Heating Of Low-energy Electrons Research Articles

Compton scattering of self-absorbed synchrotron emission

Synchrotron self-Compton (SSC) scattering is an important emission mechanism in many astronomical sources, such as gamma-ray bursts (GRBs) and active galactic nuclei (AGNs). We give a complete presentation of the analytical approximations for the Compton scattering of synchrotron emission with both weak and strong synchrotron self-absorption. All possible orders of the characteristic synchrotron spectral breaks ($\nu_{\rm a}$, $\nu_{\rm m}$, and $\nu_{\rm c}$) are studied. In the weak self-absorption regime, i.e., $\nu_{\rm a} < \nu_c$, the electron energy distribution is not modified by the self-absorption process. The shape of the SSC component broadly resembles that of synchrotron, but with the following features: The SSC flux increases linearly with frequency up to the SSC break frequency corresponding to the self-absorption frequency $\nu_{\rm a}$; and the presence of a logarithmic term in the high-frequency range of the SSC spectra makes it harder than the power-law approximation. In the strong absorption regime, i.e. $\nu_{\rm a} > \nu_{\rm c}$, heating of low energy electrons due to synchrotron absorption leads to pile-up of electrons, and form a thermal component besides the broken power-law component. This leads to two-component (thermal + non-thermal) spectra for both the synchrotron and SSC spectral components. For $\nu_{\rm c} < \nu_{\rm a} < \nu_{\rm m}$, the spectrum is thermal (non-thermal) -dominated if $\nu_a > \sqrt{\nu_m \nu_c}$ ($\nu_a < \sqrt{\nu_m \nu_c}$). Similar to the weak-absorption regime, the SSC spectral component is broader than the simple broken power law approximation. We derive the critical condition for strong absorption (electron pile-up), and discuss a case of GRB reverse shock emission in a wind medium, which invokes $\nu_{\rm a} > {\rm max} (\nu_{\rm m}, \nu_{\rm c})$.

Variation of the electron energy distribution with He dilution in an inductively coupled argon discharge

We present experimental evidence of different behaviors of plasma parameters depending on changes in the electron energy distribution (EED), caused by an electron heating mechanism and electron-neutral collision processes in an Ar/He mixture inductively coupled plasma. At a low gas pressure of 3 mTorr, where the electron neutral collision frequency νm is much smaller than the driving frequency ωRF, the EEDs evolved from a bi-Maxwellian distribution to a Maxwellian distribution, due to the efficient heating of low energy electrons when the He flow rate increased at a fixed total gas pressure. The plasma density slowly decreased with the He flow rate portion ([He]/[Ar] + [He]) in a range of 0%–70%, while the plasma density largely decreased in the He flow rate portion of 70%–100%. On the other hand, at a high gas pressure of 350 mTorr where νm ≫ ωRF, the EEDs evolved from a Druyvesteyn-like distribution to a Maxwellian distribution, due to a cooling of low energy electrons and an increase in the population of high energy electrons, when the mixing ratio of the He gas is increased. In this case, plasma density abruptly decreased for a He flow rate ratio of 0%–30%. This result directly shows that the EEDs significantly affect the different variations of plasma parameters, even in the case of the same mixing ratio of the gases.

Observation of collisionless heating of low energy electrons in low pressure inductively coupled argon plasmas

Collisionless heating of low energy electrons was observed in low pressure argon rf-biased inductively coupled plasmas (ICPs) by measurement of the electron energy distribution function (EEDF). When only capacitive power (bias) was supplied, the EEDF in the discharge was a bi-Maxwellian distribution with two electron groups. It was found that the low energy electrons were heated up significantly even with a little inductive power (&amp;lt;20 W) even when the discharge was in E mode. Due to the low gas pressure and low temperature of low energy electrons (close to the energy of the Ramsauer minimum), the collisional heating of low energy electrons appears to be negligible. Therefore, this effective heating of the low energy electrons showed a direct experimental evidence of the collisionless heating by inductive field. The significant heating of low energy electrons in E mode indicates that collisionless heating in the skin layer is an important electron heating mechanism of low pressure ICP even when the discharge is in E mode.

Theoretical investigation of the evolution of electron energy distribution functions in inductively coupled discharges

The inductively coupled plasma (ICP) device is one of the high-density plasma sources being used for the present integrated circuit etching process on an ultra-large scale. Here, we develop a Fokker–Planck code for the ICP source, and evaluate an electron energy diffusion coefficient based on the solution of the wave equations for an ICP reactor. The electron energy distribution function (EEDF) dependence on various external parameters, such as wave frequency, gas pressure and magnetic field, is investigated using the Fokker–Planck code. The effects of changing external parameters on ICP heating characteristics are also discussed. It is shown that the heating of low-energy electrons is enhanced with increasing system collisionality, which is defined as the ratio of collision frequency to effective wave frequency.

Evolution of the electron energy distribution function in a planar inductive argon discharge

The evolution of the electron energy distribution function (EEDF) over a pressure range of 5–100 mTorr is investigated in a planar inductive argon discharge. It is found that the EEDF, which appears to be a bi-Maxwellian distribution with a two-temperature structure at low pressures, evolves into a Druyvesteyn-like distribution as the pressure increases. The electron energy diffusion coefficient, which describes electron heating, is calculated under the same discharge conditions using discharge parameters and numerical results show that the heating of low-energy electrons is enhanced as the pressure increases resulting in a transition of the EEDF to a Druyvesteyn distribution.