Member of the Helmholtz Association Carbon based materials: fuel retention and erosion under ITER-like mixed species plasma conditions Arkadi Kreter et al

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July CFC in ITER ITER divertor cassette mock-up Carbon Fibre Composite (CFC) foreseen for strike plates – regions of highest heat loads Tungsten Investigation of fuel retention in CFC for ITER- relevant conditions is necessary ITER-relevant divertor conditions High D flux (up to D ~10 24 m -2 s -1 ) High D fluence ( >10 26 m -2 for 1 ITER pulse at SP) Low incident energy (E i ~10 eV) Range of wall temperatures (T s = 500 K K) Impurities Beryllium from the main wall He from D-T reactions Ar for divertor cooling

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Exposures in PISCES-A/-B linear plasma devices Eroded Material PISCES (-A, -B) schematic view Steady-state plasma n e = (2-3)·10 18 m -3 ; T e = 7-15 eV = (3-6)·10 22 D/m 2 s Variations of: E i = eV = 1· ·10 26 D/m 2 (~1 ITER pulse at strike point) T s = 370 K ('cold' ITER wall) K (ITER strike point) Controlled Be, He and Ar seeding All experiments in erosion-dominated conditions PISCES-A plasma and target Ex-situ analysis of samples for retention Thermal desorption spectrometry (TDS) Nuclear reaction analysis (NRA) with 3 He beam B Optical spectroscopy as the main tool to quantify carbon erosion

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Wide database for carbon materials exposed to pure D plasma Retention [m -2 ] Ion fluence [m -2 ] NB41 PISCES-A N11 PISCES-A NB31 TEXTOR DMS780 TEXTOR EK98 TEXTOR ATJ PISCES-A Retention vs incident D fluence, exposures at T s = 470 K, E i = 120 eV Retention [D/m 2 ] Ion fluence [D/m 2 ] Total D retention for exposures at different temperatures in PISCES-A and -B [1] R. Pugno et al., JNM 375 (2008) 168 Saturation NB41 Ts=370K NB41 Ts=470K NB41 Ts=820K DMS701 Ts=1070K [1]

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Retention in CFCs and FGGs for pure D plasma In-bulk retention is similar in different CFCs and fine-grain graphites CFCs and fine-grain graphites are porous In-bulk retention is higher for lower exposure temperatures Additional trapping sites at lower temperatures Higher population of available trapping sites at lower temperatures In-bulk retention scales as fluence with depending on temperature: <~0.5 for low T s (surface diffusion along pores) =0 for T s >~800K – saturation of retention for < D/m 2 (few sec of ITER pulse) In-bulk retention is higher for higher incident ion energies Higher D concentration in implantation layer [Staudenmaier JNM79] leads to higher D amount in bulk In-bulk retention is higher for lower fluxes Amount of diffused deuterium t, longer time available for D to diffuse in for the same fluence

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Influence of Be on retention D2 TDS spectra for ATJ exposed w/o and with Be (T s =720K, E i =35 eV) Be carbide layer appears to prevent increase of retention with fluence Sensitive to exposure parameters Total Deuterium Retention With Be, =0.5e26 D/m 2 before Be, =2e26 D/m 2 total: 1.9e21 D/m 2 Pure D, =2e26 D/m 2 : 2.3e21 D/m 2 Pure D, =0.5e26 D/m 2 : 1.6e21 D/m pure D =0.5e26m -2 pure D =2e26m -2 Be containing plasma D 2 desorption flux [x10 19 D/m 2 s] Temperature [K] Scenario of Be experiment 1.Establishing background plasma ( =0.5e26 D/m 2 ) 2.Be injection from oven (total =2e26 D/m 2 ) 0.5 K/s

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Influence of Be+He on retention – low energy case D2 TDS spectra for ATJ exposed to pure D, D+Be, D+Be+He (T s =720K, E i =35 eV, f He =16%) He appears to change the retention mechanism and reduce retention for low incident energies Total Deuterium Retention D+Be, =0.5e26 D/m 2 before Be, =2e26 D/m 2 total: 1.8e21 D/m 2 Pure D, =0.5e26 D/m 2 : 1.6e21 D/m 2 D+Be+He, =0.4e26 D/m 2 before Be, =1.7e26 D/m 2 total: 0.5e21 D/m K/s

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Influence of Be+He on retention – high energy case D2 TDS spectra for ATJ exposed to pure D, D+Be, D+Be+He (T s =720K) E i =80 eV vs E i =35 eV No effect of reduced retention due to He at high incident energy Retention is higher presumably due to higher incident energy Total Deuterium Retention D+Be, =0.5e26 D/m 2 before Be, =2e26 D/m 2 total: 1.8e21 D/m 2 Pure D, =0.5e26 D/m 2 : 1.6e21 D/m 2 D+Be+He, =0.4e26 D/m 2 before Be, =1.7e26 D/m 2 total: 0.5e21 D/m 2 D+Be+He, =0.3e26 D/m 2 before Be, =1.2e26 D/m 2 total: 3.6e21 D/m K/s

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Influence of Be+Ar on retention D2 TDS spectra for ATJ exposed to pure D, D+Be, D+Be+Ar (T s =720K, E i =35 eV, f Ar =10%) Ar appears to change the retention mechanism and reduce retention for both low and high incident energies Total Deuterium Retention D+Be, =0.5e26 D/m 2 before Be, =2e26 D/m 2 total: 1.8e21 D/m 2 Pure D, =0.5e26 D/m 2 : 1.6e21 D/m 2 D+Be+Ar, =0.1e26 D/m 2 before Be, =0.5e26 D/m 2 total: 0.8e21 D/m 2 D+Be+Ar, =0.1e26 D/m 2 before Be, =0.5e26 D/m 2 total: 0.5e21 D/m K/s

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Influence of impurities on retention in carbon materials 1.Incident deuterium ions saturate the implantation layer (~ nm) 2.The level of saturation is defined by the balance between adsorption and ion-induced desorption for a given number of available trapping sites 3.From the implantation layer, D 'diffuses' further in-bulk along surfaces of the pores With addition of Be no further increase of in-bulk retention Be carbide layer appears to suppress the in-bulk penetration of deuterium (Be 2 C layer thickness is a few 100 nm) However, Be itself can cause fuel uptake He and Ar impurities decrease the in-bulk retention for certain exposure conditions Presumably due to depletion (ion-induced detrapping) of the implantation layer, from where it otherwise moves deeper in the bulk

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Erosion of carbon by mixed species plasma: background Attributed to build up of Be carbide surface layer CD band light emission in front of FGG target as a measure of chemical erosion Scaling [Nishijima JNM 2007] obtained for D plasma with various c Be, T s, E i, I How the addition of other impurities (Ar, He) would influence the effect of reduced erosion? Reproduce the experiments from [Nishijima JNM 2007] with addition of Ar and He Chemical sputtering of carbon materials due to combined bombardment by ions and atomic hydrogen [W. Jacob et al, Phys. Scr. T124 (2006) 32] Chemical sputtering: Simultaneous interaction of low-energy ions and atomic hydrogen It causes a significantly higher erosion than the sum of the individual processes - chemical erosion due to atomic hydrogen alone and physical sputtering due to ions Does it mean that impurities in ITER (e.g. Ar) will cause enhanced erosion? Check for the effect of Ar and He on erosion of graphite target in PISCES Mitigation of chemical erosion by Be seeding [M.J. Baldwin, R.P. Doerner, Nucl. Fusion 46 (2006) 444, D. Nishijima et al, J. Nucl. Mater. 363–365 (2007) 1261]

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Ar and He do not influence C erosion significantly Initial value (before Be injection) similar Chemical sputtering due to Ar not visible Decay time similar with and without Ar No influence on Be carbide build up Addition of Argon (and Helium) does not appear to affect carbon erosion Normalized CD light emission (T s =700K, E i =35 eV, f Ar =10%) Similar behaviour w/ and w/o Ar also for high Ei; Also the case for He injection instead of Ar

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Retention in CFCs and FGGs for pure D plasma Comparison of CD band decay during Be seeding onto C target: scaling [Nishijima JNM 2007] obtained w/o Argon experiment with additional Ar seeding (~10%) High ion energy case Low ion energy case CD band decay time similar with and without argon Argon (and Helium) does not appear to affect carbon erosion

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Extensive database on retention in CFC/FGG Analysed materials: CFC NB41 (new EU ITER grade) [1] CFC NB31 (former ITER grade) [2] CFC DMS780 (JET) [2] CFC DMS701 (ASDEX Upgrade) [3] CFC N11 (Tore Supra) [4] fine-grain graphite (FGG) EK98 (i.e. TEXTOR) [2] FGG IG-430U (ALT-II TEXTOR) FGG ATJ (DIII-D) [1] Exposures in: PISCES-A/B [1,3,4], TEXTOR test limiter [2], TEXTOR ALT-II main limiter [1] A. Kreter et al., PFMC-12, Jülich 2009 [2] A. Kreter et al., J. Phys.: Conf. Series 100 (2008) [3] R. Pugno et al., J. Nucl. Mater. 375 (2008) 168 [4] J. Roth et al., J. Nucl. Mater. 363–365 (2007) 822

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Retention in NB41: Fluence dependence M=4 (D 2 ) desorption spectra (T s = 470 K, E i = 120 eV) =50e25 D/m 2 10e25 3e25 1e K Retention [D/m 2 ] Ion fluence [D/m 2 ] NB41 PISCES-A [1] N11 PISCES-A [2] NB31 TEXTOR [3] DMS780 TEXTOR [3] EK98 TEXTOR [3] Total D retention for exposures at T s = 470 K, E i = 120 eV No saturation up to = D/m 2 ATJ PISCES-A [1] [2] A. Kreter et al., J. Phys.: Conf. Series 100 (2008) [1] J. Roth et al., J. Nucl. Mater. 363–365 (2007) 822 Similar behaviour for different CFCs and fine-grain graphites 0.5 K/s [1] A. Kreter et al., PFMC-12, Jülich 2009

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July D 2 desorption flux [x10 19 D/m 2 s] Temperature [K] T s =470 K T s =370 K T s =820 K 470 K 370 K 820 K Higher retention for lower exposure temperatures due to additional trapping sites and higher population M=4 (D 2 ) desorption spectra (E i = 120 eV, = 2.4e26 D/m 2 ) Retention [D/m 2 ] Ion fluence [D/m 2 ] Total D retention for exposures at different temperatures in PISCES-A and -B Higher retention for lower T s Saturation for T s > ~800 K [1] R. Pugno et al., JNM 375 (2008) 168 Saturation NB41 Ts=370K NB41 Ts=470K NB41 Ts=820K DMS701 Ts=1070K [1] Retention in NB41: dependence on exposure temperature 0.5 K/s

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Retention in NB41: dependence on ion energy Higher retention for higher incident ion energy Total D retention vs E i ( = 1e26 D/m 2, T s = 470 K) M=4 (D 2 ) desorption spectra ( = 1e26 D/m 2, T s = 470 K) E i =20eV E i =50eV E i =120eV 0.5 K/s

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Retention in C: dependence on ion flux Data from PISCES and TEXTOR exposures compared to data from ion-beam facilities [J. Roth et al., JNM 363–365 (2007) 822] Ion beam data show higher retention than data from plasma devices Can be attributed to flux dependence (higher retention for lower fluxes) Typical ion fluxes Ion beam: m -2 s -1 PISCES, TEXTOR: ~ m -2 s -1

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Retention in NB41: D penetrates deep in bulk NRA depth profile of NB41 ( = D/m 2, T s =470 K, E i =120 eV) 18 at% of D at surface saturated implantation layer Penetration in bulk -NRA 2D mapping of D in NB31 exposed in TEXTOR for = D/m 2 at T s = 500 K Inhomogeneous penetration of D in bulk over tens of m A. Kreter et al., J. Phys.: Conf. Series 100 (2008) P. Petersson et al., PFMC-12, Juelich MeV 3 He + beam

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Summary and discussion: pure D plasma (I) In-bulk retention is similar in different CFCs and fine-grain graphites CFCs and fine-grain graphites are porous In-bulk retention is higher for lower exposure temperatures Additional trapping sites at lower temperatures Higher population of available trapping sites at lower temperatures In-bulk retention scales as fluence with depending on temperature: <~0.5 for low T s (surface diffusion along pores) =0 for T s >~800K – saturation of retention for < D/m 2 (few sec of ITER pulse) 1.Incident deuterium ions saturate the implantation layer (~ nm) 2.The level of saturation is defined by the balance between adsorption and ion-induced desorption for a given number of available trapping sites 3.From the implantation layer, D 'diffuses' further in-bulk along surfaces of the pores

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Summary and discussion: pure D plasma (II) 1.Incident deuterium ions saturate the implantation layer (~ nm) 2.The level of saturation is defined by the balance between adsorption and ion-induced desorption for a given number of available trapping sites 3.From the implantation layer, D 'diffuses' further in-bulk along surfaces of the pores In-bulk retention is higher for higher incident ion energies Higher D concentration in implantation layer [Staudenmaier JNM79] leads to higher D amount in bulk In-bulk retention is higher for lower fluxes Amount of diffused deuterium t, longer time available for D to diffuse in for the same fluence

Arkadi Kreter et al. EU-TF PWI SEWG on material migration and material mixing Culham 8 July Estimations of retention in ITER [Roth et al., PPCF 50 (2008) ] Estimated T inventory for initial wall material choice (C/W/Be) with CFC NB31 (CFC bulk retention scaled up from ion beam data) Estimated T inventory for different wall material choices NB41 has similar retention characteristics as other CFCs incl. NB31 Generally, estimations for retention in ITER done for NB31 still valid However, Roth et al overestimated retention in CFC significantly, because of neglecting the flux dependence and saturation at high temperatures Effects of impurities are also favourable, maybe even for co-deposition