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SPACE RESOLVED MEASUREMENTS OF THE EMISSION OF INTERMEDIATE AND HIGH
IONIZATION STATES OBTAINED FROM VACUUM SPARK PLASMAS USING XUV MULTILAYER
MIRRORS AND A RbAP CRYSTAL
M. Finkenthal, D. Stutman, J. Schwob, J. Underwood
To cite this version:
M. Finkenthal, D. Stutman, J. Schwob, J. Underwood. SPACE RESOLVED MEASUREMENTS OF THE EMISSION OF INTERMEDIATE AND HIGH IONIZATION STATES OBTAINED FROM VACUUM SPARK PLASMAS USING XUV MULTILAYER MIRRORS AND A RbAP CRYSTAL.
Journal de Physique Colloques, 1988, 49 (C1), pp.C1-87-C1-90. �10.1051/jphyscol:1988117�. �jpa-
00227437�
JOURNAL DE PHYSIQUE
Colloque C1, Supplement au n03, Tome 49, Mars 1988
SPACE RESOLVED MEASUREMENTS OF THE EMISSION OF INTERMEDIATE AND HIGH IONIZATION STATES OBTAINED FROM VACUUM SPARK PLASMAS USING X W MULTILAYER MIRRORS AND A RbAP CRYSTAL
M. FINKENTHAL, D. STUTMAN, J.L. SCHWOB and J.H. UNDERWOOD' R a c a h
Institute
of Physics, TheHebrew
University,IL-91904
Jerusalem, Israel
'center for
X - r a y O p t i c s ,Lawrence
Berkeley L a b o r a t o r y , Berkeley, CA 94720, U.S.A.Rksumb
On dktermine 1'6tendue spatiale des rbgions Qmettant les raies spectrales des degrbs dlionisation interm6diaires ( tels quo ceux de la s6quence de NeI ) et kIev0s ( s6quence d e 1.i I ) des C.16ments. Ti, V, Fe, Co et Ni au sein d l u n e 6tincelle sous vide
A
faible inductance et ti courant &lev&, en uti- lisant des miroirs ti couches multiples ou ii l t a i d e d t u n cristal RbAP. Les spectres ont 6tk enregistrbs dans le domaine 10 ii 308 .
L a conclusion princi- pale d e cette &tude est que la r6gion d16mission des degrks dlionisation interm6diaires est de l'ordre de 200 microns, tandis que les 6tats dtionisa- tion extr&mes sont formEs dnns le plasma pouctuel produit par striction.Abstract
The spatial extent of the regions from which lines of the intermediate, NeI-like and high Li I-like ionization states of Ti, V, Fe, Co, and Ni, are emitted in a low inductance high-current vacuum spark plasma, has been determined using multilayer mirrors and a RbAP crystal. The spectra have been recorded in the 10-30 A range.
The main conclusion of the present work is that the intermediate ionization states are emitted from an extended plasma region, of the order of 200 microns, while the highly ionized states are formed, as it has been previously stated, in the minute plasma pinch.
The present work was motivated by the authors interest in the possible use of the vacuum spark, or a similar, more powerful pulsed source, as a "flashlamp" for XUV optical pumping experiments (1). Indeed, in low-inductance high-current vacuum spark discharges, narrow and bright quasi-continuum bands emitted by rare-earth ions have been recorded in the soft X-ray range, 70-100 A (2). To evaluate the potential of these bands for optical pumping, it is essential to know the spatial extent of the emitting region and its electron density and temperature. The transient nature of the spark discharge and the minute dimensions of the plasma region emitting the highly ionized states ( 3 - 6 ) * , make these measurements difficult. Furthermore, the complexity of the spectra of the heavy elements above mentioned (2>60), makes the use of the line emission for plasma diagnostics even more difficult. In order to cope with these difficulties, in a first stage, presented here, discharges using anodes made of elements with 2=22-28 were produced. The intermediate ionization states of these elements, formed in the spark plasma, i.e. ions with the 2pk shell open, have ionization potentials similar (or slightly higher) than those of the rare-earth ions of interest produced in a similar discharge. Therefore,any information about the plasma region fromwhichthe NeI,FI,etc.,ions emit will beof interest in the above mentioned context.
The present paper describes the imaging experiments in which two multilayer mirrors(~~~),boron nitrate and tungsten-carbon, and a RbAP crystal have been used to evaluate the extent of the plasma region emitting the spectra of the above mentioned intermediate states. The conclusion is that the region emitting the ionization states centered around Nel, is a relatively extended plasma zone of 200 microns; its electron density and temperature are estimated to be several times 10'' cm-' and 200- 400eV, respectively (these results are presented and discussed in a different paper (7))* The present work also confirmed that high charge states, such as Li I-like (and He I and H I-like, as shown in a previous work (4)) are emitted from the high- d e n s l t y / h i g h - t e m p e r a t u r e , minute plasma point source (pinch). Our experimental results agree well with thooe predicted by the "radiative collapsen model (8).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988117
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PHYSIQUET h e low-inductance high-voltage vacuum spark system is similar to that described in reference (4). A 24 pF capacitor bank was charged to (maximum) 17kV and an externally triggered discharge was initiated in an anode-cathode gap of 3-5 mm.
The peak discharge current may reach 250 kA and the discharge cycle is 5 usec long.
In the experiments discussed here, the discharge was imaged simultaneously by a soft X-ray pinhole camera and the MLM or the crystal system. High resolution spectra, which enabled the temperature and density estimates, have also been obtained in a single discharge simultaneously with hard X-ray pinhole camera imaging, using either the RbAP crystal or a 2m grazing incidence Schwob-Fraenkel spectrometer. The pinhole camera was mounted inside the discharge chamber; a 5 micron Ak filter and a 2 micron polycarbonate coated with 3000 A aluminum filter, have been used in front of X-ray Kodak films. The pinholes were several microns in diameter. The first filter enabled to image the discharge in X-rays up to 15 A, while the second gave an image, roughly up to 70 A.
BN/V and W/C multilayer mirrors produced at the Lawrence Berkeley Laboratory Center for X-ray Optics, have been used to obtain space and energy resolved images of the spark plasma. The d value, representing the sum of the spacer and reflecting layer width, is 36.1 A and 22.1 A for the BN/V and W/C mirrors respectively. In both cases the total number of layers is 50, thus giving a spectral resolution of the slitless system of the order of 0.5-1 A , in the range of interest. The RbAP crystal, produced by Quartz & Silice (France) has a 2d value of 26.12 A. Both mirrors and crystal were placed at 23 cm from the source and a filter, used to eliminate visible and EUV light and protect them from debris ejected by the spark, was inserted between them and the transverse slit. The Bragg angle of incidence at the center of the dispersive elements was varied from 15O to '15" enabling thus an overall spectral coverage in various experiments, from 15 A to 30 A. A Kodak SC5 plate fragment was used to record the spectral image reflected by the mirror.
Spectra of vanadium, iron, cobalt and nickel, have been recorded in the 10-20 A range, using the RbAP crystal as dispersive element; spectra of the same elements have also been obtained using the W/C mirror. The titanium spectrum in the 20-30 A range, has been recorded by means of the BN/V mirror. Figure 1 shows the comparison between the high resolution iron spectrum obtained in the 12-18 A range with the qrystal and the low resolution MLM spectrum of iron in the same Spectral range. This low-resolution spectrum is still very informative: it enables the identification of the main emitting charge states, and inside a given ionization state the comparison of the two different transitions. For instance, the well separated 2p-3s and 2p-3d transitions in the Ne I-like ion at 15 A and 17 A , can yield information on the emitting plasma parameters.
The comparison of the X-ray and soft X-ray pinhole images of various discharges indicated already that the charge states emitting in the 20-100 A range, represent an extended source. A low resolution, titanium spectrum obtained using the BN/V mirror.
narrowed down the focus and showed that the Ti spectrum in the 20-30 A range is emitted by a plasma having a spatial extent of the order of 200 microns. Figure 2 presents the image of the vanadium plasma at various wavelengths, obtained using the imaging system with the RbAP crystal. It is clear that the Li I-likeVXXI lines at 14 A are emitted by a point source, while the 2p-5d. Ne I-like VXIV lines at 16 A c o m e from a n extended source. C o n s i d e r i n g Lhe geornctry of the system, a magnification of seven of the emitting region is obtained if the emitting zone is larger than the narrow transversal slit (50 urn). The extent of the "blurb?"
perpendicular to the dispersion direction is 1.5 mm; one concludes therefore that the region of plasma emitting the Ne I and F I-like ion lines, is of the order of 200 pm.
The "radiative collapseT1 model predicts indeed that the spark plasma column is constricted during several tens of nanoseconds and reaches a quasi-equilibrium with a radial extent of the order of 100 um. During this time, the electron temperature increases to 300-400 eV and the maximum ionization state is FI-like. At these conditions, the electron density of the emitting region, as predicted by the model, is of the order of 1OZ0cm-\ The further collapse towards the micro-pinch state, where temperatures of several keV and densities of the order of 1 0 2 * c m ~ s a r e predicted, happens on a time scale of 1 nsec. The extent of the first quasi- equilibrium zone and the emitting ionization states agree well with those predicted by the model. The fast transition from the pre-pinch to the micro-pinch, prevents the gradual formation of the "missing" N I-to F3 I-like states.
Figure 1. Comparison of t h e vacuum spark iron spectra obtained: (a) with the RbAP crystal and (b) W/C nlultilayer mirror.
Figure 2. The Vananliurn spark spectrum obtained using the R b A P crystal as dispersive ele~nent; the narrow features a t 14.5R are LiI-like, V XXI images of two plasma pinches while t h e features a t 16
A,
are NeI and FI-like V XIV and V XV images of extended pre-pinch region. Under our experime~ltal conditions (see text), this region is of t h e order of 200 pmREFERENCES
1. M. Finkenthal, J.L. Schwob and P. Mandelbaum, Journal de Physique (19861, 129.
2. P. Mandelbaum, M. Finkentha1,J.L. Schwob, and M. Klapisch, Phys.Rev.A L ( 1 9 8 7 ) 5051
.
*
The number of reports on the subject is quite large; the following are only a few representative examples.3. T.N. Lie and H.C. Elton, Phys.Rev.A.3- (1971 1, 865.
C1-90 JOURNAL
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PHYSIQUE11. B.S. F r a e n k e l and J . L . Schwob, Phys. L e t t . ( 1 9 7 2 ) , 81 a n d P h y s . L e t t . 4OA ( 1 9 7 2 ) , 83.
5. W.A. C i l l i e r s , R . U . D a t l a and H.R. Griem, Phys.Rev.A
12
( 1 9 7 5 ) . 1408.6. C.R. Negus and 1.I.T. Peacock, J . Phys. D
12
( 1 9 7 9 ) . 91.7 . M. F i n k e n t h a l , D. S t u t m a n , J . L . Schwob, P. Mandelbaum a n d B. G o l d s t e i n , M. C h e n , M. K l a p i s c h , t o b e p u b l i s h e d , J. Phys. B A t . and Mol. P h y s i c s .
8. V.V. V i k h r e v , V . V . Ivanov and K.N. Koshelev, Sov. J . P l a s m a P h y s . 8(6) ( 1 9 8 2 ) , 6 8 8 a n d K . N . K o s h e l e v , Yu,V. S i d e l n i k o v , V.V. V i k h r e v , Acad.Sci. USSR. p r e p r i n t //I, 1985.
