Speaker
Description
Dust in D-T fusion reactors will be radioactive due to interactions with 14MeV neutrons and retention of tritium, which undergoes β-decay into helium with a 12.32yrs half-life. Tritium radioactive decay releases 18.6keV partitioned between an electron and anti-neutrino. The mean electron kinetic energy of 5.7keV suffices to traverse 200nm of W, implying that tritiated W dust constantly emits electrons.
It has been documented that tritiated carbon dust collected from TFTR post D-T operation exhibited unusual mobility, levitated in presence of electrostatic fields and even spontaneously boiled [1]. Many consequences of tritiated dust self-charging have been conjectured [2]. Nevertheless, even very recent Monte Carlo simulations of β-decay induced self-charging have been oversimplified [3]. They were based on the EmOpt4 Geant4 package that is incapable of treating secondary electron emission due to beta electrons and, thus, significantly underestimate self-charging.
We performed Monte Carlo simulations of the self-charging rate of tritiated W dust with the MicroElec Geant4 extension that is tailor made for the relevant electron energy range [4], since it utilizes single scattering algorithms, employs the dielectric formalism that treats secondary electron generation due to electron excitation & plasmon decay and considers reflection of internal electrons from the surface potential barrier. Moreover, we empirically modified the surface barrier so that it reproduces reliable electron emission data at normal incidence and validated the updated model against reliable electron emission data at oblique incidence. Finally, we introduced a hybrid physics list implementation that smoothly combines MicroElec for <7keV with EmOpt4 for higher energies circumventing practical limitations in MicroElec’s applicability.
Self-charging rates are evaluated for different W dust radii after averaging over the uniform angular distribution & known energy spectra of the beta electrons and over experimental tritium depth profiles. Regardless of dust size, an empirical expression is obtained for the depth resolved electron escape yield that stems from the elementary theory of secondary electron emission. Moreover, up to one-order-of magnitude differences are revealed between the MicroElec and EmOpt4 predictions. Finally, the effect of the magnetic field due to prompt electron re-deposition and the effect of positive dust charge due to Coulomb attraction on the self-charging rates is evaluated.
[1] C. Skinner et al., Fusion Sci. Technol. 45, 11 (2004).
[2] J. Winter, Phys. Plasmas 7, 3862 (2000).
[3] C. Grisolia et al., Nucl. Fusion 59, 086061 (2019).
[4] Q. Gibaru et al., Nucl. Instrum. Meth. Phys. Res. B 487, 66 (2021).