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SOFT X-RAY EMISSION FROM GAS PUFF IMPLOSIONS
P. Burkhalter, G. Mehlman, F. Young, S. Stephanakis, V. Scherrer, D.
Newman
To cite this version:
P. Burkhalter, G. Mehlman, F. Young, S. Stephanakis, V. Scherrer, et al.. SOFT X-RAY EMISSION FROM GAS PUFF IMPLOSIONS. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-247-C6-252.
�10.1051/jphyscol:1986631�. �jpa-00225874�
C o l l o q u e C6, s u p p l 6 m e n t au no 10, T o m e 47, octobre 1986
SOFT X-RAY EMISSION FROM GAS PUFF IMPLOSIONS
P.G. BURKHALTER, G. MEHLMAN*, F.C. Y O U N G , S. J. S T E P H A N A K I S , V.E. S C H E R R E R and D.A. NEWMAN*
Naval Research Laboratory, Washington, DC 20375-5000, U.S.A.
' ~ a c h s / ~ r e e m a n Associates, Inc., Landover, MD 20785, U.S.A.
Resume - Un generateur d'impulsions de 2TW produit, B partir d'une bouffee de gaz, une colome de plasma de densite BlevBe. On a etudie ces implosions de bouffees de neon et d'argon du point de vue de la realisation d'un milieu laser. L'appareillage utilise pour les diagnostics permet l'observation radiale du rayonnement pour estlmer l'uniformite et le degre d'ionisation des plasmas. L'interpretation des donnees spectroscopiques obtenues sur le neon introduit sous forme de trace dans les plasmas d'argon. indique une temp6rature trop &levee (200-300 eV) pour une production efficace d'ions Ar IX (isoelectronique du neon). Le rayonnement des plasmas de neon - produits B des pressions s6lectionnees fournit 2 & 3 kJ dans les transitions de 1'Blectron Is, selon une repartition uniforme dans l'espace interelectrode de 4 cm.
On discute des efforts d'optimisation de cette source. On a obtenu, avec une decharge dans un tube capillaire associee au g6n6rateur d'impulsions, une emission de 30 GGI
pour la raie 1s-2p de l'ion Na X, B 11 A .
Abstract - Dense plasma columns are produced in a 2-TW pulsed-power generator retrofitted with a gas-puff valve. Z-pinch implosions of neon and argon gas puffs are studied as potential x-ray lasing media. X-ray diagnostics record radially the emitted radiation to evaluate the plasma uniformity and the degree of ionization.
Interpretation of spectra from argon implosions containing neon tracer indicates /
plasma temperatures too high (200-300 eV) for efficient production of Ne-like ArIX transitions. Neon plasmas at selected gas pressures emit 2-3 ~ J I of K-shell line radiation with good uniformity along the 4-cm interelectrode gap. Erforts towards source optimization are discussed. Replacing the gas puff with a capillary discharge source for sodium implosion experiments produced -30 GW of the 11 A He-a line from Na X I - INTRODUCTION
High current pulsed discharges of exploded wires and gas puff columns, provide intense sources of keV x-ray and subkeV ( X W ) radiation. Gas-puff implosions of neon driven by the 2-TW pulsed power generator (Gamble 11) at the Naval Research Laboratory have been studied. l2 Annular gas columns of argon, neon, or mixtures are imploded with 1.0-1.5 MA driving currents to produce 4-cm long 2-pinch implosions in the interelectrode gap. In this work, x-ray intensities, plasma parameters, uniformity of implosion, and the degree of ionization are evaluated with diagnostics which provide temporal, spatial, and spectral measurements.
This effort was directed towards producing a suitable XW laser gain medium by studying the plasma implosions obtained with Gamble I1 as the load
parameters were varied. Two schemes for population inversion (in XW laser scenarios) are potentially achievable:
excitation of neon-like argon for lasing at long wavelength (3p-3s at 460 A) ,l3
and coincidence photopumping of Ne IX by the 11 A line from NaX for lasing at 230
A (n=4 to 3 transitions) .l4
The Gamble I1 generator is configured with a gas puff valve and a supersonic nozzle, as shown in Fig. 1 in order to inject a hollow annular gas column across the 4-cm interelectrode gap. The 2.5 cm diameter gas puff nozzle is located on the machine axis and is surrounded by the return-current posts on the 7 cm diameter circle. Wire stretched across these posts serves as Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986631
C6-248 JOURNAL DE PHYSIQUE
the anode. The gas is preionized with two W flashboards prior to current discharge. Normally, the driving current has a 70 ns risetime; however the risetime can be reduced to 20 ns with the use of a lasma erosion opening switch (PEOS) . l5 3 The PEOS consists of a low density plasma injected across the vacuum gap of the electrode assembly as indicated in Fig. 1. The use of a PEOS to eliminate prepulse and to sharpen the current risetime has led to improved plasma uniformity in argon and neon implosions.
Fig. 1. Schematic drawing of the pulsed-power load assembly for generating imploding plasma.
Diagnostics are mounted around the vacuum chamber to view radially the imploded plasma. The location of the following diagnostics is shown in a photograph of the front end of Gamble I1
(Fig. 2). The diagnostics include: 1) filtered x-ray pinhole cameras for time integrated images of the implosion, 2) filtered x-ray diodes (XRD's) for intensity and time history measurements, 3) convex-curved crystal spectrographs to collect x-ray spectra in the 4-14 A
region, and 4) a l-m grazing incidence spectrograph to record 500-40 eV (20-300
A) XUV spectra. A fast closure valve is used as a debris shield for the grazing incidence spectrograph.
Filtered XRD's were used to record X-ra emission in selected spectral regions. 13
For argon implosions an Al-cathode XRD filtered with 0.7-pm Ti was used to measure L-shell emission from 0.25 to 0.46 keV (Fig. 3). This filter was also used on a pinhole camera with Kodak 101- 05 film to record L-shell images of the
--
Fig. 2. Photograph viewing the plasma forming region (nozzle) and the diag- nostic instrumentation on Gamble 11.
imploded plasma. For neon implosions, a Ni-cathode XRD with a 1.0-pm Cu filter was used to measure the neon He-a line.
The crystal spectrographs employed curved mica with good reflectivities in the 4-17 A region from the 002 planes and curved KAP that has strong
reflections from the 001 planes and also from the 013 and 014 diffraction
planes. l8 The different diffraction planes in mica and KAP gave a wide recording sensitivity for soft x-ray
I ARGON I
01 I I I I I
0 20 4 0 60 80 100
PLENUM PRESSURE ( P S I A)
Fig. 3. The argon L-shell emission as a function of filling pressure.
0.25 mm spatial slit positioned
perpendicular to the axis of the plasma column. Spectra were recorded on Kodak direct exposure film (DEF) for which the film response is known. l9 Integral reflection coefficients of the crvstal were computed from a model for the curved KAP crystal2' and taken from
published values for mica.21 Spectral Fig- 4. Pinhole images of argon ~lasma.
traces were digitized with a scanning microdensitometer and processed by computer to yield x-ray intensities.
XW spectra collected with the grazing incidence spectrograph were recorded on uncalibrated Kodak type 101-01 film.
The scans are presented as film density traces. To aid in the identification of spectral lines, atomic structure
calculations were performed with a code from the Los Alamos National
Laboratory. 22 Besides, argon and neon transitions have been observed and classified for example in the early work by Peacock et using a plasma focus device and more recently by a group at the Imperial College in London using a gas puff device. (A.E. Dangor, private communication)
I11 - RESULTS A. Argon Plasma
Pinhole images of the argon L-shell plasma emission shown in Fig. 4 were collected with and without the PEOS near the optimum gas filling pressure of 55 psiA (see Fig. 3). Images recorded without the switch (Fig. 4-upper) show considerable flaring and large emission diameters (>l cm) along the inter- electrode gap. An axial channel of intense emission, about 1 mm in diameter is observable in the upper image of Fig.
4. Discharges with the PEOS produce narrow, more uniform images of Ar L- shell emission (Fig. 4 bottom image).
An absence of Ar K-shell emission was noted in shots with the PEOS operated with gas loadings near or above the optimum pressure for Ar L-shell
emission. The observable plasma length for argon implosions both with and without the PEOS was about 3.5 cm. The Ar K-shell emission, which is maximum for a gas filling pressure of about 15 psiA, is an order of magnitude less intense chan the maximum L-shell emission.
The grazing incidence spectrograph was used to record the argon L-shell spectra in order to identify the stages of ionization formed in argon implosions.
Spectra were recorded in the 25-300 A
wavelength region. The spectral portion between 25 and 50 A (250-460 eV)
corresponds to the range recorded by the Ti-filtered XRD (An=l transitions).
Detailed identifications revealed arrays of 2p-3s, 2p-3d transitions in Ne-like ArIX through N-like Ar XI1 as indicated
in Fig. 5. At longer wavelengths, (Fig.
6) intense 2s-2p transitions from Ar X and Ar XI are observed. A few weak lines believed to be Na-like ArVIII exist in this wavelength range. These lines were observed with both machine conditions but were slightly enhanced in shots with the PEOS. The existence of such ionization stages indicates that the argon implosions were well beyond the desired 50-70 eV plasma temperature for efficient excitation of Ne-like argon (J. P. Apruzese, P. C. Kepple, and J. Davis, private communication).
A technique which used neon as a tracer in argon was developed to determine the temperature of imploded argon plasmas.
Spectra of argon implosions with 5% and 1% neon tracers were collected with a curved KAP crystal spectrograph. Figure 7 shows the argon K lines diffracted by the 001, 002, 013, and 014 planes in KAP. The plasma-to-spectrograph distances were 150 and 100 cm for the 5%
Fig. 5. XW argon spectrum showing 2p-nl transitions.
C6-25 0 JOURNAL DE PHYSIQUE
2s-2p TRANSITIONS Ar XI Ar X
1 I
Fig. 6. X W argon spectrum showing 2s-2p transitions.
and 1% neon tracers respectively. Under these conditions the argon K lines are overexposed in first order but readable in the higher order planes (particularly the 002 planes). Spectral line
densities were recorded in first order between 10 and 14 A. The neon K-line intensities from Ne X and Ne IX were measured to interpret plasma
temperatures from the a to p line ratio.
The lower spectrum in Fig. 7 recorded with the spectrograph positioned on the vacuum chamber housing as seen in Fig. 2
(at a distance of about 50 cm)
registered, in the 9-12 A region, copper L-lines from the brass nozzle (cathode).
As the gas filling pressure was varied between 15 and 60 psiA, the neon La/Hea lfne intensity ratio was found to vary from 0.26 to 1.1. The imploded argon plasma temperatures were estimated between 200 and to 350 eV. These temperatures are significantly higher than needed for Ne-like Ar production but are consistent with the ionization stages identified in the grazing incidence spectrum.
Fig. 7 . Spectrograms from argon with neon tracer plasmas (curved KAP crystal).
A narrow space-resolving slit was not used to collect these spectra in order to record readable neon a and P lines.
B. Neon Plasma
Spatially-resolved spectra were
collected to study and measure the x-ray line emission in neon gas implosions.
Time-integrated spectra were recorded on DEF film using both curved KAP and curved mica diffraction crystal spectrographs and the derived spectral intensities were used to determine plasma conditions. Spectra were acquired for machine currents of 1.0 - 1.5 MA with and without a PEOS using the largest diameter (2.5 cm) nozzle. Data were also acquired for neon plasma implosions generated using smaller nozzles and no switch. Pinhole images record a plasma column of 3.5-4 cm length.
Neon spectra were processed to obtain integrated line intensities. Figure 8 shows the neon plasma spectral intensity for two typical shots. The integrated line intensities obtained for both shots are comparable within a factor of two and the total yields are similar except that more continuum emission
(bremsstrahlung) occurs in the shots without the PEOS; also the spectral lines are noticeably narrower when the
850 1000 1150 1300 ENERGY (ev)
Fig. 8. Intensity trace from neon (upper: w/o PEOS, peak current 1.2 MA, lower: with PEOS, peak current 0.83 MA)
feature of neon plasma emission, for both shots, is that most of the line radiation, i.e., at least 90%, is contained in both a lines. Neon
implosions yield high radiation emission and, under the appropriate machine conditions, uniform plasmas. In shots without the PEOS, we measured energies of 2 to 3 kJ due to K-shell line radiation into 4s from the whole neon plasma column. The total radiated energy was 4-5 W. The use of the PEOS significantly reduces the plasma column diameter and improves the plasma
uniformity. The factor of two intensity difference between the traces in Fig. 8 is related to the difference between peak currents; the latter was about 30%
less in the shot with the PEOS.
Improved implosion stability and plasma uniformity is observed in neon gas implosions generated with the PEOS or with a smaller diameter nozzle. This insight was gained from the degree of plasma flaring or pinching in pinhole images and from spatial variations of a line intensities. With the largest diameter (2.5 cm) nozzle and no switch the La/Hea line ratio decreased from around 1.0-1.3 near the cathode to nearly unity at midgap, and to about 0 . 3 near the anode. With the PEOS, this ratio was about 1.1 with a uniformity of + 30% across the entire interelectrode
-
gap. Without the PEOS, but with a smaller diameter nozzle (1.75 cm), the plasma uniformity improved. In this case the a-line ratio varied from 1.0 to 1.3 near the cathode to a value of 0.7- 0.8 near the anode. As shown in pinhole images (Fig. 9) the plasma flaring is reduced with the smaller nozzle.
developed to produce a plasma from solid dielectric materials such as sodium fluoride. A small capacitor bank generates a discharge along the interior wall of a NaF capillary tube and the generated plasma is injected axially, through a 2 cm diameter nozzle, into the 4 cm electrode gap of Gamble 11. (1.2 MA current) This plasma implodes and emits x-rays about 100 nsec after the beginning of the current pulse. This x- ray emission was recorded with pinhole cameras, x-ray diodes, and crystal spectrographs. The pulse duration, (FWHM), as recorded by an aluminum filtered XRD detector is 20 nsec for this K-shell radiation. Pinhole images indicate that the imploded plasma fills the 4 cm interelectrode gap.
The NaF spectrum was collected with moderate resolution using the mica crystal spectrograph with spatial resolution. Figure 10 shows the sodium K-shell radiation (above 1 keV) from a typical NaF spectrum obtained with this instrument. The spectra were processed to obtain the sodium line intensities from which plasma conditions and uniformity could be estimated. The fluorine emission below about 900 eV was absorbed strongly by the 10 pm Be window on the spectrograph and only F IX higher Rydberg transitions were recorded. A few Cu L-lines were observed originating from the brass nozzle of the capillary discharge. The Na X and Na XI a-lines dominate the spectra in the same manner as in the neon gas puff spectra.
The sodium a transitPons were recorded siniultaneously with high resolution using the second-order diffraction from a curved KAP crystal spectrograph. A typical spectrogram is shown in Fig. 11.
A narrow slit was used to provide spatial resolution along the axis of the
N a X Ib
i i
Fig. 9 . Pinhole images of neon plasmas Fig. 10. Mica crystal spectrogram from for two different diameter nozzles NaF plasma generated with the capillary (upper 2.5 cm, lower 1.75 cm). discharge.
C6-252 JOURNAL DE PHYSIQUE
implosion. The spectral line shapes can Appropriate configurations and be correlated to the plasma diameter and conditions can produce uniform
therefore exhibit some resemblance with implosions of several cm in length. The the pinhole images (see Fig. 11). plasma uniformity and stability can be
improved by decreasing the risetime of
PINHOLE
h% L, Heg the current during the implosion and by adjusting the diameter of the gas-puff nozzle. Argon is efficiently imploded with 1-MA driving currents, but is overheated and ionized well beyond the Ne-like argon stage that is desired for x-ray laser experiments; therefore, Fig. 11. KAP crystal (002) spectrogram
showing spatial uniformity along the sodium lines. The associated plasma pinhole image is shown on the left, the capillary nozzle being on top.
Sodium line intensities were obtained from both spectrograph images to measure line emission variations along the interelectrode gap and line ratios. We obtained good agreement between both spectrograph measurements for intensity variations. The He-like transitions show a maximum near midgap and decrease towards either electrode. The La emission shows two broad intensity maxima. Both a line integrated
intensities vary by a factor of 2-2.5.
The La/Ha intensity ratio varies from around 1 along a plasma region of more than 1 cm length on the nozzle side, to about 0.5 on the cathode side over about 1.8 cm of the plasma column. The estimated sodium plasma temperature appears in excess of 200 eV based on the measured line intensity ratios (a, p and
-y transitions).
Absolute line intensities were obtained by integration over the line profile with an average value reached by summation over the individual scans.
Most of the sodium line radiation occurs in the a transitions with an energy between 0.7 and 1 W. The corresponding power in these two lines is 42 + 10 GW
based on a 20 nsec pulse duration from the XRD measurements. The power in the Hea line (11.0 A) is 27 GW in agreement with an independent measurement of 25 GW obtained with a Ge-filtered x-ray diode.
IV - CONCLUSIONS
Imploding plasmas driven by pulsed power generators are found to be efficient emitters of keV energy x-rays for both the gas puff and the capillary
discharges studied in this work.
higher atomic number elements, e.g., iron or chromium are indicated for future work. Implosions of sodium plasmas using a capillary-discharge source have produced an intense pump source of 11.0 A photons appropriate for Na/Ne line coincidence x-ray laser experiments.
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