Abstract
Ion velocity distribution functions (ivdfs) are investigated by laser induced fluorescence in Ar–Xe and Ar–He expanding helicon plasmas as a function of gas composition. In the case of Ar–Xe plasma, it was found that in the helicon source, both the and vdfs are unimodal. Their parallel speeds are subsonic and unaffected by changes in gas composition. At the end of the source, the argon ivdf shows a bimodal structure indicative of an electric double layer upstream of the measurement location. The fast argon ion component parallel velocity increases with Xe fraction from 6.7 to 8 km/s as the Xe fraction increases from 0% to 4%. In the expansion region, the bimodal character of Ar ivdf is maintained with a supersonic fast component reaching parallel speeds of 10.5 km/s. For all the studied plasma conditions and different spatial locations, the vdf exhibits a unimodal structure with a maximum parallel flow velocity of 2.2 km/s at the end of the source. For Ar–He plasma, the Ar ivdf is bimodal with the fast ion component parallel velocity increasing from 5.2 to 7.8 km/s as the He fraction increases from 0% to 30%. For the same He fraction range, the slow argon ion population distribution changes from a single Gaussian to a wide distribution extending all the way from the speed of the fast population to 0 m/s.
I.A.B thanks Dr. X. Sun now at TriAlpha Co., Dr. C. Biloiu now at Varian Semiconductor Equipment Associates, Dr. A. M. Keesee, and Dr. A. Hansen for many enlightening discussions, suggestions, and critical analyses regarding this work. This work was supported by NSF Award No. PHY0611571.
I. INTRODUCTION
II. LASER INDUCED FLUORESCENCEDIAGNOSTICS
A. LIF principles and basic hardware
B. Argon ion LIF
C. Xenon ion LIF
III. ION ACCELERATION IN TWOELECTROPOSITIVE GAS SPECIES PLASMAS
A. vdf in Ar–Xe plasma
1. Effect of Ar/Xe ratio on vdf in the plasma source
2. Effect of Ar/Xe ratio on vdf in the expansion region
B. vdf in argonxenon plasma
IV. ION VDFS IN Ar–He PLASMA
V. DISCUSSION
Key Topics
 Laser induced fluorescence
 85.0
 Magnetic fields
 22.0
 Double layers
 20.0
 Plasma temperature
 20.0
 Plasma flows
 17.0
Figures
(a) Threelevel LIF scheme and (b) hyperfine structure of 605.278 nm line of the odd isotopes due to nuclear spin splitting.
(a) Threelevel LIF scheme and (b) hyperfine structure of 605.278 nm line of the odd isotopes due to nuclear spin splitting.
Argon ion parallel velocity distribution functions in (a) HELIX at and (b) HELIX at . The thick black lines are LIF signals; the thin lines are Gaussian fits assuming a single and a dual ion population line; and at the top is the iodine reference spectrum. The ivdf evolves from unimodal at into bimodal at .
Argon ion parallel velocity distribution functions in (a) HELIX at and (b) HELIX at . The thick black lines are LIF signals; the thin lines are Gaussian fits assuming a single and a dual ion population line; and at the top is the iodine reference spectrum. The ivdf evolves from unimodal at into bimodal at .
(a) Individual ion sound speeds (circles for and triangles for ) and the system sound speed (open squares) as function of xenon fraction in HELIX at . (b) Parallel ion flow speeds in HELIX at vs xenon fraction (same symbol assignment).
(a) Individual ion sound speeds (circles for and triangles for ) and the system sound speed (open squares) as function of xenon fraction in HELIX at . (b) Parallel ion flow speeds in HELIX at vs xenon fraction (same symbol assignment).
At in HELIX, the LIF amplitude (a) and parallel flow speed (b) as function of small changes in the xenon fraction. Filled and open symbols denote the fast and slow ion group, respectively. (b) The (solid line) and system (dotted line) sound speeds calculated based on measurements at .
At in HELIX, the LIF amplitude (a) and parallel flow speed (b) as function of small changes in the xenon fraction. Filled and open symbols denote the fast and slow ion group, respectively. (b) The (solid line) and system (dotted line) sound speeds calculated based on measurements at .
(a) LIF amplitude and (b) parallel flow speed as function of xenon fraction at in LEIA. Filled and open symbols denote the fast and slow ion groups, respectively. (b) The (solid line) and system (dotted line) sound speeds calculated based on measurements at .
(a) LIF amplitude and (b) parallel flow speed as function of xenon fraction at in LEIA. Filled and open symbols denote the fast and slow ion groups, respectively. (b) The (solid line) and system (dotted line) sound speeds calculated based on measurements at .
(a) Normalized LIF amplitude and (b) parallel flow speed at (open symbols) and (filled symbols) in HELIX vs argon fraction, respectively. Also shown in (b) are the (solid line) and system (dotted line) sound speeds based on local measurements.
(a) Normalized LIF amplitude and (b) parallel flow speed at (open symbols) and (filled symbols) in HELIX vs argon fraction, respectively. Also shown in (b) are the (solid line) and system (dotted line) sound speeds based on local measurements.
(a) Normalized LIF signal amplitude and (b) parallel flow speed vs input rf power in HELIX at (open symbols) and (filled symbols). (b) Bohm speed at . Operating conditions: , , , and .
(a) Normalized LIF signal amplitude and (b) parallel flow speed vs input rf power in HELIX at (open symbols) and (filled symbols). (b) Bohm speed at . Operating conditions: , , , and .
Effect of increasing helium fraction on the parallel ivdf in HELIX. Measurements were obtained at .
Effect of increasing helium fraction on the parallel ivdf in HELIX. Measurements were obtained at .
(a) Dependence of the metastable population obtained from integration of the ivdfs and normalized to the pure Ar case (filled squares) and ratio of slow/fast LIF signals (light filled squares) vs helium fraction. (b) Parallel velocity of the fast (filled circles) and slow (open circles) populations; the (solid line) and system Bohm (dotted line) speeds, respectively; the dashed line intersecting the full circles is the theoretical dependence . (c) Bimodal ivdf with fast and slow populations for 0% He. (d) for a 30% He fraction the ivdf exhibits a long tail characteristic of chargeexchange collisions. Measurements obtained at in HELIX.
(a) Dependence of the metastable population obtained from integration of the ivdfs and normalized to the pure Ar case (filled squares) and ratio of slow/fast LIF signals (light filled squares) vs helium fraction. (b) Parallel velocity of the fast (filled circles) and slow (open circles) populations; the (solid line) and system Bohm (dotted line) speeds, respectively; the dashed line intersecting the full circles is the theoretical dependence . (c) Bimodal ivdf with fast and slow populations for 0% He. (d) for a 30% He fraction the ivdf exhibits a long tail characteristic of chargeexchange collisions. Measurements obtained at in HELIX.
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