24P086-P
Effects of Implanted Helium on Radiation Damage in Tungsten by Heavy ions
タングステンにおける重イオン照射損傷に与える注入ヘリウムの効果
Naoki Futagami, Hideo Watanabe, Naoaki Yoshida, Yuji Hatano, Qiu Xu
二神直樹, 渡辺英雄, 吉田直亮, 波多野雄治, 徐虬Interdisciplinary Graduate School of Engineering Sciences of Kyushu University
Kasugakouen6-1, Kasuga, Fukuoka 816-8580, Japan
九州大学総合理工学府 〒816-8580 福岡県春日市春日公園6-1
Helium is considered to affect irradiation damage by neutrons, because it has strong interaction withdefects. In the present work, therefore, synergistic irradiation effects of low energy helium and high energy
heavy ions in tungsten was studied by comparing damages with helium irradiation, heavy ion irradiationand the simultaneous irradiation. In case of the simultaneous irradiation, small helium bubbles were formed
preferentially near the incident surface because, implanted helium was trapped in vacancy clusters andvoids formed by heavy ions irradiation.

1. Introduction
In a fusion reactor, plasma-facing materials arebombarded with 14MeV-neutrons and particles(hydrogen isotope, helium) with high flux. It isknown that the former cause displacement damage
and nuclear reaction the latter cause various phenomena in materials. In particular, helium will affectirradiation damage with neutrons because helium
diffuses easily in metallic materials and has strong
interaction with lattice defects. Though these irradiations occur simultaneously, their synergistic
effects have not been investigated extensively so far.
The objective of the present work is, therefore, tomake clear the synergistic irradiation effects of low
energy helium and high energy heavy ions intungsten, which is a promising candidate of PFMs,
by comparing damages with helium irradiation,
heavy ion irradiation and the simultaneous irradiations.
2. Experimental Procedures
Tungsten sheet (0.1mm-thick) of nominal purity of99.95% was used. Samples for TEM observation
were manufactured as following. Disks of 3mm
were punched out from the sheet, and heated for 15
minutes at about 2273K in a good vacuum torecrystallize. These samples are almost defects free.
Samples for analysis of depth distribution of helium
were prepared as following. Platelets of 12×12mm
were made by cutting the sheet and heated for 3
hours at 1273K to release residual stress.
Conditions of helium irradiations are following;
irradiation energy is from 0.25keV to 5keV,
irradiation temperature from room temperature to1073K, and irradiation dose from 1020 to1021He+/m2. On the other hand, those of heavy ion(Cu2+) irradiations are following; irradiation energyis 2.4MeV, irradiation temperature from room
temperature to 1073K, and irradiation dose from0.01 to 1dpa. Calculation using TRIM-code
indicates that displacement damage distributes up toabout 600nm with a peak at 300nm. The irradiation
conditions were selected by taking into account thecircumstances of fusion reactors. Microstructures ofthe samples were observed by using TEM and FIB,
and depth distribution of helium was analyzed by
GD-OES.
3. Results and Discussions
3.1 Heavy ions irradiation
Fig.1 shows a typical TEM image of a crosssectional sample irradiated with 2.4MeV Cu2+ to 1
× 1019 Cu2+/m2 at room temperature. Small
dislocation loops and their clusters arraying wereformed. They distributed up to 1000nm, which ismuch deeper than that estimated by the TRIM-code.
Density of the interstitial loops is about two ordersof magnitude higher than that estimated by using a
rate theory, in which interstitial loops are assumed
to be nucleated by combining two free interstitials.
This means that most of the dislocation loops mustbe nucleated by cascade collisions. Furthermore,each loop can not grow larger individually butaligned loops grow by coalescing. It is worth tonote that nano-voids were also densely formed near
the incident surface.
3.2 He irradiation
Fig.2 shows cross-sectional TEM image and depth
distribution of helium in tungsten irradiated with
0.25keV to 2.0×1021He+/m2 at 1073K. In regard tograph of analysis by GD-OES, horizontal axis is
depth from the incident surface and vertical axis isratio of helium and tungsten (He/W) which iscalculated on the assumption that all of theimplanted helium stay in tungsten. In TEM image,helium bubbles are observed up to a depth of
100nm. In contrast, GD-OES also detects helium up
to a depth of 100nm. The region that heliumbubbles exist in TEM image corresponds well withdepth distribution from these results. In addition,large helium bubbles with tens of nm are observedin TEM image. It is considered that they becomelarger by migration and coalescence of small
helium bubbles because they become mobile at1073K. Furthermore, because concentration ofhelium decreases in large bubbles, He/W in theregion where large bubbles exist is small in comparison with region that dense small bubbles exist.
3.3 Simultaneous irradiation
Fig.3 shows depth distribution of helium intungsten irradiated with 0.25keV-He+ and 2.4
MeV-Cu2+ to 2.0×1021He+/m2 and 1×1019 Cu2+/m2 at 1073K simultaneously and that of helium intungsten irradiated by helium alone. The lower
graph shows detail at shallow region less than
100nm in depth. In case of the simultaneousirradiation, helium is detected greater than helium
only irradiation in neighborhood of surface. This
indicates that small helium bubbles were formed
because implanted helium was trapped in vacancy
clusters and voids induced by heavy ions irradiation.
In addition, in case of simultaneous irradiation,
profile is not completely flat in deeper region. This
indicates that helium may be trapped up to detection
limit by GD-OES.
In the presentation, not only analysis by GD-OESof the simultaneous irradiation but also TEM
images of that will be presented.

Fig.2 TEM image and depth distribution of
He of W irradiated by He+

Fig.3

Fig.1 TEM image of W irradiated by Cu2+

depth distribution of He ofsimultaneous and He irradiation