Mulser, P.

Sigel, R.

Witkowski, S.

English

MPG.PuRe

LASER-PRODUCED PLASMAS 113 Experimental Investigation of Plasma Production by Irradiating Solid Hydrogen Folls with an Intense Pulse Laser R. Sigel, P. Mulser and S. Wltkowslcl Institut rUr P!ssmaphyslk 8046 Garohlng bel MUnchen , Oermany It ls already known [1,2,,] that interaction between intense laser light and solid hydrogen in a one -dimensional model yields three regions (0,1,2) with different states of matter (Fig. la), In a strongly absorbing deflsgrstlon front F a hot , transparent plasma of moderate density ls produced and streams towards the l aser with high velocity (Region 2, V<tll07 om/sec) . F may be regarded as a piston almost impervioua to matter that generates an intense Shook wave in the solid because of the re-action of the escaping plasma . The compressed matter (Region 1, v_ 106 cm/sec) streaming away rrom the laser with F is separa -ted rrom the undisturbed solld with denslty ~o (Region 0) by the shock rront S . Most of the laser energy is transrerred to Region 2 . The effeot or a focused laser beam on a plane target oan be des-cribed by a piston model based on the one-dimenslonal model . The a b LASER x o Fig. 1 a) Sohematic denSity profile for a solid hydrogen target irra-diated with an intense laser beam (one-dimensional model) . b) PiSton model describing the pene tration of a solid hydrogen target (foil) by a focused laser beam. rocused beam is represented in Fig. lb as a parallel beam or finite cross seotion , the numbers denoting the regions 0 - 2 as in the one-dimensional model. The beam penetrates the target like a piston, the face of the piston being formed by F. The shook wave generated by the piston resembles a bow shock. The rnatter caught up by the Shook front S is pushed aside owing to the high pressure between Sand F [2] and thus transfers the reaction o r the plasma to the matter surrounding the piston . The plasma produced in F is eJepted towards the laser and ex-pands in the front half- space . The ions and electrona are ex-pected to expand at the same velooity . Sinoe the plasma 000113 during expaneion , its energy can be found far rrom the target as directed kinetic energy of the ions. Fig . 2 Streak picture or a laser-irra-diated solid hydrogen foil (time mark ~ corresponds to the mo-ment, when the foil becomes trans-parent to the laser light) . In the experiment a ruby laser waa focused on solid hydrogen foils [4] in high vacuum. The intenSity in the focus was about 1012 w/cm2 over an area of dia-meter 0 .1 mm. The mOdel was veriried in the follow -ing respects: 1) Absorption measurements [5J revealed the existence of the strong -ly absorbing deflagration front F , which moves with a velocity or ) x 106 cm/ sec. 2) Streak pictures (Fig. 2) taken tangentially to the surfaoe of the fOil show luminous phenomena caused by the penetrating piston on the front of the foil at the start of the laser pulse. The back or the roil is not affected until the piston appears there a few nanoseconds before the fall be-comes tran.sparent . ) The plasma produced by the laser was ob -served at various tlmes after the maximum of the laser pulse by 8~ Fig . ) Interferogram ('reconstructed from a hologram) of the plasma produced by a laser f'rom a solid hydl'oeen fo i l ( 18 nsec af -ter maximum of 186er pulse). The roil (th1c)mess 1 . 5 1M1) is not pierced by the_ laser, thererore no plasma is observed on the back of the foil. means or hol ographic lnterrerometry [6] (Fig . }). In the casa or thick f oils that are not plerced. by the laser the number of free electrons in the plasma attains the fina l value of 4 x 1016 arter }O nS( . . 4) Two oharge separating probes each consisting of an earthed perforated plate and a colleotor at negative po-tent~l were mounted 20 cm in rront or and behind the target at an angle of 450 to the laser axis ror making time resolved measurements of the number of impinging i ons . As in the i nter -f'erometric measurements , it was round in agreement with the model that with thin foils as well the bulk of the plasma was ejected into the front halr - space. Integration over the whole sol1d angle with the data of [7J yielded 4 x 101.6 i ons in the case of thick rolls. This number agrees well with the value determined interferometrically for eleotrons . 'Ihe mean k inetic energy or the ions was 200 eV . Thus 45 " of the Isser energy was reoovcred as kinetic energy of the ions .• Cons i dering that part of the laser energy ia transferred to the dense matter as work done by the p1aema at the interfaCe between t he pia ton and denae matter and that ion1~ation energy has to be provided and r adiation los ses covered, this result appears satisfaotory and confirms the described model like the other results . This work was performed as part of the jOint researoh progrll/Tlllle between the Institut fUr Plasmaphysik and Euratom. References Yu . V. AFANASIEV , O.N. KROKHIN and G.V. SKLIZKOV, IEEE J . Q . El., QE-2 ( 1966) 118) . 2 A. CARUSO and R. GRATTON , Int . Rep . L . O. I. 68/2 (1968) . ) P. MULSER and S. WITKOWSKI, Fhys . Lott . ~ (1~68) 151. R. SI GEL, H. KRAUSE and S . WITKOWSKI, J. Sci. Inetr. , Ser . 2 , Vol. 2 ( 1969) 187 . 5 R. SIGEL , K. BUECHL, P. M1JL9ER and S. WITKOWSKI, Fhya. Lett . ~ (1968) 498 . 6 J .M. BURCH , J .W. GATES , R.O. N. HALL and L.H. TANNER , Nature ill (1966) 1)117 . H. OPOWER and W. PRESS , Z . Naturrorsohg. 21A (1966) }IIII. 

Experimental Investigation of Plasma Production by Irradiating Solid Hydrogen Foils with an Intense Pulse Laser

http://hdl.handle.net/21.11116/0000-0008-C1B9-D

Experimental Investigation of Plasma Production by Irradiating Solid Hydrogen Foils with an Intense Pulse Laser

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