Plasma interactions with Be surfaces - PowerPoint Presentation

Slide1

Plasma interactions with Be surfaces

R

. P.

Doerner

, D.

Nishijima

, T. Schwarz-

Selinger

and members of the PISCES Team

Center for Energy Research, University of California – San Diego, USA

Work performed as part of

:

Plasma-Surface Interaction Science Center (MIT and

UTenn

)

US-EU Collaboration on Mixed-Material PMI Effects for

ITER

Slide2

The PISCES-B divertor plasma simulator is used to investigate ITER mixed materials PSI.

PISCES-B is contained within an isolated safety enclosure to prevent the release of Be dust. Slide3

PISCES-B has been modified to allow exposure of samples to Be seeded plasma

P-B experiments simulate

Be erosion from ITER wall,

subsequent sol transport

and interaction with W baffles

or C dump plates, as well as

investigation of codeposited

materials using witness plates

PISCESSlide4

Outline of Presentation

Erosion in the plasma environment

Comparison to TRIM and ion beam data

Surface characterizationRole of morphologyRedeposition

/sticking efficiencySummary

PISCESSlide5

Be erosion yield measurements in PISCES

Two techniques are used to measure physical sputtering yield

Weight loss measurements,

use low density plasma to reduce

redeposition

(i.e. long ionization mean free path).

Line emission spectroscopy,

uses high density plasma to minimize geometrical loss terms (i.e. short ionization mean free path).

These presentation will focus on the weight loss technique and changes to spectroscopic measurements that are normalized to weight loss measurements

.

[He plasma on Be, weight loss are 5-10 times lower than TRIM calculations, He plasma on C, weight loss agree with TRIM calculations].

PISCESSlide6

Significant variations in the Be sputtering

yield are measured

Incident ion energy ~100

eV

J. Roth et al., FED 37(1997)465.

PISCES

discrepancy between - JET - PISCES-B - ion beam – TRIM - sputter

yields

(< 45%) (<

0.7%)

(< 8%) (< 3.5%) Slide7

Uncertainties in sputtering yield measurements

Uncertainties in weight loss measurements:

Surface contamination

Incident ion energy

Ion flux measurement

Ion species mix (molecular ions)

Redeposition

fraction

Surface morphology changes

Atomic D adsorption on the surface (secondary ion yield)

Uncertainties in absolute spectroscopic measurements:

Atomic physics database

Electron energy distribution (non-

Maxwellian

)

Angular distribution of sputtered particles

Geometrical loss fraction

PISCESSlide8

Native beryllium oxide surface is removed early during the plasma exposure

Distinctive oxygen lines near 777 nm can monitor erosion of O from surface

Background helium plasma does not change (second order He I line)

Larger ion energy will remove oxide layer quicker

BeO (0,0) molecular band emission (@ 470.86 nm) is not detectable

PISCESSlide9

AES reveals a relatively ‘clean’ Be surface after sputtering yield measurements

PISCESSlide10

Incident ion energy is corrected for plasma potential. Ion flux is uniform over sample surface.

From B. LaBombard et al, JNM 162-164 (1989) 314.

PISCES-A space potential

measurements show

V

pl

~ 1-2 T

e

E

ion

= |

V

bias | - 1.5Te

Target

From D. Whyte et al, NF 41 (2001) 47.

Ion flux calculated

from

upstream Langmuir

probe agrees

with total current collected

on

the

sample manipulator

when biased

into

I

sat

(with ~10%)

PISCESSlide11

Molecular ion fractions are calculated from zero-d rate balanced model, based on measurements

Model uses the atomic and molecular processes to the right

Rate equations predict molecular ion fractions based on n

e

, T

e

, B, N

H2

Turbulent radial transport is assumed (1/B scaling)

Model is verified with molecular ion species measurements

From E.

Hollmann

et al., Phys.

Plasmas 9 (2002) 4330.

PISCESSlide12

Molecular ion effects change both the shape and magnitude of sputtering yield curve

PISCES

Shape of measured yield agrees with molecular ion model predictions.

Magnitude is a factor of ~5 too low.Slide13

ERO calculates 10-20% ionization of sputtered Be in the PISCES-B plasma

Shape of Be I (457 nm) axial profile agrees well with experimental profiles

Magnitude of profiles are normalized

Molecular ion fractions provided by previous model

No Be deposition observed on W or C targets exposed to D plasma (no wall source)

PISCES

From D. Borodin et al, Phys. Scr. T128 (2007) 127.Slide14

Be surface

before plasma

exposure

after

Surface morphology evolution

with time /

fluence

spectroscopy:

mass loss:

 morphology

change can

account for a factor

of

2 in reduction of the yield

PISCESSlide15

Similar yield evolution

with

time/

fluence is documented in the literature

 morphology

change can

account for a

factor

of

2

reduction of the yield

1keV,



H

2+7.3E21 ions/cm2

Mattox and Sharp, J.

Nucl

. Mater. 1979:

PISCESSlide16

“maximum” – static TRIM + MD

“minimum”

–

SDTrimSP

with 50% of D (reasonable limit)Plasma atoms remaining in the near surface also can reduce the sputtering yield by a factor of 2-3

From C.

Björkas

PISCESSlide17

Gross erosion is expected to remain constant with increasing Be redeposition

/influx

N = G * (1-R), where N = net erosion, G = gross erosion, R =

redeposited fractionIn PISCES-B, Gross = net erosion with no Be seeding (

λion is large compared to rplasma

, so redeposition is small)Ion influx from Be oven is identical to sputtered atoms that are ionized in the plasma and redeposited

on the target, this allow a controlled and independent variation of the Be influx to the target

PISCESSlide18

Erosion/deposition balance in Be seeded

high flux D discharges

Use Be oven seeding to balance surface erosion to test input parameters of material migration models

Mass loss measures net erosion

Spectroscopy measures gross erosion (Be I line)Y Be→Be

≈ Y D→Be, and low concentration of Be

When incident/seeded Be ion flux = sputtered flux of Be, net erosion should = 0.

Mass loss

ion

fluence

:



10

22

/cm2target temperature < 320K

PISCESSlide19

No change in mass loss is measured when Be seeding flux equals sputtering of Be by D

ADAS database is used

Be flux from Be II (313.1 nm) and background plasma flow velocity

(E. Hollman JNM, PSI-19)Be ion flux is verified during no bias discharges, when weight gain is measured (net deposition)

Net erosion stays constant, implying gross erosion must increaseErosion yield of 0.15% can only be compensated by seeding 2.8% Be

Mass loss

ion

fluence

:



10

22

/cm

2

D/Be plasmatarget temperature < 320K

2.8%

PISCESSlide20

Beryllium seeded He discharges

target:

bias: < -40V results in E ≈ 30eV

He ion flux: 5

·1018 cm

-2s-1Be seeding: nBe

/

n

D

= 0 – 4 %

sputter yield He on Be @ 30eV:

Y = 0.15

% (measured)

target

oven

PISCES

: During ‘no Be seeding’ discharge,

Gross ≈ Net erosion

Gross

erosionSlide21

Different behavior is observed experimentally

Net erosion stays constant until influx >> sputtering rate

Gross erosion increases with increasing Be influx to target

Reduced sticking of depositing Be, or increased re-erosion could explain observations

Similar reduced sticking needed to model CD

4

and WF

6

injection experiments in TEXTOR (from A.

Kirschner

)

Possibly similar to C-MOD modeling difficulties reported by J. Brooks (PSI19)

PISCESSlide22

What PMI issues are still unresolved

Sputtering Yield – we believe we can explain the observed, low sputtering yields in PISCES-B, due to surface morphology and fuel atoms within the target surface (no Be seeding)

How many gas atoms are in the surface during exposure?

Gross/Net Erosion – drastic differences in behavior of gross erosion during Be seeding indicates the simple theory regarding the benefits of prompt

redeposition may need to be revisited10 times more influx is needed to balance erosion and force net erosion to zero

Low sticking probability or high re-erosion possible explanations

PISCESSlide23

Be seeding fraction (i.e. influx) is at most in the percent range,

so Be self-sputtering and Be reflection can be neglected