb film growth on sapphire and Nb

Reflectometry, XRD and EXAFS measurements have been performed for film of various thicknesses. Experimental results will be interpreted by ab initio calculations. Only rough results here

time

2h

18h

PbO

6

4

2

(cou

nts)

45403530252 θ [deg]

Lattice constant vs thickness

Two lead phases

oxidation


Conclusions

We gained:

Reproducible processes (4 times for 1,6-cell cavities)

Micro-droplets removal

Optimised laser cleaning procedure

Reasonable QE

We will continue with:

Beam tests of Pb/Nb injector at Hobicat at HZB (end of 2010)

Contaminations removal

Fatting the film

System reconstruction in order to shorten the plasma channel

M.10.4.1 Lead deposition on samples for photocathode development.10.4.1D / QE data for Pb Nb deposited photo cathode samples

M. 10.4.2 - 1.6- Lead deposition on the half cell and cell cavities.D 10.4.3 Cold test results for the test cavities with and without the deposited lead photo cathode


http://eucard.web.cern.ch/eucard/index.html

Thanks for the attention

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

4 5 6 7

QEPhoton Energy (eV)

After Laser Cleaning

Before Laser Cleaning

1.E+09

1.E+10

1.E+11

0 10 20 30 40Epeak [MV/m]

Q

Q

QE

together

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Nietubyć - thin film pb photocathodes prepared with the cathodic arc

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