EUV irradiation of exoplanet atmospheres occurs on Gyr time-scales

ABSTRACT Exoplanet atmospheres are known to be vulnerable to mass-loss through irradiation by stellar X-ray and extreme-ultraviolet (EUV) emission. We investigate how this high-energy irradiation varies with time by combining an empirical relation describing stellar X-ray emission with a second relation describing the ratio of solar X-ray to EUV emission. In contrast to assumptions commonly made when modelling atmospheric escape, we find that the decline in stellar EUV emission is much slower than in X-rays, and that the total EUV irradiation of planetary atmospheres is dominated by emission after the saturated phase of high-energy emission (which lasts around 100 Myr after the formation of the star). The EUV spectrum also becomes much softer during this slow decline. Furthermore, we find that the total combined X-ray and EUV emission of stars occurs mostly after the saturated phase. Our results suggest that models of atmospheric escape that focus on the saturated phase of high-energy emission are oversimplified, and when considering the evolution of planetary atmospheres it is necessary to follow EUV-driven escape on Gyr time-scales. This may make it more difficult to use stellar age to separate the effects of photoevaporation and core-powered mass-loss when considering the origin of the planet radius valley.

Time-resolved studies of low-temperature, EUV-induced plasmas: EUV emission in selected spectral ranges

Interaction of extreme ultraviolet (EUV) pulses of high intensity with gases results in the creation of non-thermalized plasmas. Energies of the driving photons and photoelectrons are sufficient for creation of excited states, followed by emission of the EUV photons. In most cases, decay times of these states are short comparing to the driving EUV pulse. It means that just after stopping of the driving pulse, the EUV emission corresponding to the excited states should also stop. From our earlier measurements in the optical range, however, it can be concluded that lifetimes of such plasmas exceed a time duration of the driving pulse even two orders of magnitude. Hence, it can be expected that the time duration of the EUV emission can be also significantly longer than the irradiation time. In this work, EUV-induced, low-temperature helium (He), krypton, and xenon plasmas were investigated. EUV emission from these plasmas was studied, using a specially prepared detection system, allowing for time-resolved measurements, in selected spectral ranges. The detection system was based on a paraboloidal collector and a semiconductor photodiode, sensitive for the EUV photons. For spectral selection, the corresponding filters or multilayer mirrors were employed. In most cases, the time duration of the EUV emission was significantly longer than the driving EUV pulse. In case of He plasmas, the emission corresponding to excited atoms was detected even hundreds of nanoseconds after the irradiation. It was also shown that the corresponding time profiles depended on densities of gases to be ionized.

Influence of Pre-Ionized Plasma on the Dynamics of a Tin Laser-Triggered Discharge-Plasma

The effect of laser-current delay on extreme ultraviolet emission by laser-triggered discharge-plasma has been investigated. Typical waveforms for current, voltage, laser signals, and X-ray signals have been compared. Theoretical tin spectra were simulated among the electron temperature ranges from 10 to 50 eV to compare with the experimental results. The results show that longer laser-current delay time is propitious to increase the steady-state time of plasma at high temperatures, and it increases the intensity and spectral purity of 13.5 nm emission in 2% band. The 13.5 nm radiation intensity increases about 120% with the delay increasing from 0.7 to 5 μs, and the extreme ultraviolet (EUV) emission conversion efficiency (CE) increases from 0.5% to 1.1%.

Time-resolved measurements of extreme ultraviolet (EUV) emission, from EUV-induced He, Ne, and Ar plasmas

Irradiation of gases with intense pulses of extreme ultraviolet (EUV) can result in the formation of low-temperature plasmas. During the time of irradiation, various non-thermal processes driven by the EUV photons and photoelectrons take place, leading to the creation of excited states of atoms and ions. Fast relaxation of these states should result in EUV emission within a time comparable to the driving EUV pulse. On the other hand, from our earlier works, a time duration of the emission in an optical range is over an order of magnitude longer. It can be thus expected that the time of EUV emission can be also relatively long. In this work, time-resolved measurements of the EUV emission from low-temperature plasmas induced in He, Ne, and Ar gases were performed. Due to a low intensity of the emitted radiation, a specially prepared detection system, based on an EUV collector and an EUV sensitive photodiode, was employed. In all cases, a time duration of the EUV emission was much longer compared with the driving EUV pulse. Time profiles of the corresponding signals were specific for particular gases. In case of He and Ne plasmas, these time profiles varied with initial densities of gases to be ionized. The corresponding dependence was especially visible in case of plasmas induced in helium. In case of Ar plasmas, such dependence was not revealed.

Эмиссионные свойства лазерной плазмы при ее возбуждении на молекулярно-кластерных струях углекислоты

The paper presents the results of studies of a gas-jet laser-plasma source of extreme ultraviolet radiation (EUV) based on a conical supersonic nozzle. Molecular cluster CO2 jets were used as targets. By changing the parameters of the gas at the nozzle entrance, various modes of gas outflow were obtained, the influence of these modes on the emission and technical characteristics of the radiation source was studied. An experimentally shown is an increase in EUV emission with an increase in the amount of clustered matter in the jet. A possible relationship was observed between the increase in the intensity of EUV radiation with an increase in the number of clustered matter in the jet.

Temporal measurements of extreme ultraviolet (EUV) emission, from low temperature, EUV-induced plasmas

In this work, extreme ultraviolet (EUV) emission, from EUV induced, low-temperature microplasmas, were investigated. To perform temporal measurements of the EUV pulses of low intensity, in a medium vacuum, under the pressure of the order of 0.1–0.01 mbar a special detection system was prepared. The system was based on an EUV collector and a semiconductor detector, sensitive for the EUV photons. The collector consisted of two identical grazing incidence, paraboloidal mirrors, and allowed to focus a part of the radiation emitted from the microplasma onto the detector surface. An absorption filter, mounted between the collector and the detector, allowed for selection of an interesting wavelength range. Plasmas were created by irradiation of small portions of gases, injected into the vacuum chamber, using a laser produced plasma EUV source. Three gases were used for the EUV induced plasma formation: neon, krypton, and xenon. Low-temperature plasmas, created in these gases, contained multiply charged ions, emitting radiation in similar wavelength ranges. Two detectors, AXUV20HS1 and AXUVHS5, were used for the measurements. It was shown that differences between the corresponding signal profiles, obtained using both detectors, were not very significant. Moreover, it was demonstrated that the duration of the EUV emission from plasmas, created in different gases, were comparable with the duration of the driving EUV pulse. The longest EUV emission was observed for Kr plasmas, approximately 1.5 times the full width half maximum of the driving EUV pulse.