DEVELOPMENT OF LASER ABLATION FOR H - PowerPoint Presentation

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DEVELOPMENT OF LASER ABLATION FOR H2 CONTENT MEASUREMENT IN METALS

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Irene Martini

Outline

Motivation

CLIC accelerating structures: static and dynamic pressure requirements

Future plans: measurement of H

2

in thin filmsStudy of Hydrogen Content measurement by laser ablation Experimental setup: vacuum and optical systemExperimental sequenceLimits: outgassing and dynamic hydrogen backgroundDetection of electronsTrialsTest on CopperTest on TitaniumConclusion

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Motivation

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-

Modular structures assembled by bonding cycles performed at high temperature and under H2

Diffusion of hydrogen in the copperPOSSIBLE STATIC AND DYNAMIC PRESSURE EFFECTS DURING OPERATIONCLIC ACCELERATING STRUCTURESSTRICT REQUIREMENTS TO LIMIT INTERACTION OF THE BEAM WITH RESIDUAL MOLECULES

Laser ablation setup 1/2TE-VSC

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VACUUM SYSTEM

2 chambers + by-pass

(static vacuum in AB, released gas pumping through a small conductance

)Purposedetect small pressure increaseAB

AB

AN

AN

RGA

SAMPLES

By-pass

SAMPLES

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Experimental setup 2/2

OPTICAL SYSTEM

Purpose:

have

a focused square spot with uniform energy density on the targetHomogenizerto have a flat intensity profileAttenuator

to operate at difference

fluence

Converging Lens

to take an image of the slit

Pyroelectric

Joulemeter

to measure the pulse energy

( not

shown in the

scheme)

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Experimental sequence

Crater profile

(after ablation)

Pressure increase recorded by RGA

SAMPLES

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System Limit

OUTGASSING FROM THE CHAMBER

LIMIT OF DETECTION

Time

for

ablation

experiment

100 s

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Dynamic H

2

Background

Ablation on Silicon

Measured:Source of H2 not related to actual content in the sample Hp: Desorption stimulated by photons and by charged particles (produce in laser-target interaction) Expected: negligible

Investigation

of

photodesorption

(

shot

sample

holder

with

unfocussed

laser

beam

) : NO

increase

of

H

2

partial

pressure

Desorption stimulated by charged particles

Purpose

:

Investigate, remove or at least reduce the H

2

source

Stop or slow down charged particle

with ‘Buffer gas’ (Xe, CO

2

)

Trap

charged

particles

with

electric

field

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Confirmation of

plasma threshold

found in literature The influence of buffer gas is negligible Discussion: Setup: Biased filament (up to

100 V)

Tipical

pulsed

shape

signal

recorded

on oscilloscope

Purpose

: Detection

of

electrons

while

shotting

samples

with

laser

30

μ

s

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Trials

Investigation

of photodesorption NO INFLUENCE on desorption signalStop or slow down charged particles with ‘Buffer gas’ (Xe, CO2) NO EFFECTIVE Attempt

to

detect

ions

in the

same

way

of

electrons

MEASUREMENT NOT REPRODUCIBLE

Trap charged particles

(positive and negative bias on the sample holder)

same

desorption

signal

as

grounded

sample

holder

configuration

The H

2

source is STIMULATED DESORPTION from the wall of the chamber

The attempts done to reduce it were

unsuccessful

Ablation on CopperTE-VSC

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Confirmation of huge parasitic source of H2

Confirmation

that buffer gas is not effectiveH2 solubility in Copper:Expected

WITH BUFFER GAS

(10 shots, 5 Hz)

UHV CONDITION

Copper

sample

treated

in the

same

condition

of

bonding

cycle

(

for

P=1 bar, T=1040˚C)

χ

H

= 76 ppm

Q

H

2

~10

-7

mbar

· l

(

crater

depth

~

μ

m)

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Ablation on Titanium

H2 solubility in Titanium:

No

measured difference between H2 release from treated sample and untreated sampleExpectedTREATED TITANIUM

UNTREATED TITANIUM

χ

H

~

4 · 10

4

ppm

(

crater

depth

~

μ

m)

(

for

P=25

mbar

, T=1020˚C)

Q

H

2

~10

-4

mbar

· l

Test on

treated

and

untreated

titanium

samples

to

compare the

results

.

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Attempts

:

Investigation of photodesorption Buffer gas (Xe, CO2) Bias on the sample holder to trap charged particlesTest on Titanium (higher content

)

Limits

:

Outgassing

of

the

chamber

Dynamic

hydrogen

pressure

The

method

is

not

applicable

due

to

the

induced

desorption

from

the

wall

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