Measurement of EEDF and Distribution of Primary Electron in a Bucket Ion Source

Transcription

1 Fourth IAEA Technical Meeting on "Negative Ion Based NBIs" May , Padova - Italy Measurement of EEDF and Distribution of Primary Electron in a Bucket Ion Source S. Miyamoto, F. Kanayama, T. Minami, T. Mune, and H. Horiike Graduate School of Engineering, Osaka University Contents EEDF measurment by Langmuir probe - AC superposition, noise reduction by lock-in-amplifire Slowing down of primary electron in the filter field - change in the tail of EEDFs Loss of electrons at cusp lines - difference between bulk and primary electrons

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6 View of the Device o 0 o 90 DC

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11 Spatial variation of EEDF in filter field Outline The filter magnetic field is produced by PG current and variable. Bi-temperature Maxwellian distribution functions are observed. High energy component fall off sensitively.

12 Components of the Ion source filament S N S Z X back plate N S N S N Y S N S SmCo magnet Y 0 < X < < Y < < Z < 18 Z N S driver region B N S H2 Gas plasma grid DC X DC production region plasma grid B plasma grid extraction grid PG current (0A to 1000A) H - beam ground

13 Filter magnetic field 50 Y=2, Z=6probe position 50 B [G] B [G] 0 0 BackPlate PlasmaGrid X X [cm] Z 30 PlasmaGrid 0 Y BackPlate -4 apertures Ipg=1000A 0 4 Y [cm]

14 Variation of electron temperature in the filter field distance [cm] Back Plate Degradation of temperature is observed Plasma Grid

15 Pressure dependence of electron temperature Back Plate distance [cm] Temperature lowers in accordance with increasing gas pressure because of collision. Plasma Grid

16 Spatial variation of plasma density Ion saturation current Back Plate distance [cm] Plasma Grid electron density Electron density decreases toward PG.

17 Effect of magnetic filter on high energy tail low temperature component high temperature component Back Plate distance [cm] Temperature of high energy tail drops at weaker field Plasma Grid

18 High energy tail ratio in density density Back Plate distance [cm] Plasma Grid The ratio of high energy tail electron does not depend on filter field, while the tail temperature drops. Ratio of High energy tail

19 Mean free path and Larmor radius of electron ρa λfree [m] Larmor radius 1G 5G 10G 1mTorr 2mTorr 4mTorr mean free path energy [ev]

20 Why high energy tail is affected more sensitively by magnetic field low energy component high energy component w/o filter ~λ free <<L s ~L s with filter ~ρ a <<L s <<L s : scale length of electron diffusion, L s : system scale length (ion source)

21 Loss of electron at cusp lines Electron energy distribution functions are measured in the cusp magnetic field with a micro-meter driven electro-static probe. gas injection port magnets stainless tube SUS field line micro-meter probe probe S N N S tungsten probe ceramic insulator S N magnet field line

22 Basic property 1 Te ev) cusp center B probe psition Electron temperature

23 Basic property 2 Js (A) cusp center B probe position Ion saturation current

24 Basic property 3 Vf cusp center B probe position Floating potential filament

25 Energy distribution function Electron energy distribution function at 0 mm. The structure of measured distribution function is rather complicated but fitted by bi-temperature Maxwellian.

26 Energy distribution function 2 Electron energy distribution function at -2 mm. The structure of measured distribution function is rather complicated but fitted by bi-temperature Maxwellian.

27 Ratio of high energy tail electron in density density (m -3 ) ratio probe position (mm) filament High energy electrons are more localized than bulk electrons. The peak of the high energy electron distribution shifts from the center of the cusp.

28 Picture of electron flows at cusps high energy component low energy component flows along field lines field lines wall magnets

29 Effect of filament configuration filament 1 ratio of high energy electron filament 2 probe position (mm) The observed high energy electron peaks coincide with primary electron discharged from filaments.

30 Summary Electron energy distribution function is measured using Langmuir probes. Effect of magnetic filter is investigated using a variable magnetic field. In low gas pressures (< 2mTorr), energy distribution function is bi-temperature Maxwellian. High energy electrons are more sensitive to magnetic field than low energy electron. When electron flux goes across field lines, high energy part is attenuated quickly. Anode area for primary electron is smaller than that for thermal electron. Slowing down process in magnetic field might relates to the different behavior between primary and thermal electrons.