Experimental Study Of Cascaded Arc Plasma Based On The Laser Thomson Scattering

Cascaded arc plasmas are characterized by high density,high particles flux and steady-state operation and have been widely applied in many research fields,such as deposition of thin films,modification of material surface,synthesis of nano-materials and magnetic confinement fusion.Therefore,it requires to accurately measure the parameters of cascaded arc plasma.As the most fundamental two plasma parameters,electron density(ne)and electron temperature(Te)often have the significant influence on other plasma parameters and the local thermodynamic equilibrium(LTE)in cascaded arc plasmas.So precise measurements of ne and Te in cascaded arc plasma are of vital significance for understanding in-depth of complex physical mechanisms in plasma and optimization of the plasma applications.Among the plasma diagnostic techniques,laser Thomson scattering(LTS)has been acknowledged as one of the most accurate methods to give access to ne and Te.This method has the advantages of non-invasive measurement,high spatial and temporal resolution and independence of LTE state in plasma.Hence,the LTS is an ideal candidate to precisely measure ne and Te of cascaded arc plasma.In this thesis,the LTS systems have been developed and employed to accurately measure ne and Te under various discharge conditions,especially in the case of adding the reactive gases,and some novel electron behaviors in cascaded arc plasma have been found.Since the Te obtained by LTS does not depend on whether the plasma is in LTE while the electron excitation temperature(Texc)measured by optical emission spectroscopy(OES)is equal to Te only when the plasma is in LTE.In this thesis,a method to study the fundamental non-LTE issues in cascaded arc plasma is proposed via comparing the Te and Texc.The thesis are organized as follows:In chapter 2,a Single Grating Spectrometer Laser Thomson Scattering(SGS-LTS)system and a Triple Grating Spectrometer Laser Thomson Scattering(TGS-LTS)system have been built,respectively.The SGS-LTS system mainly consists of a single grating spectrometer and a laser sub-system.The SGS-LTS system features simple equipment and easy operation.But,the single grating spectrometer does not function as a notch filter,leading to that the SGS-LTS system cannot eliminate the intense stray light signal which can totally swamp the weak LTS signal.To suppress the strong stray light,a triple grating spectrometer(TGS)with the function of a narrow-band notch filter has been established in this thesis and the TGS is combined with the laser sub-system to constitute the TGS-LTS system.Compared with SGS-LTS system,TGS-LTS system has higher detectivity and the detective limit is as low as 1×1017 m-3.Thereby,the TGS-LTS system can be extensively applied in the precise measurements of ne and Te in low-density plasma.Besides,in this chapter the cascaded arc plasma generator is briefly introduced.In chapter 3,the SGS-LTS system has been utilized to accurately measure ne and Te in cascaded arc argon(Ar)plasma.In addition,the OES is applied to determine Texc in cascaded arc Ar plasma.The non-LTE characteristics of cascaded arc Ar plasma is investigated through comparing Te and Texc The LTS measurements show that in the typical discharge conditions,ne of cascaded arc Ar plasma is about 1019 m-3 to 1020 m-3 and Te is around 0.3 eV to 0.6 eV.With the increase of discharge current,gas flow and background pressure,both of ne and Te increase.As increasing the discharge current and gas flow,more power is input into the cascaded arc plasma source(CAPS)so that ne and Te in the CAPS increase,causing the increase of ne and Te in the downstream plasma.The increase of background pressure reduces the volume of plasma beam so that the radial transport of plasma is weakened and electrons are constrained in the central zone of plasma,leading to the increase of ne.The growth of ne enhances the three-body recombination reaction in which the internal energy of Ar ion is converted into the kinetic energy of electron,that is,Te increases.In the cascaded arc plasma,Texc is always greater than Te,and Texc gradually approaches to Te with the increase of ne.This is because the cascaded arc Ar plasma belongs to recombining plasma and the Ar atomic levels are underpopulated with respect to the populations in Saha-Boltzmann balance.With increasing ne,both the collision between electrons and Ar atoms and the three-body recombination reaction are enhanced.As a consequence,the populations of Ar atomic levels increase and approach to the values in Saha-Boltzmann balance.Subsequently,the plasma moves toward LTE so that Texc closes to Te.In chapter 4,the TGS-LTS system has been adopted to investigate the influence of N2 on ne and Te in cascaded arc Ar plasma as well as the influence on LTE in plasma.Meanwhile,the physical mechanisms of the effect of N2 on ne,Te and LTE in plasma are qualitatively proposed.The results show that ne drastically drops from 1020 m-3 to 1018 m-3 when the N2 gas ratios increase from 0%to 10%at the background pressures of 150 Pa and 300 Pa.Nevertheless,at the higher background pressures(500 Pa and 800 P a)ne firstly sharply declines and then slightly increases.The dissociative recombination reactions between electrons and N2 molecular ions can consume a large number of electrons,resulting in the sharp decrease of ne.The associative ionization reactions between the metastable N2 molecules can produce electrons,which can bring about the increase of ne when the associative ionization reactions are dominant.The Te presents the behavior that firstly increasing and then decreasing with the increase of N2 gas ratios in all the given discharge conditions.This can be attributed to the synergy of super-elastic collision reaction and electron impact excitation reaction between electrons and nitrogen molecules.The super-elastic collision reaction between electrons and highly vibrational excited N2 molecules can convert the vibrational energy of N2 molecules into the kinetic energy of electrons to enhance Te while the electron impact excitation to N2 molecules in ground state induces the loss of electron kinetic energy to reduce Te.Additionally,Texc is further away from the Te with the increase of N2 gas ratios.The introduction of N2 gas induces the drastic reduction of ne so that the collision between electrons and Ar atoms becomes weak which leads to that the populations of Ar atomic levels further depart from the values in Saha-Boltzmann balance.Hence,the cascaded arc plasma more severely deviates from LTE.In chapter 5,the TGS-LTS system has been employed to directly diagnose ne and Te in cascaded arc Ar and oxygen(O2)mixture plasma to investigate the influence of O2 gas on ne and Te.It is found that with the increase of O2 gas ratio from 0%to 10%,ne rapidly drops and the falling range can exceed two orders of magnitude,while Te has minor changes and only in high background pressure and low discharge current can the decrease of Te occur at higher O2 ratios.The decrease of ne is mainly attributed to the dissociative recombination reactions with O2 molecular ions.Meanwhile,a part of electrons are consumed by the formations of negative ions(O2" and O-)owing to the electronegative O2 gas,which further reduces ne as well.With the increase of O2,the super-elastic collision between electrons and high vibrational excited O2 molecules balances with the electron impact excitation to O2 molecules in the ground state.The energy gain from the super-elastic collision is comparable to the energy loss induced by electron impact excitation.Therefore,Te changes a little with the increase of O2 gas ratios.In the case of high background pressures and low discharge currents,the density of O2 molecules in ground state is high at higher O2 ratios so that the electron impact excitation reaction dominates which causes the decrease of Te.