1. Investigate Laser induced desorption (LID) of hydrogen retained in co-deposited layers on JT-60 open-divertor tile 20 ps-pulsed Nd:YAG laser for wide laser-intensity I L regions (3 orders of magnitude) with  = 1064 nm (  ) and 266 nm (4  ) emission study properties of desorbed gases & emitted particles to find optimum laser condition (I L & ) for LID Purpose 7th ITPA meeting on SOL/divertor physics, November 6-9, 2006 Removal of redeposited carbon layers by laser ablation Y. Sakawa, ILL. Osaka Univ. D. Watanabe Nagoya Univ. K. Sugiyama, T. Tanabe, Kyushu Univ.

2. Ablation Sublimation of C (3800K) ■ Thermal desorption of H 2 Ionization (C + : 11.26 eV) Multi-photon ionization (3-photon process of 266nm) enhancing release of ions, surpressing : neutrals Inverse bremsstrahlung absorption:  ↑ ⇒ increase Laser Intensity ( =266nm ） 10 W/cm 2 10 11 W/cm 2 Non-ablation Weak-ablation Strong-ablation Three regimes are distinguished ■ Bond breaking by laser photon :  ↑ ⇒ decrease Bonding energy : C-H (4.5 eV), C=C (7.24 eV) Photon energy : 266 nm (4.66 eV) 1064 nm (1.17 eV)

3. Y. Hirohata, et al., J. Nucl. Mater. 337-339, 609-613 (2005) Tile Co-deposits Plasma Co-deposits on JT-60 open-divertor tile

4. ■ Sample JT-60 open-divertor tile Exposed to 1800 H 2 discharge (June ~ October 1988) H/C ~ 0.03 (1.4 x 10 21 atoms m -2 mm -1 ) Thickness of co-deposits : 35 ~ 40 mm ■ Laser Nd:YAG laser (Continum Custom PY61C-10) Pulse duration : 20 ps Repetition rate : 10 Hz Wave length : 266 nm (4  ), 1064 nm (  ), Energy : < 3 mJ/pulse (4  ), 35 mJ/pulse (  ) Intensity : < 9 x 10 11 W/cm 2 (4  ), 6 x 10 12 W/cm 2 (w) ■ Measurements Emitted ions : Time-of-flight mass spectrometer (TOFMS) Desorbed gases : Quadrupole mass spectrometer (QMS) Vissible light emission: optical spectroscopy Ablation spot size : Scanning electron microscope (SEM) Ablation depth : Optical microscope Experimental setup Vacuum pressure < 3 x 10 -8 Torr

5. Beer’s low : I L : Laser intensity I ablation : Ablation threshold  ： Absorption coefficient  d = ln 1 I L  I ablation Ablation threshold ~10GW both for 266nm and 1064 nm. Ablation depth is larger for 1064 nm with the identical I L. 266 nm:  =35  m -1 1064nm:  =15  m -1 Non- ablation region

6. Weak-Ablation Region (WAR 266 ) I ablation < I L < I ionization I ionization = 9 x 10 10 W/cm 2 Strong-Ablated Region (SAR 266 ) I ionization < I L Three regimes for 266 nm laser I ablation = 9 x 10 9 W/cm 2 Non-Ablation Region (NAR 266 ) I L < I ablation I ablation I ionization Ablation threshold Ionization threshold CII

7. SAR 266 : Emission from C 2, C, C +, and C 2+ WAR 266 : Emission from C 2 Visible emission SAR 266 WAR 266

8. SAR 266 : C +, C 2+ ions are emitted WAR 266 : Carbon clusters (C n + ) are emitted TOFMS SAR 266 WAR 266

9. Weak- ablation region Strong- ablation region Non- ablation region Weak- ablation region Strong- ablation region Non- ablation region Ionization Energy of C = 11.26 eV 266 nm Photon Energy = 4.66 eV ⇩ 3-Photon Ionization 1064 nm Photon Energy = 1.17 eV ⇩ Need 11 photons I ionization is lower for 266 nm owing to 3-photon ionization. I ablation I ionization I ablation I ionization 226nm 1064m

10. 266 nm : Number of desorbed H 2 / Spot size increases by increasing I L. 10Hz laser irradiation H 2 desorption rate Spot size Integrate over 30 s (300 shot) Weak- ablation region Strong- ablation region Number of desorbed H 2 Spot size

11. N retained : Hydrogen atoms retained in the ablated volume using density of hydrogen in co-deposits ~ 1.4 x 10 21 atom / m 2  m [Hirohata et al., J. Nucl. Mater. 337-339 (2005) 60.] N desorbed : Number of desorbed hydrogen H 2 desorption efficiency is larger for 266 nm because of bond-breaking & ionization by laser photons Thermal desorption from surrounding region

12. C 2 H 2 production rate for 266 nm: Increases by increasing I L in Weak-ablation region Decreases by increasing I L in Strong-ablation region C 2 H 2 production rate = Number of desorbed C 2 H 2 Number of desorbed H 2

13. Conclusions LID of hydrogen retained co-deposits on JT-60 open-divertor tile Ablation by 266 nm and 1064 nm of a 20 ps-Nd:YAG laser are compared 1) Ablation properties ◆ Three regions of ILD were distinguished (NAR, WAR, SAR) ◆ I ablation was nearly identical for 266 nm and 1064 nm. ◆ Co-deposits can be removed faster for 1064 nm. ◆ I ionization was lower for 266 nm because of effective 3-photon ionization. 2) Hydrogen desorption properties ◆ Hydrogen-removal efficiency was largest in SAR and larger for 266 nm ◆ C 2 H 2 production rate decreased by increasing I L in SAR for 266 nm For the ablative removal of hydrogen, a short-wavelength and high-power laser irradiation is desirable.

14. 266 nm: Desorption of H 2, C n H m by laser irradiation Desorption of H 2 O is small.

15. 266 nm : Desorption of H 2 per shot  I[H2] is expressed by fast- and slow-decay processes Δ I[H 2 ] = Δ I fast exp(- t /  fast ) + Δ I slow exp(- t /  slow ) Δ I[H2] QMS

16.  slow ～ 500 s 266 nm  I fast decreases in SAR,  fast increases in SAR  I slow proportional to I L,  slow is independent on I L SARWAR  fast QMS

17. 1064 nm ： Desorbed H 2 decreases faster than 266 nm ⇨ because of larger ablation depth 10Hz laser irradiation Δ I[H2] Desorbed H 2 per laser shot QMS