Hi,
Any idea about the relation between the plasma pressure and the magnetic field if a magnet is used to mitigate the debris particles in laser plasma sources especially tape drive targets?
Many thanks,
Rad
Few questions first, what type of laser (wavelength, duration, energy)? What is your intensity on target and focal spot? How thick is the tape.
Unfortunately, my answer will likely be not much, as most of the damage to optics and such comes from the debris plume which includes many hot (molten), but relatively uncharged particles and vapor which will deposit on the optics anyway. Your best friend is distance (and gravity). The magnet will help with the hot electrons and ions from causing radiation damage, but this (making an assumption about your rep rate and energies) so long term compared to the fogging and chunk damage that you will probably never notice.
Also, the other best thing to due is add a low pressure He fill to reduce debris, a few Torr will really help, and won't affect the laser. (also assuming you are in a vacuum! :-) )
Hi kirk,
Many thanks for your answer. So the laser used is IR 1054nm long pulse 800ps and 50Hz rep, whereas the tape is 15um thick. In such low thicknesses you would expect your shockwave to penetrate through and not much of debris emission occur. The use of buffer has two effects, the first would be the collision of debris particles with the gas molecules resulting a reduction in the kinetic energy of debris, hence lower range and less adhesability to surrounding optics which I have already used (N2). Whereas the other possible scenario is the interaction between the gas and the ablated beam in an early time scale before ablating the target and laser energy coupling to the plasma process takes place which in turns makes the energy of the beam cooking the target with less energy, hence lower debris emission. Back to the magnet, where I noticed that the use of magnetic field only is not preventing big debris particles however reducing the TOF(speed). which makes me wonder whether the use of magnet is efficient or not.
Cheers,
Rad
Radhwan,
These guys tried both an E-field and a B-field.(see attached)
What is your application, x-ray production?
Using a 500 fs 80J laser, I find only distance and moving the target normal away from the focusing optic to be reliable, but still have to replace the optic ($20k) after 1000 shots.
What is the N2 pressure, don't you get ionization defocusing in you gas? What is your intensity? And what is the energy of your laser, 10s of Js?
I would try reducing the tape thickness to help as well. Going the He would allow higher pressure back-fill without any other deleterious effects on the laser or interaction.
Well the gas pressure is 4.5mbar and you should not use high irradiance where you exceed the breakdown threshold of the gas to have it ionized prior to reaching the target as in the case of short pulse. I agree with you by manipulating the distance between your target and delicate expensive optics, however a question of compromising especially if you work in the soft x-ray region with smaller range (BEUV
Maybe I will not answer the question, but one could use standard quartz protection window between target and focusing optics, as it is used in common laser processing machines. Then you will have to replace cheap window instead of objective optics. If the window is thin, aberrations may be neglected.
Unfortunately neutral particles are main component of the debris in picosecond pulse treating and one need very high magnetic field strength to deflect them.
Hi Mikhail,
Many thanks for your answer. Well I am actually using a a protective 1mm glass between the focusing lens and the target, which is quite efficient in mitigating the debris and can be used at the same time as debris collector which I have successfuly used to study the distribution and size of the particles. Please look at the design in my previous published work in the link below
https://www.researchgate.net/publication/259239042_King%27s_College_Laser_Plasma_X-Ray_Source_Design
However, one need to think of protecting the rest of the optics in the system such as reflectors, condensers or zone-plates in if it is needed to expand the generated x-ray further for application purposes and there comes the idea of protecting these parts inside your system with magnet for example!
Cheers,
Rad
Conference Paper King's College Laser Plasma X-Ray Source Design
Radhwan, thanks.
As I can see, better way to deflect debris particles may be through ionizing them by means of specially organized electron beam (it is very possible under vacuum conditions, see reference below for instance). Charged particles are much more easily operated by electric or magnetic fields, which may not be so strong.
http://link.springer.com/article/10.1134%2FS1063785010120230
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