Ion Drag Force Measurements with PK4 at MPE

1. Ion Drag Force Measurements with PK4 at MPE Markus H. Thoma,* Max-Planck-Institut für extraterrestrische Physik and Centre for Interdisciplinary Plasma Science • The PK4 Experiment • The Ion Drag Force – Theory and Experiment • Measurements and Results * Supported by DLR (BMBF)

2. 1. The PK4 Experiment Complex plasma in a combined DC/RF discharge in a glass tube Laboratory predevelopment phase (2002 – 2004) in collaboration between MPE and IHED Aim: Microgravity experiments on board of the ISS (2007?)

3. PK4 chamber

4. DCdischarges in plasmachamber Neon gas at 300 K and 20 – 200 Pa DC current: 0.1 – 5.0 mA ginvestigation of liquid state, e.g. transition from laminar streaming to turbulence, forces on particles, dispersion relations, particle charges, cloud collisions, lane formation, nozzle simulation

5. 2. The Ion Drag Force – Theory and Experiment External forces on microparticles 1. Gravity 2. Electrostatic force 3. Neutral gas (Epstein) friction 4. Ion drag Momentum transfer between ions and dust: ion collection (collection force) and Coulomb interaction (orbital force) Assumption: standard theory of Coulomb scattering g Debye length as cutoff (Barnes et al., 1992)

6. Ion-dust collisions g standard Coulomb scattering theory not valid because of high particle charge g r not much smaller than lD Ions with impact parameter > Debye length contribute to orbital force ( S. Khrapak et al., 2002) • Ion drag force typically an order of magnitude larger than predicted by • standard Coulomb theory • possible explanation of void formation observed in PKE-Nefedov on the ISS

7. Experimental determination of ion drag force by Zafiu, Melzer, and Piel (2003) Measurement of the deflection of falling particles in an RF-discharge Result: ion drag force agrees well with standard formula by Barnes et al.if electron Debye length is used Results agree also with improved formula (S. Khrapak et al., 2003)

8. 3. Measurements and Results Method: Measurement of particle velocities in PK4-chamber for different particle sizes, pressures, and currents Stationary velocity is quickly reached (< 0.2 s) due to neutral friction Force balance: Experiment: Neon, pure DC-mode Particle radius: 0.6 to 5.5 microns Pressure: 20 – 120 Pa Current: 0.5 – 1.5 mA

9. Vertical tube position: all particle sizes in bulk plasma, however gravity dominates g determination of ion drag force difficult Horizontal tube position: only small particles (up to 1.7 microns) flow through middle of tube (bulk plasma), larger particles (radius 3.4 and 5.5 microns) g microgravity (so far only parabolic flight experiments) Determination of particle charge from MD simulation by Zobnin et al.(2000) (together with IHED probe measurements for electron temperature and density) in good agreement with dust wave experiments by Ratynskaia et al. (2004): particle charge is significantly smaller than predicted by OML Determination of longitudinal electric field by IHED probe measurements: Dominant error in ion drag force from electrostatic force due to uncertainties in the probe measurements and the theoretical prediction of the charge

10. Determination of neutral drag coefficient by “falling particle” experiment without plasma at MPE and IHED: larger than predicted by Epstein corresponding to

11. Velocity measurement of particles from track length or positions in • consecutive pictures • Exposure time per frame 8 ms, distance per pixel 9.9 microns • pixel corresponds to 1.24 mm/s (4.13 mm/s for lens 0.3) 15 – 100 tracks for each parameter, velocity between 1 and 10 cm/s Particle velocities independent of number density g no Havnes effect?

12. MPE laboratory experiments Particle radius 0.64 microns, I = 1 mA Rough agreement of experiment (2) with Khrapak’s prediction (3), no agreement with Barnes’formula: (4) ion Debye length, (1) electron Debye length No indication of current dependence in contrast to theory (30% velocity reduction due to current dependence of ion drag from 0.5 to 1.5 mA predicted)

13. a = 0.98 mm a = 1.28 mm • Barnes’ formula with electron Debye length • Experiment • Khrapak’s formula • Barnes formula with ion Debye length

14. Particle radius 1.71 microns, I = 1 mA • Barnes’ formula with electron Debye length • Experiment • Khrapak’s formula • Barnes formula with ion Debye length

15. Particle size and pressure dependence • Dependence on particle size in good agreement with theory • Dependence on pressure not in agreement with theory: the ion drag • decreases less with increasing pressure than predicted g collisions • at higher pressure not negligible

16. Results from Parabolic Flights (ESA PF36 Campaign, March 2004)

17. Particle radius 3.43 microns, I= 1 mA • Barnes’ formula with electron Debye length • Experiment • Khrapak’s formula • Barnes formula with ion Debye length Conclusions similar as in the case of laboratory experiments

18. Conclusions and Outlook • Ion drag measurements with PK4 in agreement with • theoretical predictions by Khrapak et al. g ion drag force significantly • larger than predicted by standard Coulomb scattering • Barnes’ formula with ion or electron Debye length excluded • Good agreement with theory in particular at low pressure (20 Pa) • (no ion-neutral collisions in theory); weaker dependence on pressure than • predicted • Size dependence as theoretically predicted • No indication of current dependence in contrast to theory • No indication for negative ion drag at larger pressures as predicted by • by Schweigert et al. • Neutral drag force about 10% larger than predicted by Epstein theory • More accurate determination of particle charge and electric field • desirable g smaller error in electrostatic force • Further experiments in laboratory and under microgravity • (parabolic flight campaign, ISS) planned • Automated velocity measurements, systematic investigations, and better • statistics

19. Collaborators (PK-4 Team): MPE: M. Fink, H. Höfner, S.A. Khrapak, M. Kretschmer, G.E. Morfill, R.A. Quinn, S. Ratynskaia, K. Tarantik IHED: V. Fortov, Y. Gerasimov, O. Petrov, A. Usachev, A. Zobnin