Hi,
I wonder if it is possible to utilize long-pulse lasers to accelerate protons where the intensities are below 10^15 w.cm^-2, and why?
Thanks
Rad
If you mean long pulses as typically exceeding 100 fs. read this article
http://www.nature.com/ncomms/journal/v3/n6/full/ncomms1883.html
Hi Hamid,
I am speaking on laser pulses in the nano second scale and intensity not exceeding 10^15Wcm^-2. The article you mentioned is about a sub-picosecond class laser systems for ultrashort, pulse duration ~30 fs and intensity ~10^21 W cm^−2.
Cheers,
Rad
Well..I don't think it is possible as the laser electric field is low weak while proton mass is very heavy compared to the electron. Actually this intensity is also not enough to accelerate electrons to get high quality beam.
It depends on what you mean by "accelerate". Certainly you ionize and heat the electrons which will then expand in a Deby-sheath and drag ions behind them. However, you will get only few electronvolts ion energy, which I wouldn't call "accelerated".
We extracted low-charge ions by a plasma excited by 100-ns long excimer laser pulses. A comparison between our results and those of shorter laser pulsewidth can be found in https://www.academia.edu/4087129/Novel_laser_ion_sources
We are talking about ions. Protons are unlikely to be extracted/accelerated by ns-pulse laser. You may give a look at the results of alfa particle emission by 3-ns pulses of ABC laser at Frascati, see http://iopscience.iop.org/1742-6596/508/1/012033/pdf/jpconf14_508_012033.pdf
Hope these papers may help
Actually, protons could be easily obtained with low intensity ns lasers striking on solid targets. They come essentially from hydrogen-containing impurities, adsorbed in the first surface or subsurface layers of the material. Choosing good hydrogen-adsorber targets, you could obtain bunches of 10^8 protons per pulse.
For what concerns "acceleration", you should consider, for example, that with a laser pulse of 10^9 W/cm^2 irradiance you could hardly obtain peak energies greater than a few hundreds of eV (tipically, near 100 eV). You could go further increasing the irradiance, nevertheless, due to the nature of ns laser-matter interaction, you'll hardly reach the MeV range.
Post-acceleration is generally needed to utilize the beams obtained with low-intensity lasers: https://www.researchgate.net/publication/275955071_Post-acceleration_of_laser-induced_ion_beams?ev=prf_pub
Article Post-acceleration of laser-induced ion beams
Laser driven proton acceleration demands MeV class protons. For that at least TV/m electric field is required, which is unlikely by a nanosecond pulse of 10^15 W/cm2. The scale length of the plasma increases which hinders the acceleration gradient. However, recently attempts are made with structures targets to accelerate protons using two pulses. Please go through the articles for details.
1. Phys. Plasmas 8, 542–549 (2001)
2. Phys. Rev. Lett. 106, 134801 (2011); Phys. Rev. Lett. 110, 215004 (2013)
Surely, to obtain MeV energy protons, ultra-high power lasers (from hundreds of TW to PW) are needed. Needs are relaxed if you want lower energy protons. It is just a matter of the purpose you follow, but in any case you obtain particles that, initially at rest on target surface, reach an energy whose extent depends on the power per area deposited on target.
Ion acceleration can be observed with nanosecond pulse lasers (due to Double Layer formation for example). However at intensities on the order of 10^12 W/cm2 the energy of the ions is on the order of maximum few KeV. We measured this ion acceleration with Langmuir Probe measurements. The results are published in:
http://iopscience.iop.org/0295-5075/100/4/45003.
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