366 / 2019-02-28 09:00:56

Ultrafast measurements of ion temperature in high-energy-density plasmas by nuclear resonance fluorescence

Nuclear resonance fluorescence,High-energy-density plasmas,Ion temperature

全体主题 > Fundamental Physics at Extremes

Diagnosing high-energy-density plasmas is a challenge for inertial confinement fusion (ICF), magnetic confinement fusion and experimental astrophysics. Especially for few-neutron-yielding plasmas and almost fully ionized plasmas, the two main diagnosing methods, namely neutron time-of-flight diagnostics and X-ray line spectroscopy, will be difficult to work.
The talk will introduce a new method to diagnose the high-energy-density plasmas. Using a gamma ray to excite the nuclear energy levels, characteristic gamma line emission (nuclear resonance fluorescence, NRF) will be generated. The Doppler broadening of NRF spectra depends on the ion temperature of the plasmas, thus it can be used as a diagnosing method. Diagnosing plasmas by NRF is actively because it actively excites the nuclear energy levels, while other two methods rely on the plasmas themselves to emit neutrons or X-ray lines.
As a demo, the resonance energy level of 6Li at 3.56 MeV was excited to generate NRF photons by Geant4 simulation. For 6Li plasmas with an areal density of 1 g cm-2 and an ion temperature of 10 keV, a collimated gamma beam with spectral density 100 photons/keV at 3.56 MeV can generate one NRF photon and has an emission spectral width of approximately 6 keV for NRF spectra. Both quasi-monoenergetic gamma-rays from Compton-Scattering sources and bremsstrahlung sources can be used to excite NRF, when a proper detection angle (for example, >90° from the incident direction of gamma rays) is adopted to mitigate the continuous gamma background induced by Compton scattering of incident gamma rays in the plasmas.
Since ultrafast gamma rays can be generated by intense laser, this method could be used to probe the ultrafast dynamic change of the ion temperature. In ICF, The confinement time is of the order of picoseconds and the target size is less than 1 mm. Thus a collimated gamma-ray pulse with brilliance exceeds 1014 photons/s/mm2/keV at the resonance energy of 3.56 MeV is required to generate one NRF photon. The challenge of that how to measure the ultrafast NRF gamma spectrum can be solved by gamma tracking arrays such as AGATA and GRETA.