Carbon materials are used as plasma facing materials in ITER and future D-T fusion reactors. Because of large tritium retention in carbon, a significant fraction of tritium is expected to retain in the vacuum vessel as co-deposits with carbon, and it is necessary to remove tritium periodically. One of the methods to remove tritium is the laser ablation, in which high-intensity laser beam is irradiated. In this paper, we have investigated the pulsed laser ablation of the graphite target, in which 10 keV hydrogen ion are implanted, and examined effects of laser wavelength and intensity on the ablation.
An ArF excimer laser (Lambda Physik OPTex, wavelength: λ = 193 nm, energy: E_L < 13 mJ/pulse, pulse length: τ∼8 ns, intensity: I_L < 2x10⁹ W/cm², repetition rate: 5 Hz) and Nd:YAG laser (Continuum Custom PY61C-10, λ = 266 nm, energy: E_L < 3 mJ/pulse, τ∼20 ps, I_L < 2x10¹¹ W/cm², repetition rate: 10 Hz) were irradiated on the target with the incident angle of 45° through a focusing lens of 300 mm in focal length. Emitted ions and desorbed gases were measured by a time-of-flight mass spectrometer (TOFMS) and a quadrupole mass spectrometer (QMS), respectively. Visible light emissions were also monitored by a spectrometer (Hamamatsu C7473).
When the ArF laser of I_L∼10⁹ W/cm² was irradiated, H₂, CH₄, C₂H₂, C₂H₆, were the dominant molecules desorbed, and mostly disappeared within 30 shots indicating removal of implanted hydrogen. In TOFMS, the mass number of ions was widely distributed with averaged mass number of ∼200 for initial few shots and of ∼80 after 30 shots. In the visible emission spectra, only C₂ Swan band was observed. When 266 nm YAG laser of I_L∼10¹¹ W/cm² was irradiated, in contrast, the ion mass of TOFMS was mostly less than 40, and the emissions of carbon atoms and ions like CI, CII, and CIII were dominant. Thus the ablation phenomena were significantly varied with laser characters. Changing the intensity of the laser pulse, the details of the ablation mechanism of the hydrogen-implanted graphite will be discussed.