Report from NLCTA

• CERN Built T18’s1. 820 degC Vacuum Braze

2. 1050 degC H2 Braze (SLAC procedure) - New

• T24s1. CERN Built (cells pre-fired)

2. KEK/SLAC Built - New

3. Comparison to T26

• NLCTA S-band Gun– Cathode breakdown damage and laser pulse heating - New

Chris Adolphsen, 10/20/10

CERN Built T18Cells made by Kugler, no etch, 820 degC vacuum braze at CERN,

installed in a vacuum can at SLAC

Copper grain size small due to ‘low’

braze temperature

Photo of Iris

T18_CERN_Disk Vacuum History in Jan 09 During Operation

0 10 20 30 40 50 60 70 80 900

50

100

Ave

rag

e U

nlo

ad

ed

Gra

die

nt: M

V/m

0 10 20 30 40 50 60 70 80 900

50

100 Bre

akd

ow

n R

ate

: 1

/hr70ns 230ns 200ns70ns

CERN T18 Processing History

Time (hr)

Breakdown Characteristics T18_Disk_1 during last 500 hrs, ~ 115 MV/m, 220 ns

T18_Disk_CERN during last 40 hrs, ~ 50 MV/m, 200 ns

-100 0 100-5

0

5

10

15

20

25

30

35

40

Reflected Phase: Deg

Fill

ing

tim

e fo

r d

iffe

ren

t ce

ll: n

s

0 10 20 300

0.2

0.4

0.6

0.8

1Missing Energy -vs- Breakdown Position (cell #)

-100 0 10010

-2

10-1

100Reflected Power -vs- Reflected Phase (deg)

0 50 100 150 200 250

0

0.2

0.4

0.6

0.8

1Missing Energy -vs- Time of Breakdown (ns)

Time of Breakdown (ns)

Fra

ctio

nal M

issi

ng E

nerg

y

Loca

tion

of B

reak

dow

n (n

s)

Iris 12 Autopsy

Iris 12 Autopsy

A 200 micron long calcium and carbon rich object that appears to have caused surface melting over a 1 mm wide area on this iris

0 2 4 6 8 10Energy (keV)

0

2

4

6

8

10

cps

Ca

OCu

Cu

SiS

Ca

Ca

Cu

Cu

0 2 4 6 8 10Energy (keV)

0

2

4

6

cps

C

CaTiO

Cu

Cu

Mg

Si

SK

K

Ca

Ca

Ti

Ti Fe CuCu

Dirty region which came after the test ?Dirty region which came after the test ?

Iris 11 Autopsy

2 4 6 8 10Energy (keV)

0

10

20

30

cps

O

Cu

Cu

Cu

Cu

0 2 4 6 8 10Energy (keV)

0

5

10

15

20

cps

C

O

Cu

Cu

Cu

Cu

Dirty particles which came after the test ?Dirty particles which came after the test ?

Iris 13 Autopsy

9

Second CERN-Built T18Improved Packaging

Use SLAC/KEK Assembly Cycle

The Adaptors That Launched a 1000 Emails

Start of Operation 08/02/10

Start of OperationStart of Operation 01/06/09

Vacuum History of Two T18_Disk Structures Made by CERN

#2 #1

0 100 200 300 400 5000

20

40

60

80

100

120

Time with rf on (hours)

Ave

rag

e U

nlo

ad

ed

Gra

die

nt (

MV

/m)

0 100 200 300 400 500

-5.5-5-4.5-4-3.5-3-2.5

Lg

(BD

R)

(1/p

uls

e/m

)

55ns65ns

100ns150ns

200ns 230ns

Processing History of T18_CERN_21e-3(1/pulse/m) = 34.6/hour at 60 Hz for 0.16 m

Final BDR at 90 MV/m@230ns is 1.3e-6/pulse/m

Start of Operation 08/02/10

Vacuum History of The Structure

Breakdown Waveforms

1500 1600 1700 1800 1900 2000

10-2

100

Event 21 at 23-Aug-2010 15:20:28

1300 1400 1500 1600 1700 1800

10-2

100

1500 1600 1700 1800 1900 2000-400

-200

0

200

400

1300 1400 1500 1600 1700 1800-400

-200

0

200

400

Reflected Power Reflected Phase

Output Power Output Phase

Blue: reference, Green: breakdown, Red: difference

Time (ns) Time (ns)

Breakdown Distribution for T18_CERN_2

-150 -100 -50 0 50 100 150-5

0

5

10

15

20

25

30

35

40

Reflected Phase: Deg

Fill

ing

time

for

diff

eren

t ce

ll: n

s

0 2 4 6 8 10 12 14 16 180

50

100

150

Cell #

BK

D E

vent

s

BDR Gradient Dependence

88 90 92 94 96 98 100 10210

-7

10-6

10-5

10-4

10-3

Gradient (MV/m)

BD

R (

1/p

uls

e/m

)

Measurement from 343 to 477 hours

Measurement from 477 to 567 hours

230 ns Pulse Width

90 95 100 105 11010

-7

10-6

10-5

10-4

10-3

Gradient (MV/m)

BD

R (

1/p

uls

e/m

)

TD18 700 hr

T18 SLAC11200 hr

T18 SLAC1900 hr

T18 CERN2343~567 hr

T18 SLAC1250 hr

T18 SLAC1500 hr

T18 SLAC11400 hr

BDR Gradient Dependence for T18_CERN2, TD18, T18_SLAC1 at 230 ns

4 6 8 10 12 140

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Position (a.u.)

E fie

ld (

a.u

.)

11.427 GHz FROM INPUT

CERN-Built T24 Disk StructureCells 24+input+output

Filling Time: ns 61

a/λ (%) 12.6 - 9.4

vg/c (%) 1.8 - 0.9

Phase Advance Per Cell 2/3

Power Needed <Ea> = 100 MV/m 42.4 MW

Unloaded Ea(out)/Ea(in) 108/90

Es/Ea 2

Pulse Heating ΔT: K (<100MV/m>@100ns)

7.5 – 8.4

Field Profile Along the Structure

T24 Fabrication• Manufactured

at VDL, Q4 ‘08 • Assembly at CERN with

“new” procedure (following T18 task force ) Apr-Jun 09

• Pre-fire of disks at 1040 ˚C. Resulting uneven surface caused braze leaks

• Cells oxidized at one point but it was removed with 650 ˚C bake

• Now back at CERN for evaluation

Assembly Cycle

- Intermediate transport in dedicated boxes - Intermediate assembly work under laminar flow atmosphere

- Intermediate transport in dedicated boxes - Intermediate assembly work under laminar flow atmosphere

Pre-Fire and Brazing

COUPLERS

• Pre-fire of couplers at 1040 ˚C

• Brazing of couplers 850 ˚C, SCP2

• Machining of couplers

STRUCTURE

• Pre-fire of disks at 1040 ˚C

• Brazing of all structure 815 ˚C, SCP1

• Brazing of cooling circuit 790 ˚C, eutectic AgCu – spots of braze filler material

Coupler surface after pre-fire

EDMS#10000659EDS analysis of brazing filler metal dropped out - The drops of filler metal in structure 11WNSDvg1.8 have a composition close to the Ag-Cu eutectic with no evidence of Pd.

RF Check and Tank Installation • RF check and tuning in building 112 clean room

• Problem occurred in the clean room: oxidation of the structure (all the parts made of annealed copper which were lying in the lab had blue or purple spots and those which were not annealed preserved their Cu color)

• Baking 650 ˚C for ½ day

– Witness piece analyzed (EDMS#1009001) –

• Before baking: traces of Cl and S

• After baking: traces of S (traces of Cl disappeared)

• Installation in the tank to verify part compatibility (EDMS#1003726)

• Shipment to SLAC (all parts under N2) – cover design incompatible with transport loads

Vacuum Histories

SLAC T18_1CERN T24_1

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

Time with RF On (hr)

Average Unloaded Gradient (MV/m)BKD Rate (1/hr)

T24 Processing History at 100 ns and Shorter Pulse Lengths

0.2

0.4

0.6

0.8

30

210

60

240

90

270

120

300

150

330

180 0

Reflected Phase -vs- Relfected Power (max)

0 10 20 30 40 50 60-10

0

10

20

30

40

50

Bkd Event Number

BK

D P

ositi

on (

ns)

Breakdown Locations

T24_SLAC_1 in NLCTA

T24 Structure

To SLED

Input Arms

5 Cell Output Loads

Magic T & 10 Cell Load

Floating strong-back(above beam line)

Beam Phase Monitor

(hidden from view inthis picture)

Start of Operation 09/21/10

Vacuum History

Start of Operation 08/02/10

T18_CERN_2T24_SLAC_Disk1

T24_SLAC_Disk1 RF Processing Results1e-3(1/pulse/m) = 51.8/hour at 60 Hz for 0.24 m

Currently BDR at 92 MV/m@200ns is 9.8e-5/pulse/m (~5BKD/hr)

0 50 100 150 200 2500

20

40

60

80

100

120

Ave

rag

e U

nlo

ad

ed

G (

MV

/m)

0 50 100 150 200 250

-5.5-5-4.5-4-3.5-3-2.5 L

g(B

DR

) (1

/pu

lse

/m)

Time with rf on (hr)

80 ns

100 ns150 ns

200 ns230ns

-150 -100 -50 0 50 100 150

0

10

20

30

40

50

60

Reflected Phase: Deg

Fill

ing

time

for

diff

eren

t ce

ll: n

s

Breakdown Distribution for T24_SLAC_Disk1

0 5 10 15 20 250

20

40

60

0 5 10 15 20 250

10

20

30

Cell No.

Bre

akdo

wn

Eve

nts

-100 0 100

0

10

20

30

40

50

60

Reflected Phase: Deg

Fill

ing

time

for

diff

eren

t ce

ll: n

s

0 ~ 200 hours 200 ~ 230 hours

T26 vs T24 ParametersStructure

TypeTotalAcc.Cells

vg

% c

2amm

Pin for 100

MV/m

Tf

ns

T26 30 3.30 - 1.62 7.8 - 6.3 84 36

T24 24 1.83 - 0.92 6.6 - 4.9 42 58

4 6 8 10 12 140

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Position (a.u.)

E fie

ld (

a.u

.)

11.427 GHz FROM INPUT

T26 T24

SLAC T26 (Even Number T53 cells)

0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

Time: hrs

Ave

rage

Unl

oade

d G

radi

ent:

MV

/m

0 50 100 150 200 250 300 350 400 450 500-6

-5.5

-5

-4.5

-4

-3.5

-3

log1

0(B

DR

): 1/

puls

e/m

50ns

70ns

100ns

150ns

200ns 150ns

100ns 200ns100ns 150ns

(a)

(b)

T26 Structure Process History Profile

98 100 102 104 106 1082

4

6

8

10

12

14x 10

-5

Unloaded Gradient: MV/m

BD

R:1

/pul

se/m

Pulse Wdith: 100nsPulse Width: 150ns

T26 BDR Gradient Dependence

0 10 200

2

4

6

8

Cell #

Per

cent

of B

reak

dow

n E

vent

s

-100 0 100

0

10

20

30

40

Reflected Phase: Deg

Filli

ng ti

me

for d

iffer

ent c

ell:

ns

0 10 200

2

4

6

8

10

Cell #

Per

cent

of B

reak

dow

n E

vent

s

-100 0 100

0

10

20

30

40

Reflected Phase: Deg

Filli

ng ti

me

for d

iffer

ent c

ell:

ns

0~250hrs

250~500hrs

NLCTA S-Band Gun Cathode

Ran at 105 MV/m without interlocks (~ 1 us fill)

Recently removed since dark current become too large

Laser Pulse Heating• Estimate of pulse heating in the NLCTA S-band rf gun from the laser (1 ps, 260

nm):• Assuming the laser induced heat does not diffuse much (< 14 nm) during the 1

ps laser pulse the temperature rise of the copper is (Laser energy * Absorption fraction / (Copper density * Heated Volume)) / Heat capacity where

• Laser energy = 10 uJ• Copper density = 9 gm/cm^3• Laser volume = pi * r^2 * depth where r = 0.5 mm and the laser penetration

depth is 14.3 nm • Heat capacity = .38 J/(gm*K)• Absorption fraction = 50%

• This yields a pulse temperature rise of 10e-6*0.5/(9*pi*.05^2*14.3e-7)/.38 ~ 130 degC

.

Laser Ablation• As shown in a paper at the url below on short pulse (0.5 ps), 248 nm

laser ablation of metals, they find it starts to occur (i.e. > 1 nm

removal per pulse) at a laser fluence of about 100 mJ/cm^2 versus

~ 1 mJ/cm^2 for the NLCTA gun.

• They assume the threshold of ablation is when the evaporation

enthalpy at 298 degK is exceeded, which is about 50 kJ/cm^3 for

copper - using the above copper heat capacity and density, this

would be equivalent to a 14e3 degK temperature rise, and is thus

consistent with the above computed temperature rise of 130 degC

with 1/100 of the laser fluence.

• http://www.springerlink.com/content/p6t732u61452t6t7/fulltext.pdf

Summary• Second CERN-built T18 did much better than the first, but has

breakdown rate over two orders of magnitude larger than the

SLAC/KEK built T18’s for the same operation parameters and

processing time

• The CERN-built T24 preformed poorly and the SLAC/KEK version is

doing OK after 230 hours of processing

• Based on earlier T26 results, would expect better performance of the

T24 thus far.

• Lasers pulses used for photocathodes can raise surface temperature

above 100 degC. However, the pulses are only about 1 ps long, and

the heat quickly dissipates. Nonetheless, cathode breakdowns

appear clustered in the area exposed to the laser.