ION DISTRIBUTIONS IN LASERPRODUCED PLASMAS WITH SMALL DIAMETER FOCAL SPOTS

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ION DISTRIBUTIONS IN LASERPRODUCED PLASMAS WITH SMALL DIAMETER FOCAL

SPOTS

K. Rohr

To cite this version:

K. Rohr. ION DISTRIBUTIONS IN LASERPRODUCED PLASMAS WITH SMALL DI- AMETER FOCAL SPOTS. Journal de Physique Colloques, 1989, 50 (C1), pp.C1-577-C1-581.

�10.1051/jphyscol:1989162�. �jpa-00229361�

JOURNAL DE PHYSIQUE

Colloque C1, supplkment au n o l , Tome 50, janvier 1989

ION DISTRIBUTIONS I N LASERPRODUCED PLASMAS WITH SMALL DIAMETER FOCAL SPOTS

K. R O H R

Universitat Kaiserslautern, FB Physik, 0-6750 Kaiserslautern, F.R.G.

Abstract Ion emission from a laserproduced plasma with very small focal spots (20 flm full l/e width) has been investigated, charge, angle and velocity being resolved. The experiments have been performed for different polarizations and wavelengths (1.06 pm and 0.53 pm) of the laser (Nd-YAG mode locking system). In the case of p-polarization an enhancement of the total ion expansion energy by a factor of 3.5 is obtained compared with s-polarization. This is due to resonance absorption. While most of the additionally absorbed energy leads to an increase in the number of emitted ions, the velocity spectrum is reduced and the average charge is only weakly influenced. The result is quite different when the wavelength of the laser is changed instead of the polarization. For the first harmonic (0.53 pm) an increase in the ion expansion energy is likewise observed, which, however, in this case is preferentially due to an increase in the mean kinetic energy, the ablated mass remaining essentially unchanged.

The use of high power lasers for the production of ions has serveral advantages compared to conventional methods. It allows the creation and investigation of ions from nearly all materials and in such high charge states not accessible e.g. by discharge techniques. In addition , the ion current is naturally pulsed which is of advantage in many cases, where the ions are used for secondary scattering experiments. In the following paper the influence of polarization and wavelength on ion spectra when the laser is focussed to a very small diameter spot is investigated. It can be expected that small focal spots lead to considerable changes in ion expansion dynamics

(Estabrook, 1986).

Laser Pulse

Figure 1. Experimental arrangement.

The experimental setup is shown, in principle, in figure 1. The light pulse of an Nd-YAG mode locking system is focussed onto a solid target (plexiglas and tantalum) to a focal size of 20 pm (full l/e width) whereby the polarization of the laser is either s or p and the wavelength is 1.06 flm or frequency doubled (0.53 pm). The pulse length

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989162

C1-578 JOURNAL DE PHYSIQUE

is 45 ps (for 1.06 fim). The experiments have been ~erformed with focal intensities in the 1013 ~ / c m - ~ range. Ions can be detected state of charge, angular and energy resolved by a combined TOF-CDA apparatus.

Details of the coaxial dynamical analyser ( C D A ) have been published by Eicher et al. (1983). The experimental method has been described in foregoing papers e. g. Dinger et al. (1987).

2 s

-

Polarization

-

_a

i 1

- -

E -

O

L L

3

⁴

C o

-

Polarization

-

3

2 1

n

0 5 10 15 20 25 30 35

Time of Flight [psl

Figure 2. Ion spectra from. a plexiglas target. The laser intensity is 6.10i3 W / c m h n d the pulse length is 45 ps. The units are drawn to scale.

Figure 2 shows typical ion current spectra for s-polarized (upper part) and p-polarized (lower part) laser light, respectively. The target angle 6 as well as the detector angle h are set to 350, which means that detection is in the direction of the target normal. While during the measurements this position of the target angle is fixed at

3 5 3 , which is in the maximum for resonance absorption for the present

experimental situation of a small focus and steep density gradient (Dinger et al. 19871, the angular dependence of ion emission is investigated in a range from 15" to 65". In all cases the focal intensity is fixed at 6.10'~ ~ c m - ~ .

The enveloping curves of the current in figure 2 show a structured behaviour for both polarizations, whereby the different structures, fast and thermal ion groups, can be correlated to different plasma electron temperatures which are known to coexist in the focal region (Eidmann, 1975; Stenz et al., 1977). It is apparent from figure 2 that in the case of p-polarized laser light the emitted charge is clearly enhanced and in addition, some although weaker increase of the fast velocity ion group is observed. However,in contrast to well known large focal spot results (e.g. Wagli and Donaldson, 1977) the enhancement is not selectively concentrated to one special group of ions, but leads to an essentially uniform strengthening of the whole signal. In particular it is found that, except for the largest emission angle detected (65O), the ratio of the heights of the two main peaks at about 5 fis and 10 fis is nearly constant and equal for both polarizations of the laser.

Similarily the increase of the velocities is nearly equal for both peaks and only less than 10 % for the smaller emission angles.

o : Laser p-pot.

400

0

0 0 . 15 25 35 45 55 65 Emission Angle ideg 1

Figure 3. Angular dependent emission of the H

+

, 02+,

c4+

ions for the two different polarization of the laser. The particle numbers are in relative units.

The situation in figure 2 is found to be general both for most emission angles and ion species. As an example the emission characteristics for H+, 02+ and

c4+

ions are shown in figure 3 in the angular range from 15" to 65':'. Common to all curves is, that maxiumum particle emission occurs in an angular range close to the target normal (35")

.

^However,

2 +

in the case of 0 and

c4+

the emission cone is tilted towards the direction of the incoming laser beam. Except for the 65" direction, an effective increase ih the number of ions is observed for p-polarized laser light. Averaged over all angles and ion species the enhancement is about 2.5. Together with an increase of the average velocity by a factor of about 1.2 the factor of the total increase of the plasma expansion energy can be estimated td be larger than 3.5. Averaged over all angles, the stoichiometric compositions is maintained within 20%

for both polarizations.

The influence of the laser polarization on the degree of ionizations of the C and 0 ions (4a,b) and of the total plasma (4c) is shown in figure 4. Suprisingly in each case, the degree of ionization is higher, for s-polarized laser light, the effect being stronger for the C than for the 0 ions. Averaged over all angles the differences are 27 % , 5 % and 12 % for the C- and 0-ions and the total plasma, respectively.

This result can be understood qualitatively if, for the present situation, the relevant absorption mechanisms (resonance absorption and inverse Bremsstrahlung) and the dominating interactions in the expanding plasma (above all three body recombination) are considered.

If on the other hand, the wavelength of the laser is changed co the first harmonic ( 0 . 5 3 pm) by maintaining a constant focal intensity and polarization (s), an increase in the ion expansion energy is likewise observed (Dinqer et al. 1986).

JOURNAL DE PHYSIQUE

o : Laser p - p o l

x : Laser s- pol. 0-ions

a a B B 8 8

0 0 15 25 3 5 45 55 65

Emission Angle Ideg I

Figure 4. Angular dependence of the emitted average charge for t h e 0, C and the total target ions for the two different polarizations of the laser.

I

^{TARGET: TA}

TIME OF FLIGHT

F i g u r ~ 5. Spectra dN/dt of t h e number of ions from a tantalum target of rhe thermal i o n g r o u p (~a' t o ~ a ~ + ) . The laser intensity is 2.5 * 10 1 "

W/cm2.

As can be seen from figure 5 , where for a tantalum target the summed up spectra of the individed dN/dt distributions are compared for the fundamental wavelength and the first harmonic, this increase is preferentially due to an enhancement in the kinetic energy of the ions, the ablated aass, however, remaining essentially unchanged.

These results are important in practical applications, if e.g. the plasma current is used as an ion source for secondary experiments.

Additionally the results can give indications of the relevant atomic interactions in the plasma during the expansion.

Acknowledaement

This work has been supported by the Deutsche Forschungsgemeinschaft under the SFB 91: Energietransfer bei atomaren und molekularen Stol3prozessen.

References

Balmer J.E & Donaldson T.P. 1977, Phys. Rev. Lett. 2 , 1084

Dinger R., Rohr K., Weber H. 1986 Laser and Particle Beams,

4,

239 1987 Laser and Particle Beams, 3, 691 Eicher J., Rohr K., Weber H., 1983, J. Phys. E: Sci. Instrum. l6, 903 Eidmann K., 1975. Plasma Phys. ll, 121

Estabrook K., 1986, Phys. Fluids 2 9 , 3093

Stenz C., Popovics C., Fabre E., Virmont J., Poquerusse A. and Garban C., 1977, Journal de Phys., 18, 761

Wagli P. & Donaldson T.P., 1978, Phys. Rev. Lett., 40, 875