1. Stagnation Layers at the Collision Front between Two Colliding Plasmas: Prospects for Materials Growth and (VUV) LIBS
John T. Costello
National Centre for Plasma Science & Technology (NCPST)/ School of Physical Sciences, Dublin City University
EU COST MP0601 Salamanca May

2. Talk contains elements of the work of:
Ph.D Students
John Dardis (Imaging/Spectroscopy)
Padraig Hough (Interferometry)
Thomas ‘Mossy’ Kelly (Image Processing)
Postdoc
Paddy Hayden
International Visiting Fellow
Sivandan Harilal (Purdue University)
Former Group Members
Kevin Kavanagh (Founder - Brand Plate Media -
Hugo de Luna (Now Lecturer - Federal University of Rio de Janeiro)
Jofre Pedregosa (Now Maitre de Conference, Universite de Provence)
EU COST MP0601 Salamanca May

3. Talk contains elements of collaboration with:
Ph.D Students
Mr. Conor McLoughlin (PLD)
Postdoc
Paddy Hayden (LIBS)
Colleagues
Jean-Paul Mosnier (PLD)
Eugene Kennedy (LIBS)
Former Group Members
Eoin O’Leary (LIBS)
International
Leo Gizzi et al.
EU COST MP0601 Salamanca May

4. Outline of the Talk Colliding Plasmas - Introduction
Optical Diagnostics
Prospects for PLD & LIBS
EU COST MP0601 Salamanca May

5. Colliding Plasmas - Introduction
Making Stagnation Layers
Laser Pulse Energy: mJ/ beam
Laser Pulse duration: 170 ps, 6 ns, 15 ns
Focal Spot Size: ~ 100 mm
Irradiance: W.cm-2
EU COST MP0601 Salamanca May

6. Colliding Plasmas - Introduction
Not a new idea !
‘Seed’ Plasmas
‘Stagnation Layer’
When plasma plumes collide there are two extreme scenarios:
Interpenetration - interactions are mostly via binary collisions
Stagnation - plumes decelerated at collision plane, rapid accumulation of material, kinetic energy converted into excitation energy (glow), rapid growth of dense (stagnated) layer,………
EU COST MP0601 Salamanca May

7. Colliding Plasmas - Introduction
Huge body of literature on colliding plasma fundamentals -
but mainly from work at high power laser facilities !
Motivations - they are many and varied………..
1. Fusion (Hohlraums)
2. X-ray Lasers
3. Space Weather Observations
4. Pulsed Laser Deposition
5. Laboratory-Astrophysical Model Experiments
T R Dittrich et al., Phys. Plasmas (1999)
R W Clark et al., Phys. Plasmas (1997)
J L Horwitz and T E Moore, IEEE Trans. Plasma. Sci (2000)
C Sanchez Ake et al., J. Appl. Phys (2006)
C D Gregori et al., Ap. J (2008)
EU COST MP0601 Salamanca May

8. Colliding Plasmas - Introduction
Collisionality Parameter:
Plasma - Plasma Separation
Ion - Ion Mean Free Path (mfp)
For collisions between
opposing plumes (1, 2)
Slow moving and dense plumes are more likely to stagnate !
ii >> D Interpenetration
ii ~ D  ’Soft’ Stagnation
ii << D  ’Hard’ Stagnation
EU COST MP0601 Salamanca May

9. Colliding Plasmas - Introduction
Collisionality Parameter:
Plasma - Plasma Separation
Ion - Ion Mean Free Path (mfp)
For collisions between
opposing plumes (1, 2)
Key point: One can engineer stagnation layer characteristics; ‘hardness’, density, temperature, shape, etc. by varying geometry (D) and laser-target interaction physics (mfp, ii) - application specific…..
EU COST MP0601 Salamanca May

10. Part 2. Optical Diagnostics
Stagnation layer growth
Time resolved (ICCD) imaging
Time-space resolved spectroscopy
Faraday ion cup
Plasma Diagnostics
Time-space resolved spectroscopy - ne & Te
Time resolved interferometry - ne
EU COST MP0601 Salamanca May

11. Part 2. Optical Diagnostics
EU COST MP0601 Salamanca May

12. Optical Diagnostics Time Evolution: Tight point focus on each Ca face:
120 mJ per beam
ICCD: 5 ns gate
10 ns interval
Ca - Emission 423 nm
EU COST MP0601 Salamanca May

13. Optical Diagnostics Stagnation Layer Evolution Broadband image - 200ns
Colliding aluminium plasmas
~100 mJ/170 ps/’seed’ beam
EU COST MP0601 Salamanca May

14. Optical Diagnostics Stagnation Layer Evolution:
(Al) - Charge resolved !
‘Growth rate’ - 10 m/ns
300mJ/6ns/’seed’ beam
EU COST MP0601 Salamanca May

15. Optical Diagnostics
Stagnation Layer (Al): Electron density & temperature
‘Seed’ spectrum
Stagnation zone
‘Seed’ spectrum
EU COST MP0601 Salamanca May

16. Optical Diagnostics
Stagnation Layer (Al): Electron density & temperature
Spectroscopy - only works well for t > 100 ns
Spectra dominated by continuum emission - solution - time resolved interferometry
Experimental Setup-
Nomarski Interferometry
EU COST MP0601 Salamanca May

17. Separation of electron and ion stagnation - Ambipolar effects
Optical Diagnostics
Separation of electron and ion stagnation - Ambipolar effects
Electrons -
‘Nomarski’
Al+ ions -
ICCD
Al plume -
ICCD
EU COST MP0601 Salamanca May

18. Colliding Plasma - PLD NeoceraTM PLD system Replace single plume
with stagnation layer……..
EU COST MP0601 Salamanca May

19. Colliding Zn Plasma - PLD
Vacuum
Single Plume
Vacuum
Colliding Plumes
SEM IMAGES
1 mbar O2
Single Plume
1 mbar O2
Colliding Plumes
EU COST MP0601 Salamanca May

20. TISR-VUV-LIBS, Ambient Gas
Line: S4+ (78.65 nm)
Sulphur/Steel in N2
Background Gas
EU COST MP0601 Salamanca May

21. VUV - LIBS, Ambient N2 Sulphur/ Steel in N2 Background Gas - strong
S nm
Sulphur/ Steel in N2
Background Gas - strong
enhancement of S nm
line - S5+ + e  S4+*….?
EU COST MP0601 Salamanca May

22. LIBS with Colliding Plasmas ?
1. Results give us some confidence that colliding plasmas could also be engineered to improve the efficacy of LIBS…….
2. Also, could be used in ‘double pulse’ LIBS experiments to separate ‘sampling’ from excitation…..
EU COST MP0601 Salamanca May

23. Support Science Foundation Ireland Irish Research Council for Science,
Engineering and Technology
EU COST: MP0601
EU COST MP0601 Salamanca May

24. What have we learned to date ?
1. Strong stagnation in table top colliding plasmas
due to large value of the collisionality parameter (z)
2. Degree of confinement/ hardness of the stagnation
layer can be controlled by designing the value of z
3. Stagnation layer becomes quite uniform after 100s ns
and so looks attractive for investigation as alternative
pulsed laser materials deposition source, target for LIBS
4. Preliminary PLD results are promising…..
5. Colliding Plasma LIBS efficacy to be proven……..
EU COST MP0601 Salamanca May