Laser-produced plasma stigmatic observations in the extreme
ultraviolet by means of a CCD detector
P. VILLORESI, G. NALETTO, P. NICOLOSI, E. PACEand G. TONDELLO
Dipartimento di Elettronica e Informatica, Università di Padova Via Gradenigo 6/A, I-35131 Padova, Italy
(ricevuto l’8 Gennaio 1997; approvato il 24 Gennaio 1997)
Summary. — Observations of the emission from laser-produced plasmas of carbon, aluminum and tungsten have been performed by means of a vacuum normal-incidence stigmatic spectrograph. The detector used is a Peltier-cooled CCD, particularly treated so to be sensitive in the vacuum ultraviolet spectral range. The presented results include the spectrum emitted, in the range from 380 to 800 Å, with spectral and spatial profiles along the expanding plasmas and absolute measurements of the plasma brightness. The measurements show an outstanding capability of this type of detector, in terms of sensitivity, resolution and dynamic range, with respect to traditional devices, for UV detection, as photographic film, intensified linear arrays and scintillator coated CCDs.
PACS 52 .25.Jm – Ionization of plasmas.
PACS 52.25.Qt – Emission, absorption, and scattering of ultraviolet radiation. PACS 32.30.Jc – Visible and ultraviolet spectra.
1. – Introduction
The laser-produced plasmas are extensively studied for a rather wide range of scientific applications. Besides the fusion-oriented experiments, noteworthy are the study of high-intensity interactions of laser beams with matter, of dense plasmas, the research on XUV and X-Ray sources and their applications, on XUV and X-Ray lasers etc. [1-4].
Laser-produced plasmas emit radiation in the XUV and X-Ray spectral range with very high instantaneous brightness. The duration of the emission is strictly related to the laser pulse: with a Q-switched laser it lasts a few ns while with shorter pulses emissions having duration as short as 20 ps have been measured [1, 5]. For this reason the LPPs can be applied to the study of fast transient phenomena, the short duration of the emission allows to reduce considerably the time needed for a measurement [1]. The physical characteristics of the LPPs can be summarized shortly: the spatial scale ranges from tens of mm, comparable to the laser-target interaction region, to a 759
few mm in the expanding plasma plume; the time evolution is on the scale of ns; the density changes several orders of magnitude in a few tenths of mm, from the critical value for the laser wavelength and higher in the interaction layer drops out when the plasma expands; similarly behaves the temperature, which is 0.1–1 keV close to the target. Correspondingly, the emission shows very peculiar characteristics: both continuous and discrete spectra are emitted, their intensity is stronger near the target and gets weaker in a few tenths of mm, the spectral line profiles are affected by various perturbing mechanisms, like Stark and Doppler broadening effects and radiation trapping. It is clear that the study of the LPP requires challenging performances for the spectroscopic instruments. High spatial resolution, less than 30–50 mm, is needed to resolve different emitting regions, high spectral resolution is needed for complex spectra like those emitted by high-Z elements, high dynamic range for the simultane-ous recording of signal intensities which can differ by orders of magnitude.
In the present experiment a laser-produced plasma has been observed with an instrument combining a spectrograph of advanced design and the very ultimate type of UV CCD detector. The spectrograph mounts a toroidal grating for compensating the astigmatism typical of a spherical grating. In this way, stigmatic imaging performances are obtained on a relatively wide spectral interval [6]. The detector is a thinned laser-annealed, ion-implanted and back-illuminated CCD, that shows extremely high sensitivity in the EUV [7]. In addition, the array characteristic of the detector allows the high-resolution recording of bidimensional images.
Spectra of ions of various elements emitted by laser plasmas have been observed both with high spectral and spatial resolution. Information about the dynamic and spectroscopic behavior of the plasma has been obtained. The instrument sensitivity has been calibrated and the absolute measurement of the emitted intensity has been derived. In this way the spectroscopic performances of the complete instrument in the EUV spectral range have been fully characterized.
2. – The experiment
The experimental set-up consists of Q-switched laser, a chamber for generating the plasma and a spectrograph equipped with a bidimensional array detector. A schematic of the chamber and of the spectrograph is shown in fig. 1.
2.1. The laser. – The laser system is composed of a Nd-YAG oscillator and three Nd-glass amplifiers. The oscillator has an unstable cavity, positive branch, and the output coupler has Gaussian reflectivity profile. By the technique of the injection seeding, it operates in single longitudinal mode (SLM). The pulse width may vary between 12 and 30 ns. The amplifier chain may deliver pulses with the maximum energy of 20 J, the output beam has a 25 mm diameter and an almost flat spatial profile.
2.2. The interaction chamber. – The plasma is generated in a small interaction chamber, located in front of the spectrograph entrance slit. It is produced by focusing the laser pulse onto a plane target. The laser beam is focused with a best form lens with focal length of 190 mm, with anti-reflection coating on both sides, and operates at fO20. The lens is placed in air close to the vacuum window. This latter is also anti-reflection
Fig. 1. – Scheme of the plasma chamber and the spectrograph.
coated, with protection from the debris of the plasma, realized by a plastic thin film. The focal spot on the target is located at about 6 mm from the entrance slit of the spectrograph. This position was a good compromise between two different require-ments: keeping the plasma closer to the slit for having good spatial resolution, since the spectrograph has its focus on the slit; keeping the distance of the plasma large enough to reduce the risk of deposit on the slit jaws. In addition, particle contamination of the spectrograph, due to material debris produced in the laser ablation process, was avoided by independently evacuating the interaction chamber with a turbomolecular pump. In the present experiment the laser pulse was limited to about 3 60.2 J of energy, with a duration of 18 61 ns at half width. This in order to reduce the amount of back-reflected light which could have been reamplified in the laser chain; in fact at this time the system was not equipped with efficient optical insulation device. In addition, in some cases, like for carbon targets, the focusing lens has been defocused by about 1–2 mm. Accordingly the laser power density on the target ranged from 1011W/cm2to a few 1012W/cm2in the case of sharp focusing.
The plane of incidence of the laser beam on the target can be set either parallel to the dispersion plane of the spectrograph, that is perpendicular to the entrance slit, or perpendicular to the dispersion plane, i.e. parallel to the slit: we will refer to the first as the transverse observational mode and to the second as the longitudinal one. Due to mechanical constraints, the angle of incidence of the laser beam on the target was about 227, however the expansion of the plasma can be assumed approximately with circular symmetry around the normal to the target. Indeed, this assumption seems acceptable from previous observations, which have shown that the expansion axis of the plasma is in between the normal and the direction of the incoming laser beam. Even for the present experiment, the distribution of the debris on the chamber wall, as well as on the entrance window, leads to estimate an angle of 147 between the plasma axis and the target normal.
2.3. The spectrometer. – The spectroscopic observations of the LPPs have been performed with both spectral and spatial resolution using an imaging spectrometer at normal incidence, equipped with a toroidal grating working in the Rowland configuration [8]. The grating is Pt coated and has a ruling density of 3600 groovesOmm, its radii of curvature are R41011.1 mm in the dispersion and r4 991.8 mm in the sagittal plane, respectively, its ruled area is 63 3 63 mm2and the plate factor is 2.7 Å/mm. The angle of incidence is about 117, and the configuration provides a stigmatic interval of 100 Å, centered at 570 Å: the stigmatic range has been defined assuming as limiting value a factor two of worsening of the spatial resolution with respect to the one at the best stigmatic point. This particular optical configuration has been already tested, showing a spatial resolution better than 30 mm in the stigmatic point [9] at full aperture (approximatively fO14).
In effect, since the plasma is not on the entrance slit of the spectrograph, but it is 6 mm in front of it, the spatial resolution on the plasma is governed by the aperture of the spectrograph. So, both to keep a good spatial resolution on the plasma and also in order to avoid the saturation of the detector in the peaks of the strongest emission lines the aperture in the sagittal plane has been limited to about fO85 corresponding to a projection of a point of the focal plane onto the plasma of about 70 mm. Actually, this limits the spatial resolution in the laser-target interaction region, where physical size and disuniformities of the plasma are very small; but, beyond 300 mm from the target, the spatial resolution results well sufficient to distinguish different spatial features, in fact the gradients of plasma parameters tend to smooth out with the distance from the target.
The entrance slit width was set to 50 mm, and the aperture in the dispersion plane was fO16, corresponding to a gathered length on the plasma per pixel of about 400 mm. In order to scan the full spectral range on the detector, which was mounted on a fixed position in the spectrometer, the grating has to be moved. This is performed with an arm, materializing the grating normal, and pivoted on the center of the Rowland circle (see fig. 1). Since the source is kept fixed, the rotation changes the diffracted wavelength on the detector, but also causes some reduction in the aperture of the instrument for the extreme spectral positions. However, this will affect only the relative efficiency of the spectra out of the stigmatic interval of the spectrograph, and consequently it does not result in a significant drawback for the purposes of the present work.
2.4. The detector. – The CCD detector used for this experiment is a prototype model developed by Electric English Valve (EEV) of the CCD0206 series with the purpose to enhance its sensitivity in the VUV spectral range, where conventional CCDs are ineffective. In particular, it is not possible to use an ordinary front-illuminated CCD, since the thick polysilicon gate structure strongly absorbs photons of intermediate energy 5 EhnE100 eV. On the other hand, neither untreated thinned back-illuminated CCDs are optimal because the path length in silicon of VUV photons is extremely short and consequently the generated photoelectrons can be trapped in a native back-oxide layer, which naturally forms on the back surface of these devices generating a positive potential well, causing a reduction of the detection efficiency. The particular CCD here used went through a process of ion implantation and laser annealing in order to generate an internal electric field which drives the photoelectrons away from the back surface toward the collecting potential wells. In addition, the anodic etching of a portion of the ion-implanted region removes a small
residual dead layer which is ineffective for the photon-electron conversion. As better described elsewhere [9], this treatment permits to have a noticeable quantum efficiency (QE), from about 60% at 300 Å down to 10% at 800 Å.
Since the detector chip is cooled in order to keep the thermal noise sufficiently low, it behaves like a small cryopump in the spectrograph. This implies that some contaminants condensate on the surface of the detector causing a significant drop of its VUV sensitivity with time because of the radiation absorption. The detector QE can be recovered simply by increasing the temperature of the detector to ambient and waiting shortly for the evaporation of the contaminant layer. But, in order to limit this effect the temperature of the CCD has been kept as highest as possible compatibly with the related thermal noise; the latter, indeed, is favored by the fact that the integration time is very short since the LPP emission follows essentially the duration of the laser pulse. The CCD temperature has been kept accordingly at about 2 607 C by means of a triple stage Peltier cooler.
The CCD frame format is 385 3 578 pixels, with pixel size of 22 3 22 mm2, but the implanted region is limited only to about 320 3 500 pixels; so, since the CCD longer direction was usually set along the spectrograph dispersion plane, the detected spectrum was about 11 mm long, corresponding to about 31 Å. The CCD electronics was developed in our laboratory and it is based on the correlated-double-sampling technique for the collection of the analog signal going out from the CCD; this signal is
Fig. 2. – Spatial distribution of the emission of Al XI lines at 550.1 Å. The intensity is calibrated as in table V, and the error is 50%.
later digitally converted by a 16 bits A/D converter and sent to a 486 CPU 66 MHz personal computer which was also used for the image processing. The uniformity of sensitivity throughout the detector area has been tested in the VUV at normal incidence: while it resulted definitely uniform for wavelengths above 100 nm, this was not at shorter wavelengths where some hot spots have been found. However, since the central area of the detector has been tested to have a good uniformity, within a few percent, some care has been taken to acquire the spectra in this portion of the CCD, so avoiding noisy flat-field corrections.
3. – The aluminum spectrum
The emission from aluminum in this spectral region was already observed: in our laboratory, years ago, with another instrument (a 2 m spectrograph) using photographic plates [10], as well as in other laboratories [11]. For the present laser power densities, the plasma exhibits high ionization degree, with strong emission of ionic lines even at distance from the target of several millimeters, and a weak
Fig. 3. – The complete spectrum of aluminum acquired in the experiment. Line labels refer to table I.
continuous spectrum, detectable only in the crater region. Therefore, this element has optimal features both for the wide spectral coverage and for the spectroscopic analysis of the plasma dynamics. In addition the intense lines at 550.01 Å and 568.12 Å of the Al XI Li-like stage, are in the central part of the spectrograph region, that with the highest spatial resolution.
First we have done a spectral survey in the region from 380 to 800 Å at constant laser power density on the target, observing the plasma in the transversal mode. Then, we have carried out the analysis of the plasma expansion observing particular
TABLEI. – List of the observed transitions for the aluminum plasma. The order refers to fig. 3. The intensity reported in arbitrary units is the integral along the spatial profile of the crater region. The error in the intensities is estimated as 10%.
No. Element Wavelength (Å) Configuration Intensity
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Al VIII Al IX Al IX Al VII Al VII Al VIII Al VII Al IX Al X Al X Al IX Al X Al VIII Al X Al X Al X Al X Al IX Al IX Al IX Al IX Al VIII Al VIII Al VIII Al VIII Al XI Al XI Al IX Al IX Al X 383.785 384.06 384.95 386.09 387.52 387.97 392.07 392.40 394.83 395.36 396.05 397.76 399.57 400.43 401.12 403.55 406.31 432.03 432.66 437.46 438.09 480.11 483.03 490.44 493.18 550.01 568.12 602.18 613.10 670.06 2 s22 p2 3P 1-2 s2 p3 3D20 2 s2 p2 2D 5/2-2 p3 2D5/20 2 s2(1S)2p2P0 1/2-2 s2 p2 2D3/20 2 s2 p4 2P 1/2-2 p5 2P1/20 2 s2 p4 2P 3/2-2 p5 2P3/20 2 s22 p2 3P 2-2 s2 p3 3D30 2 s2 p4 2P 1/2-2 p5 2P3/20 2 s2(1S)2p2P0 3/2-2 s2 p2 2D5/2 2 s2 p1P0 1-2 p2 1S0 2 s2 p3P0 1-2 p2 3P2 2 s2 p2 2S 1/2-2 p3 2P1/20 2 s2 p3P0 0-2 p2 3P1 2 s2 p3 1D0 2-2 p4 1D2 2 s2 p3P0 1-2 p2 3P1 2 s2 p3P0 2-2 p2 3P2 2 s2 p3P0 1-2 p2 3P0 2 s2 p3P0 2-2 p2 3P1 2 s2 p2 2P 1/2-2 p3 2P3/20 2 s2 p2 2P 1/2-2 p3 2P1/20 2 s2 p2 2P 3/2-2 p3 2P3/20 2 s2 p2 2P 3/2-2 p3 2P1/20 2 s2 p3 3S0 1-2 p4 3P0 2 s2 p3 3S0 1-2 p4 3P1 2 s2 p3 3S0 1-2 p4 3P2 2 s2 p3 1P0 1-2 p4 1D2 1 s22 s2S 1 /2-1 s22 p2P3/20 1 s22 s2S 1 /2-1 s22 p2P1/20 2 s2 p2 2P 1/2-2 p3 2D3/20 2 s2 p2 2P 3/2-2 p3 2D5/20 2 s2 p1P0 1-2 p2 1D2 1 .1 Q 106 1 .15 Q 106 9 Q 105 3.5 Q 105 6 Q 105 1.3 Q 106 2 Q 105 1.4 Q 106 9.5 Q 105 1.2 Q 106 1.5 Q 105 9.5 Q 105 9.5 Q 105 8 Q 105 2 Q 106 1 Q 106 1.2 Q 106 2 Q 105 6 Q 105 1 .1 Q 106 3 .5 Q 105 3 Q 105 4.5 Q 105 6 Q 105 3 .5 Q 105 1 .5 Q 106 1 .2 Q 106 2 Q 105 3 .5 Q 105 2 .5 Q 105
TABLEII. – Unidentified lines in the spectrum of fig. 3. No. l ( Å ) 1 2 3 4 5 6 7 8 9 10 11 446.34 446.93 447.42 450.24 454.26 455.10 456.15 456.96 464.25 467.81 498.89
transitions at different distances from the target in both the longitudinal and the transversal mode. In the former case, with the plasma expansion axis parallel to the slit, the corona is visible entirely, whereas in the transverse mode, by moving the target and the focusing lens, it is possible to observe the corona at different distances from the target. This provides the direct measure of its transversal extension, spectrally resolved, useful to derive the expansion geometry of different ions.
In fig. 2 the Al XI transition at 550.01 Å is reported. Near the surface of the target the intensity is stronger and the line profile is broadened up to about 0.5 Å FWHM. The spatial direction is binned a factor 3 for the sake of presentation. The space scale is referred to the plasma, that is the size of the binned pixel is scaled according to the magnification of the instrument. The main causes of broadening are the Stark effect and the optical opacity of the line, both due to the high density of the plasma near the surface of the target. Correspondingly, this region emits also a continuous spectrum mainly via bremsstrahlung. The expanding corona is characterized by a lower density and temperature, and then emits only discrete lines with lower intensity and narrower line profiles, as shown in the figure, extending for several millimeters away from the target. For this transition, as for others treated in the following, the intensity is estimated on an absolute scale, as discussed later.
The spectrum emitted by the aluminum plasma observed in the longitudinal mode from 800 Å down to 380 Å, is shown in fig. 3a)-c). It has been obtained by the composition of 20 separately acquired spectra, each corresponding to an interval of about 31 Å and partially overlapping with those nearby. In addition, the acquired spectra have been corrected for the dark signal. The intensity is now in arbitrary units because the spectrograph efficiency is known only about a very limited interval around 583 Å, and it is not realistic to assume it to remain constant in the rest. The intensity is obtained by integrating the emission over the spatial region close to the target, excluding the emission of the expanding corona. Its extension corresponds to a few pixels and changes along the spectrum, it is smaller in the center, because of the loss of stigmaticity on the extremes of the spectral range. The signal between the discrete transitions is due to the continuum and the transitions with wavelength greater than 760 Å are the second order of those from 380 to 400 Å, and are, obviously, of weaker intensity.
TABLEIII. – Transitions observed in second order in fig. 3.
Ion Transition DJ Wavelength (Å)
Al V 2 s22 p5-2 s 2 p6 2P0-2S — (3/2-1/2) (1/2-1/2) 378.71 381.42 Al VI 2 s22 p4-2 s 2 p5 1D-1P0 2 s22 p4-2 s 2 p5 3P-3P0 — — — — — (2-1) (2-1) (1-0) (2-2) (1-1) (0-1) (1-2) 243.74 307.33 308.55 309.63 309.84 310.92 312.24 Al VII 2 s22 p3-2 s 2 p4 2D0-2P — 2 s22 p3-2 s 2 p4 2P0-2S — 2 s22 p3-2 s 2 p4 2D0-2D — — 2 s22 p3-2 s 2 p4 4S0-4P — — 2 s22 p3-2 s 2 p4 2P0-2P — — — 2 s 2 p4-2 p5 2D-2P0 — 2 s 2 p4-2 p5 2P-2P0 (3/2-1/2) (5/2-3/2) (1/2-1/2) (3/2-1/2) (3/2-3/2) (3/2-5/2) (5/2-5/2) (3/2-1/2) (3/2-3/2) (3/2-5/2) (1/2-1/2) (3/2-1/2) (1/2-3/2) (3/2-3/2) (3/2-1/2) (5/2-3/2) (3/2-1/2) 239.09 240.74 278.97 279.18 309.02 309.11 309.11 352.10 353.83 356.93 259.04 259.21 261.05 261.22 282.64 385.78 381.65 Al VIII 2 s22 p2-2 s 2 p3 1D-1D0 2 s22 p2-2 s 2 p3 1S-1P0 2 s22 p2-2 s 2 p3 1P-3P0 — — 2 s22 p2-2 s 2 p3 3P-3D0 2 s22 p2-2 s 2 p3 3P-3S0 — — 2 s22 p3-2 p4 3D0-3P — — (2-2) (0-1) (0-1) (1-2) (1-2) (2-2) (0-1) (0-1) (1-1) (2-1) (2-1) (3-2) (2-2) 285.44 287.02 323.44 325.17 328.11 381.12 247.40 248.45 250.14 286.55 289.06 289.06
TABLEIII (Continued).
Ion Transition DJ Wavelength (Å)
Al IX 2 s2(1S)2p-2 s 2 p2 2P0-2P — — 2 s2(1S)2p-2 s 2 p2 2P0-2S — 2 s 2 p2-2 p3 2D-2P0 — 2 s 2 p2-2 p3 2P-4S0 — — (1/2-3/2) (1/2-1/2) (3/2-1/2) (1/2-1/2) (3/2-1/2) (5/2-3/2) (3/2-1/2) (1/2-3/2) (3/2-3/2) (5/2-3/2) 280.14 282.42 286.33 300.63 305.08 306.91 307.33 316.71 318.42 320.99 Al X 2 s2-2 s 2 p 1S-1P0 (0-1) 332.80
The identifications of the transitions shown in fig. 3 are reported in table I. The numbers in the first column state correspond with fig. 3. Some lines present in the figure have not been identified, and are listed separately in table II. Nevertheless, the most of them can be attributed with relatively good certainty to the Al VI or VII ionization stages. This conclusion is based on the observations taken in the transverse mode in other similar experiments [10], and will be discussed later.
Finally, in table III the short wavelength Al lines observed only in the second-order spectrum are reported: these were definitely weak compared to the first-order spectrum. Here they have been grouped by ionization stage.
The wavelength measurements have been derived by interpolating some standard lines throughout the spectrum: in particular, some O IV lines emitted by impurities present in the plasma and also a few Al lines whose wavelength measurement can be assumed well established. The greatest uncertainty affecting these measurements is due to the width of the spectral lines, which are broadened because of the presence of the Stark effect, the thermal Doppler, the motional Doppler broadening and the optical opacity effect. The thermal Doppler effect can be assumed negligible in this case, but near the target the Stark effect is dominant and far away some residual broadening can be due to the motional Doppler for the expansion of the plasma and to the finite instrumental resolving power. The two latter contributions can be estimated globally to give about 0.15 Å, as obtained by very narrow line profiles observed far away from the target. In addition, some lines with high values of oscillator strength and having the lower level of the transition considerably populated (ground or low excited levels) can be strongly affected by radiation trapping and consequently can show saturation in the spectral profile. In conclusion, on the basis of these evaluations we can estimate that the average uncertainty in the wavelength measurements is about 0.07 Å.
Most of our measurements is in agreement with the values listed by Kelly [12] with the exception of the lines of Al IX at 602.18 Å, 613.10 Å and that of Al X at 394.83 Å which have a considerably different value [13].
4. – The carbon spectrum
The most prominent spectral lines emitted by a carbon plasma in the analyzed spectral range correspond to the transitions 1 s22 p-1 s23 s or 1 s22 p-1 s23 d in C IV, 1 s22 s 2 p-1 s22 s 3 l and 1 s22 p2-1 s2
2 p 3 l in C III with l 4s or d, and to the two transitions 3 d-4 f in C V and 3-4 in C VI between relatively high excited levels. Within the stigmatic range only the first line of the Paschen series in the C VI H-like and a few C III lines appear; for this reason the observation of the carbon plasma emission has been mostly limited to this reduced spectral interval. The most interesting lines which TABLEIV. – List of the observed transitions for the carbon plasma. Intensities and errors as in table I.
No. Element Wavelength (Å) Configuration Intensity
1 2 3 4 C III C VI C III C III 511.5225 520.6 535.2885 538.3120 2 p2 1D 2-2 p 3 d1F30 3-4 2 p2 1D 2-2 p 3 d1D20 2 s 2 p3P0 2-2 s 3 s3S1 2 Q 104 6.6 Q 104 1.8 Q 104 2.5 Q 104
Fig. 4. – Contour plot of the C III line at 511.5 Å and of C VI line at 520.6 Å, with longitudinal mode of observation.
Fig. 5. – Emission spectrum from the carbon target: a) from the crater, maximum emission, and b) 2.6 mm from the target surface.
have been detected are reported in table IV with their wavelengths, identification and relative intensity.
Since carbon is an element with a low atomic number Z, it results strongly sensitive to plasma-induced perturbations: consequently, the emitted line profiles show noticeable asymmetric broadening due to the high density and temperature and to the expansion motion of the plasma. This fact can be clearly observed in fig. 4, that shows the contour plot of the lines at 511.5 Å of C III and at 520.6 Å of C VI, taken in the longitudinal mode. Target position is at the origin of abscissae. Fine details of the spectrum are noticeable, together with the information on the spatial expansion of the plasma. In particular, the emission from the plasma plume extends several millimetres from the target, showing intensity decreasing with the distance from the target; moreover, the spectral profile is significantly broadened near the target surface, and progressively narrow and narrow at larger distance. This is also clear in fig. 5, where the most intense lines falling in the 500–540 Å interval are shown for two different conditions: when they are emitted by the plasma about the target surface, where is maximun the emission
(
marked in fig. 5 as a))
and when they are emitted 2.6 mm far away(
marked as b))
. The intensity of the spectrum emitted near the target is an order of magnitude stronger than the one emitted far away, due to the great difference of temperature and density, also confirmed by the absence of continuous background emission among the lines in the b) spectrum.The line of C VI around 520 Å that is a transition from n 43 to n44, shows a considerably broadened spectral profile. In fact it corresponds to the multiplet
Transition Wavelength (Å) f-value 3s-4p 520.298 0.4847 3p-4s 520.806 0.03225 3p-4d 520.605 0.6183 3d-4p 520.945 0.01099 3d-4f 520.704 1.018
where f is the oscillator strength. Indeed the lines 3s-4p, 3p-4d and 3d-4f are the most prominent. Because of some perturbations due to plasma effects, the fine-structure components of this multiplet cannot be clearly resolved, and some structures appear in the observed profile only a few millimetres away from the target. To investigate this relatively large line broadening, mainly in the proximity of the target, the quasi-static theory can be used: in fact, Irons [14] as well as Kepple and Griem [15] studied the lines of hydrogenic ions, like the ones of CVI, concluding that this theory gives the best description of the dependence of the observed broadening on principal quantum number, nuclear charge and electron density. In particular, Irons studied transitions between high excited levels with Dn 4 1 deriving an approximated formula of the observed broadening. So, the analysis of the 3-4 C VI line profile can be a good diagnostic tool for the measurement of the electron density and charged particle density. In fig. 6 the evolution of the line profile for this multiplet can be followed vs. the distances from the target: it can be seen that the width decreases rapidly with the distance from the target, tending to a constant value at a few millimetres from it. At this distance the residual broadening effects are mainly the motional Doppler and the finite instrumental resolution.
From the analysis of the reported FWHM, only within 0.1 mm from the target hydrogenic conditions can be fulfilled, and the quasi-static theory can be applied. Both
Irons and Griem give for this case
ne2 /34 1 .1 Q 1013Dl( Å ) , (1)
where D l is the FWHM in Å and for the other parameters we used Zp4 2 for the average charge of perturbers and Z 46 for the nuclear charge.
The measured FWHM are 1.7 Å at target position and 1 Å at 0.1 mm of height; we derive, respectively, an electron density ne4 8.2 Q 1019cm23 and ne4 3.7 Q 1019cm23. This value is consistent with other measurements performed in quite similar experimental conditions [16].
In fig. 5 we can notice a peculiar behavior of the intensity of some transition attributed to oxygen impurities, in particular they belong all to the O III ion: in the crater spectrum the intensity is very low, while in the corona spectrum they have higher intensity. A possible explanation is given by assuming that the emission of this lines comes actually from the external region of the crater, with relatively low intensity; as the plasma expands the decreasing temperature may induce the recombination of oxygen species with higher degree of ionization, that reinforces the emission intensity. A further possible factor of this strange behavior may be also the angular aperture of the plasma plume and the velocity of the expansion, that reasonably can be different with respect to the carbon ions. For this reason different density decrease ratios, in the region observed by the spectrograph, may sustain the oxygen emission.
5. – Spatial expansion of the plasma
The free expansion of the plasma from a plane target takes place in a cone-like plume. The aperture of this cone is related to the target element, the laser spot size and also to the particular ionization stage under observation. In the following we compare the aperture of the plasma plume as a function of the distance from the target, for three interesting cases that fall in the stigmatic region of the spectrograph: they are the C VI line at 520.6 Å, the C III line at 511.52 Å and the Al XI line at 550.01 Å. Their profiles have been obtained in the transverse mode described above, with reduced aperture to increase the spatial resolution. Moving both the target and the focusing lens with respect to the spectrograph entrance slit, it is possible to observe the emission from thin slices of plasma, at various distances from the target. As an example of this observation, the spatial half-width at half maximum HWHM of the C VI line is shown in fig. 7 at different distances. The profiles are integrated along all their spectral extension, to properly consider the contribution at every spatial position: the effect of the expansion, in fact, is clearly visible as a marked broadening of the profile.
The analysis of spatial profiles was performed for the cases of the three transitions mentioned above. Every profile was fitted with a Gaussian curve by the least-squares method. The HWHM of the fitting curves is then representative of the aperture of the plasma cone at that position. Figure 7 shows this half-width vs. the distance from the target for the three cases: fig. 7 (bottom) refers to the carbon plasma and fig. 7 (top) to the aluminum one.
The higher-ionized C VI is confined near the axis of the plasma more than the lower ionization stage C III, while the Al XI expands in a narrower cone up to longer distances compared to carbon.
Fig. 7. – Aperture of the plasma cone vs. the distance from the target for the aluminum and carbon ions.
6. – Intensity measurements
The observations made in the longitudinal mode make clearly visible the drop of the intensity with the distance from the target. As previously described, this is due to the decay of plasma temperature and density related to the progressive expansion and acceleration of the plasma corona.
In fig. 8 the intensities of the C VI line at 520.6 Å of the C III line at 511.52 Å and of the Al XI line at 550.01 Å, integrated over the spectral profile, are reported on a semi-logarithmic scale as a function of the distance from the target. The aluminum line detected near the target region saturates the CCD, as revealed by the flat top of its graph. After the first millimeter, all the three lines show an almost linear drop, corresponding to an exponential spatial decay lasting for more than 5 mm; in the initial part the decrease is steeper, corresponding to the acceleration phase of the plasma, during which its expansion velocity is lower. In this region the density is then higher and so is the emission.
An absolute estimate of the intensity emitted by the plasma has been derived from the efficiencies of the detector and the spectrograph. The CCD detector QE 40.156 0.02 and the conversion efficiency, e 41.160.1 electrons per ADU, were measured in
Fig. 8. – Intensity of the emission vs. the distance from the target for the aluminum and carbon ions.
the spectral range from 300 to 11 000 Å [17] with a dedicated facility in our laboratory. The spectrograph was calibrated only in correspondence of the HeI resonance line at 584.334 Å, showing a global efficiency Rsp4 6.5 6 0.6%, including the diffraction efficiency of the grating and the reflectivity of the platinum coating.
For the transitions of aluminum and carbon shown in the previous sections, the spectrograph efficiency has been assumed to remain constant at this value, as it is reasonable for the short wavelength interval. It is then possible to estimate the plasma peak brightness integrated over the plasma emission lifetime. For this calculation, the solid angle gathered by the instruments, V 49.660.9 1025sr, the spectral width D l corresponding to one pixel, that is 0.06 6 0.01 Å, and the plasma surface over which the emission has been integrated, A 41.4 6 0.07 Q 1024 cm2, that is the area of the pixel projected into the plasma, have been measured. The result has been derived according to the following relation:
B(l) 4Imax
r EQhRspDl AV
,
where Imax is the peak intensity in ADU and r is the quantum yield of the CCD for photons of energy E, that is given by
r 4 E( eV ) 3 .65 .
B(l) is given in units of photons cm22 Å21sr21. The integrated brightness for the transitions mentioned above as well as for the emission in this region of a tungsten target, that has a continuous spectrum, is reported in table V. The errors reported are
TABLEV. – Estimate of both instantaneous and time integrated brilliance for various plasmas. Ion Wavelength (Å) Time integrated brightness (phOcm2Å srad) FWHM duration (ns) Brightness (phOcm2Å srad) Al XI C VI C III W 550.01 520.6 511.5225 530 8 63Q1014 3 61Q1014 1 60.3Q1014 7 62Q1014 23 61 16 .5 6 1.5 35 62 17 61.5 4 62Q1022 2 61Q1022 3 61.5Q1021 4 62Q1022
obtained by the composition of the individual uncertainties. The Al XI transition is the most intense; this is due to the strength of the line and to the high density of this ion species. Also the tungsten spectrum shows a high value, due to the contribution of the various ion species to the emission. Then there are the two carbon lines, that are in order C VI stronger than C III, that reflects the difference in the population of the species, i.e. the degree of ionization, due to the relatively high plasma temperature.
In order to determine the instantaneous plasma brightness, the plasma lifetime has to be measured as well. For this scope, we have changed the CCD detector with a PMT detector. The detection system was realized with a slit of about 1.1 mm of aperture placed on the spectrograph focal plane. Behind this, a scintillator for the conversion of the UV light into visible was placed, it was realized with a vacuum deposition of tetraphenyl butadiene (TPB), which has a very short fluorescence time [8, 18], on a plane glass plate. Then there was a vacuum-sealed glass window to exit from the spectrograph, and finally in air was placed a photomultiplier tube (PMT), Hamamatsu mod. R500, whose photocathode QE peak matches quite well the scintillator spectral emission and has a short rise time of 1.4 ns. The PMT traces were recorded with a Tektronics analogic oscilloscope mod. 2467B, 400 MHz of bandwidth, equipped with a
digital camera system (DCS), Tektronics mod. 1002, that digitalized and saved the image of the trace for the analysis. The observations were performed in the transverse geometry, in order to avoid the recording at the same time of the emission from the crater and the corona. The laser pulse duration was, for these measurements, of 17.5 6 1 ns. The time evolution of the aluminum transition at 550.01 Å is shown in fig. 9, as an example of the typical behavior; in the figure the average of two acquisitions performed in equal conditions is shown. The duration of the emission in the VUV for the highest ionization stages of both for the C, Al and also the tungsten plasmas, appears to be comparable with that of the laser pulse, and are reported in the fourth column of table V. In the case of C III, the emission lasts longer, about 35 6 5 ns, in agreement with the interpretation related to the space-time evolution of the different species. The tungsten emission originated in a target on which we have done another shot has shown a duration longer of about 5 ns with respect of the first one. From these measurements we have derived the instantaneous plasma brightness for the different plasmas, that is reported in the last column of table V.
7. – Conclusions
The reported observations clearly show the outstanding performances of a UV-enhanced CCD detector coupled with a stigmatic imaging spectrograph. In particular, spectroscopic observations of LPPs in the vacuum ultraviolet have shown that both high spectral and spatial resolution are easily achievable in a relatively wide spectral range. In addition, the size of the detector and the optical properties of the spectrograph allowed to record spectroscopic images covering a spectral interval larger than 30 Å with spatial extension of a few mm. Signals with intensity varying more than two orders of magnitude have been detected on a single-shot base thanks to the high linearity and dynamic range of the detector. The high sensitivity of the detector allowed also the recording of very weak signals. The minimum detectable amplitude can be estimated in the following manner. In a previous measurement the detector has been characterized determining the conversion factor (1.1 e2
OADU ) the read-out noise (22 e2), the QE (30% at 584 Å, 8% at 900 Å), as well as the absolute efficiency of the spectrograph of 6.5% at 584 Å.
The measured QE value takes into account also the CCD charge collection efficiency (CCE) so the number of collected electrons per incident photon results 1.7 when the QE is multiplied by the ratio between the photon energies (at 584 Å) and the energy corresponding to an electron-hole pair generation (3.65 eV). Assuming a minimum detectable signal of about 60 ADU, it corresponds to about 30 photonsOpixel. Finally, the possibility of having a real time display of the data in a form ready for the following elaboration via computer is worth noting.
In conclusion, the observations show the superior performances in terms of linearity, dynamic range, sensitivity and resolution of this system compared to the traditional photographic detection or intensified visible CCDs coupled with a scintillator in the focal plane of the instrument.
* * *
The authors wish to thank EEV which supplied some of the CCDs, whose performances have been evaluated and presented in this report. Very important was
also the help in the realization of some mechanical components by Mr. A. CALI` and
Mr. A. PACCAGNELLA. The interest on this work of Prof. M. RODONO`, Dr. E. JANNITTI
and Dr. L. POLETTO is also acknowledged. This work was carried out within the
framework of the EC HCM Network Program no. CHRX-CT93-0361.
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