Microstructures and Deuterium-Retention Behavior of Tungsten Exposed to D+(He and/or Be) Mixture Plasmas

Abstract

ITER-relevant mixed species (D, He, Be) plasma exposure experiments on microstructural changes and their impacts on D retention behavior in W are reviewed. It was observed that seeding of He into pure D plasma resulted in a significant reduction of D retention and suppression of surface blistering. TEM observations and ellipsometric measurements revealed that nano-sized high-density He bubbles were formed and percolated in the near surface region. Based on the experimental results, a mechanism to explain the reduced D retention was proposed, i.e., implanted D atoms can easily diffuse back to the surface through the percolating bubbles and escape from the surface during plasma exposure. Additional Be seeding to D+He mixture plasma suppressed this He seeding effect. Thus, it is considered that Be seeding has a more dominant influence than He seeding on microstructures and D retention in plasma-exposed W.

Hydrogen isotope transport across tungsten surfaces exposed to a fusion relevant He ion fluence

“…At ∼500 K, the typical nanobubble size is reported to be ∼1-2 nm, as inferred from x-ray scattering [13], and the measured He void fraction in the layer is ∼20% [16]. It is under just such conditions that the He affected surface layer gives rise to the reduced inventory effect, and it is well documented for D 2 −He plasma exposure [2,10,11,15,16] with as little as percentile fractions of He ions, as well as in sequential He, followed by D 2 , plasma exposure [10,15,17], once the impacting He + fluence surpasses ∼10 25 m −2 [10].…”

“…Thus, when implanted, precipitation occurs in the vicinity of the stopping range, and with sufficient implanted fluence, bubble growth [4,5] proceeds from accumulation in preexisting traps [6,7], impurity locations [8], and even sites of self encounter [8,9]. The He is energetically bound (several eV) [6][7][8] and has been observed as nanoscale sized bubbles in W surfaces exposed to ionized He in a variety of experiments utilizing plasmas [2,[10][11][12][13] and ion beams [12,14].…”

“…In plasma studies (incident He + below 100 eV), near surface He nanobubble layers of ∼30 nm in thickness [11,13,15] are observed over a wide range of ion fluence and exposure temper ature. At ∼500 K, the typical nanobubble size is reported to be ∼1-2 nm, as inferred from x-ray scattering [13], and the measured He void fraction in the layer is ∼20% [16].…”

In situ measurements of low energy D plasma-driven permeation through He pre-damaged W

“…It is well known that He, which is the burning ash of D-T fusion reaction, will modify the surface morphology of W and affect the permeation and retention behavior of D in W [8][9][10][11][12][13][14][15][16]. Studies [8][9][10] have shown that fuzz-like structures can be formed on the W surface when the sample temperature is between 1000-2000 K, He ion energybetween 30 eV to 250 eV and He ion flux over 10 23 m −2 s −1 .…”

“…Studies [8][9][10] have shown that fuzz-like structures can be formed on the W surface when the sample temperature is between 1000-2000 K, He ion energybetween 30 eV to 250 eV and He ion flux over 10 23 m −2 s −1 . At lower sample temperature and ion flux, He irradiation can also induce bubbles on the W surface [11][12][13]. In the meantime, holes appear on the W surface when the temperature is over 2000 K [14].…”