Measurements of EEDF in Helium Flowwing Afterglow at Pressures 500 – 2000 PA

Abstract

For measurements of recombination rate coefficients of slowly recombining ions with rate coefficients below 10–7cm3s–1 in Flowing Afterglow Langmuir Probe (FALP) experiment the plasma decay time has to be long and this requires buffer gas (helium) pressures in range 500 – 2000 Pa. Application of Langmuir probe for measurements of electron energy distribution function, EEDF (f (ε)), at these pressures is not trivial. The Druyvestein formula for calculating f (ε) from the second derivative of the electron current to the probe (I″e) cannot be used. More complex analyses of measured probe characteristic are required to obtain reliable EEDF. To characterize the extent of the required corrections, the probe characteristics in well‐defined flowing afterglow plasma were measured and two different methods were used to obtain EEDF. In the first method, the first and second derivative of probe characteristics were used to obtain EEDF [1]. In the second method, numerical correction was calculated [2] and applied to values obtained from Druyvesteyn formula. By comparison of obtained EEDF with Maxwell distribution we determine a region of electron energies where advanced analysis is necessary. It was found that in a helium afterglow at 250 K, with pressure 600 – 2000 Pa, where plasma is nearly Maxwellian with T e ∼ 250 – 600 K, the correction is necessary for electron energies ε ≤ 3.5 kBTe. At higher electron energies correction is negligible and is not necessary. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cryo-FALP study of collisional-radiative recombination of Ar+ions at 40–200 K

“…The thermodynamic equilibrium is reached and instead of the temperature of electrons (T e ), ions (T i ) and neutrals (T n ) only one temperature can be used T = T e = T i = T n (for details and discussion on plasma species thermalization in afterglow plasma see refs. [11,[22][23][24]). At the beginning of section B plasma is precooled via collisions with helium buffer gas to 100 -120 K and in section C to the desired temperature adjusted in the range 40 -300K (see discussion later on).…”

Recombination of KrD+ and XeD+ ions with electrons

“…Because of high He pressure (1600 Pa), the majority of He + ions formed in the discharge are converted in a three-body association process to He 2 + ions already prior to port P1. At this stage, plasma contains He 2 + ions and electrons and also long-living helium metastables (He(2 3 S) and He(2 1 S)) formed in the discharge [20,22]. In the presented experiments, the port P1 is used to add Ar in XeD + studies and Kr in KrD + studies.…”

“…We have recently adapted our FALP apparatus [18][19][20] for measurements of the recombination rate coefficients as small as 5 × 10 −9 cm 3 s −1 [17]. In the present study, FALP is used to measure rate coefficients of KrD + and XeD + ions at 250 K. In a FALP experiment, the recombining ions are formed in the decaying afterglow plasma in helium buffer, so in principle D 2 and Kr or Xe have to be added to the carrier gas to form KrD + or XeD + , respectively.…”

Application of Langmuir Probe in Recombination Dominated Afterglow Plasma

“…Density of helium metastables is higher by at least a factor of 10, i.e. [He m ] ∼ 5 × 10 11 cm −3 [8]. Krypton can be added via port P 1 to remove the helium metastables from the decaying plasma by Penning ionisation.…”

“…When Kr is added via port P 1 as mentioned before, Kr + dominated plasma is formed in reaction of Kr with He + 2 and He m . After removing metastables from the plasma, electrons relax in collisions with buffer gas and a Maxwellian electron energy distribution is established [8]. The decay of the plasma can be calculated by solving a set of balance equations that include ion molecule reactions, Penning ionisation, ambipolar diffusion and recombination processes.…”