AP09_2.vp 1 Introduction In the region of XUV (eXtended Ultra Violet) radiation, i.e. in the region of ~ (0.2–100) nm there are two significant sub-regions. The first is the water-window region (2.3– 4.4) nm. Radiation in this region is highly absorbed in carbon, but not much absorbed in water, so it is useful for observing or- ganic samples in their native environment. The second is the region around 13.5 nm. At this wavelength there are Mo/Si multilayer mirrors with high reflectivity at a normal incidence angle, and thus radiation in this wavelength region it is conve- nient for industrial applications as XUV lithography. A typical X-ray source, the Roentgen tube, is based on bremsstrahlung of electrons and produces a broad, continuous spectrum with intensity peaks on K-lines. If we are interested in a narrow spectrum of XUV radiation, synchrotron accelerators or FELs are suitable sources. However due to their huge dimensions, build and operation costs these devices are unobtainable for most laboratories. Another approach for obtaining XUV ra- diation involves electron transitions in plasma ions. Using this way, radiation with a line spectrum is created. Some elec- tron transitions have noticeably higher probability than other transitions, and when suitable conditions arise in the plasma, these transitions become dominant and a significant peak can appear in the spectral region of interest. In addition, when the plasma forms a uniform column with length exceeding diameter in two orders, the pumping power is sufficiently high to ensure a high enough population on a meta-stable energy level, Spontaneous Amplified Emission (ASE) can appear. ASE has the properties of a laser. The Active medium of these lasers is created by laser-produced plasma or by capil- lary-produced plasma. The first capillary produced plasma XUV laser was made by Rocca et al. at Colorado University in 1994 [1]. In 2005, Heinbuch et al., from Rocca’s team, pro- duced a tabletop version of the laser [2]. 2 Discharge apparatus 2.1 Z-pinching capillary discharge In order to obtain radiation in the XUV region, the elec- tron transitions have to proceed on highly ionized states. These states appear in plasma with a electron temperature of Te ~ 100 eV. Such hot plasma with sufficient electron density Ne > 10 17 cm�3 is created by radial compression of a plasma column by the Lorentz force FL of a magnetic field B of flowing current I – a Z-pinch. First, the current flows along the walls of the capillary. The flowing current creates a magnetic field, which affects the charged particles of the plasma by a radial force. This force compresses the particles like a snowplow till the magnetic pressure is equilibrated by the plasma pressure. Schematic of a Z-pinching capillary dis- charge is shown in Fig. 1. To obtain sufficient electron density and electron temper- ature, rapid compression is needed. 2.2 Discharge circuit design Pulsed current of amplitude ~10 kA and an abrupt rise of dI/dt > 1011 As�1 is needed for the rapid compression with a high compression rate, that is necessary for obtaining suffi- ciently dense and hot plasma. Our discharge circuit is realized by a ceramic capacitor bank discharged by closing of a spark- -gap switch through ceramic capillary filled with gas. Because of the high dI/dt, all inductances in the circuit are undesirable because their impedance becomes dominant. Reduction of inductances in the circuit allows us to achieve high dI/dt with a relatively small charging voltage. Fig. 2 shows the scheme of the apparatus. A ceramic capacitor bank with a maximum capacity of 26.2 nF is pulse charged by a 2-stage Marx generator and an RLC oscillating circuit up to 100 kV (limited by the capacitors breakdown voltage). If the voltage on the capacitors exceeds the self-breakdown voltage of the spark gap, the gap is en- closed and the capacitors discharge through the capillary. This is the main discharge circuit. To reduce the inductance of this circuit, a close coaxial configuration was designed. The capillary is also shielded by a duralumin tube. This tube re- duces the energy of magnetic the field around the capillary and thus reduces its inductance. Before the main discharge a © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 53 Acta Polytechnica Vol. 49 No. 2–3/2009 Capillary Discharge XUV Radiation Source M. Nevrkla A device producing Z-pinching plasma as a source of XUV radiation is described. Here a ceramic capacitor bank pulse-charged up to 100 kV is discharged through a pre-ionized gas-filled ceramic tube 3.2 mm in diameter and 21 cm in length. The discharge current has ampli- tude of 20 kA and a rise-time of 65 ns. The apparatus will serve as experimental device for studying of capillary discharge plasma, for test- ing X-ray optics elements and for investigating the interaction of water-window radiation with biological samples. After optimization it will be able to produce 46.9 nm laser radiation with collision pumped Ne-like argon ions active medium. Keywords: Capillary discharge, Z-pinch, XUV, soft X-ray, Rogowski coil, pulsed power. Fig. 1: Z-pinching capillary discharge (30 – 40) A, (3 – 6) �s long current pulse pre-ionizes the gas in the capillary and prepares a uniform conducting channel. The capillary is made of Al2O3, which is a material with low wall ablation. The ablation of the material from the capillary walls is an undesirable effect in the gas-filled capillary, which introduces impurities into the discharge plasma. The di- mensions of capillary are: diameter – 3.2 mm and length – 210 mm. It is filled with gas through a hollow electrode on its grounded side. Radiation is also emitted through this hole. The XUV radiation is highly absorbed in any material, even a gas, so 3 mm from the capillary exit there is a 1.2 mm diameter aperture separating the discharge gas with working pressure (10 – 100) Pa from the high vacuum of 3 orders lower pressure. The charging voltage is measured by a 40 kV Tek- tronix probe. Because the charging voltage of 100 kV could damage the probe, a capacitive divider is used. This divider divides the voltage according to the ratio of the adjustable ca- pacitor capacitance and the input capacitance of the probe. The current is measured using a Rogowski coil with RL inte- gration. The breakdown voltage is adjusted by the nitrogen pressure in the spark-gap. The system is enclosed in a dur- alumin housing in order to reduce electromagnetic noise. Biodegraded oil is used to isolate the circuit and avoid un- wanted breakdowns. 2.3 Electrostatic analysis Electrostatic simulations in QuickField, finite elements analysis software, were performed in order to estimate spots predisposed to unwanted electrical break-downs. The input parameters were 100 kV on the HV electrode, closed spark-gap, and constant resistivity along the capillary, i.e. a constant voltage drop between the electrodes of the capil- lary. The electric strength E field picture in the region of E � �( . . )0 02 2 00 107 V/m with three problem spots is shown in Fig. 3. During experiments without electro-isolating oil break- downs really did occur between point A and the housing, and between point B and C in case of spark-gap switching into a non-preionized capillary. Formerly, when a DC charging volt- age was used, these breakdowns occasionally also emerged in electro-isolating oil after exceeding 70 kV. After exceeding this voltage the main trigatron spark-gap also switched on unexpectedly. There was a need to change the DC charging design with a trigatron spark-gap to a pulse charging design with a self-breakdown spark-gap. 2.4 Charging circuit and pre-ionization The ceramic capacitor bank is charged by a two-stage Marx generator and a RLC oscillating circuit. A simplified schematic of the circuit is shown in Fig. 4. The RLC cir- cuit is formed by two charging capacitors with a capacity of CC � 37 5. nF in series, ~ 1 mH charging coil, charged capaci- tor bank C, and by parasite resistivity. After closing the Marx generator, the voltage on the capacitor bank stabilizes ap- proximately at a value of UST, given by: U U C C C C C ST MARX� � , (1) where UMARX is the voltage on charging capacitor CC af- ter switching the Marx generator on. By charging through coil, the voltage on capacitor bank has a waveform of un- der-damped oscillations with a period of ~ 25 �s (depending on the capacitor bank capacity), with the first maximum at ~ 2×UST. In the case of C �CC, the voltage on the capacitor bank can reach almost 160 kV, with a charging voltage of 40 kV (doubled by Marx and again doubled in the coil). The first stage of the Marx generator is used for pre-ion- ization of the capillary. The pre-ionization current is limited by the 1 k� resistor to (30 – 40) A, and the duration of pre-ionization is given by the charging time of capacitor bank, i.e. by the time from Marx switching on to the self-breakdown spark-gap between the capacitors bank and capillary break- down. This time is typically (3 – 6 ) �s. Formerly, the capacitor bank was charged by a DC power supply and discharged by the trigatron spark-gap switched by a commercial triggering unit. Pre-ionization circuit was separately charged by second power supply and switched by second commercial triggering unit. A breakdown of the capillary at the beginning of pre-ionization causes big electro- magnetic noise. The triggering units were not resistant to this 54 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 49 No. 2–3/2009 Fig. 2: Discharge apparatus Fig. 3: QuickField electrostatic analysis Fig. 4: Charging circuit electromagnetic noise, and the main discharge was switched prematurely. This was another reason for turning to a pulse charging design with a self-breakdown spark-gap, one power supply, and one triggering unit. 2.5 Rogowski coil The current in the capillary is measured by a Rogowski coil. This is a simple current probe based on the Faraday principle of electromagnetic induction. A toroidal coil with a returning loop is placed around the current conductor. In changing current, voltage UIND will be induced on the ter- minals of the coil, according to: U t Sn I tIND d d d d � � � � � �0 , (2) where � is magnetic flux, t is time, �0 is permeability of vac- uum, S is cross-section of coil’s loop, I is measured current, and n is number of loops of coil per unit length. As seen from (2), UIND is proportional to the change of current. In order to be proportional directly to measured current, UIND has to be integrated. There are three basic types of integrating circuit, integration with an operating amplifier – Fig. 5.a, RC integration – Fig. 5.b, and RL integration – Fig. 5.c. Because of the big changes in the measured current, we do not need active integration with OA. The measured current has a basic frequency around 3 MHz. A coil with RC integra- tion should have a higher self-resonant frequency, in order to measure the current correctly. It is difficult to manufacture a coil, with sufficiently high UIND and sufficient high self- -resonant frequency. The best solution for our case is RL integration. The equivalent high-frequency circuit for a coil with RL integration is shown in Fig. 6. By solving this circuit and as- suming R L R CD X D X� �� 1 , where RD is a small integrating resistor, LX is coils inductance, and CX is the inter-turns capacitance of the coil, and according to (2), and according to the coil inductance estimation: L nV N l n VX � �� �0 0 2 , (3) where �0 is permeability of air, n is number of coil turns per unit length, V is coil’s inner volume, and N is total number of coil turns, we can obtain the sensitivity of the coil: U R R C L R R C U R N IOUT D D X X D D X IND D � � � � � � � � 1 1 . (4) More on Rogowski coil theory is available in [3]. Our Rogowski coil has the calculated properties: low-frequency limit: f R Ld � � 1 2 29 � D X kHz, (5) time resolution: t l c � � D 10 ns, (6) sensitivity: I U N ROUT D � � �137 1AV , (7) where the time resolution is limited by the finite signal propa- gation along the coil wire length of lD. The coil was calibrated by placing it in the discharge system, where the capillary was replaced by a copper tube and the capacitor was discharged over a non-inductive 10 � resistor. The discharge current was measured via differential voltage measurement on the resis- tor, and was compared with the voltage output of the coil. The experimentally determined sensitivity was: I UOUT AV� � �( )130 8 1. (8) As seen, the theoretical sensitivity (7) is in good agreement with the measured sensitivity (8). 3 Experimental results Here we present measurements of charging voltage, dis- charge current, output radiation, and pinch positions chang- ing with respect to discharge gas pressure. Fig. 7 shows the charging voltage and the discharge current. The charging voltage is measured directly using a Tektronix HV probe and an HV probe behind the capacitor divider. The capillary is re- placed by a copper tube. The capacitance of the capacitor bank is 11.2 nF. Fig. 8 shows the measured current through an argon filled capillary at pressures 13 Pa, 40 Pa, and 65 Pa. The capacity of the bank is 15 nF and the charging voltage is 75 kV. The pinch is observable as a drop on the current waveform. Fig. 8 shows, how the pinch moves forward in time with increasing pres- sure. The strongest pinch appears when the current reaches its maximum. For our experimental voltage and capacity it is at 40 Pa. Fig. 8 also shows the XUV emission. The emission is measured by a vacuum diode with a gold cathode, 15 cm from © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 55 Acta Polytechnica Vol. 49 No. 2–3/2009 UOUT R L UIND UOUTRUIND C UOUT R UIND C OP a) b) c) Fig. 5: Integrating circuits: a) active with OA, b) passive RC, c) passive RL Fig. 6: High-frequency equivalent circuit of Rog. coil the capillary end. Electrons, visible and UV radiation are fil- tered by a 0.8 �m thick aluminum foil. 4 Conclusion An apparatus for pinching capillary discharge production, as a source of XUV radiation, was designed and realized. The discharge current was measured using a calibrated Rogowski coil, the charging voltage was measured using a capacitive divider and a Tektronix HV probe, and the XUV output radi- ation was measured using a vacuum diode. The apparatus is in the final stage of construction. After completing the appa- ratus we would like to find the laser gain on Ne-like argon ions at a wavelength of 46.9 nm. 56 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 49 No. 2–3/2009 Fig. 7: Charging voltage and discharge current Fig. 8: Pinch and XUV radiation (current – solid line, XUV signal – dot-and-dash line) Acknowledgment Research described in the paper was supervised by Dr. A. Jančárek. This research is supported by Ministry of Education CR grants 102/07/0275 and 202/08/H057, and internal grant of Czech Technical University in Prague CTU0910414. I also thank Dr. Vrba for consultations on the theoretical background of Z-pinching plasma and laser gain estimation. References [1] Rocca, J. J. et al.: Demonstration of a Discharge Pumped Table-Top Soft X-Ray Laser. Physical Review Letters, Oc- tober 1994, Vol. 73 (1994), No. 16, p. 1236. [2] Heinbuch, S., Grisham, M., Martz, D., Rocca, J. J.: Demonstration of a Desk-Top Size 46.9 nm Laser at 12 Hz Repetition Rate. Optics Express, Vol. 13 (2005), No. 11, p. 4050–4055. [3] Nevrkla, M.: Design and Realisation of Apparatus to Study Capillary Discharge in Argon: Master thesis. Prague: CTU, Faculty of Nuclear Sciences and Physical Engineering, 2008. Michal Nevrkla e-mail: michal.nevrkla@fjfi.cvut.cz Department of Physical Electronics Czech Technical University in Prague Faculty of Nuclear Sciences and Physical Engineering Břehová 7 115 19 Prague 1, Czech Republic © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 57 Acta Polytechnica Vol. 49 No. 2–3/2009 << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles true /AutoRotatePages /None /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Error /CompatibilityLevel 1.4 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends true /DetectCurves 0.0000 /ColorConversionStrategy /CMYK /DoThumbnails false /EmbedAllFonts true /EmbedOpenType false /ParseICCProfilesInComments true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams false /MaxSubsetPct 100 /Optimize true /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveDICMYKValues true /PreserveEPSInfo true /PreserveFlatness true /PreserveHalftoneInfo false /PreserveOPIComments true /PreserveOverprintSettings true /StartPage 1 /SubsetFonts true /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile () /AlwaysEmbed [ true ] /NeverEmbed [ true ] /AntiAliasColorImages false /CropColorImages true /ColorImageMinResolution 300 /ColorImageMinResolutionPolicy /OK /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 300 /ColorImageDepth -1 /ColorImageMinDownsampleDepth 1 /ColorImageDownsampleThreshold 1.50000 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile () /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /CreateJDFFile false /Description << /ARA /BGR /CHS /CHT /CZE /DAN /DEU /ESP /ETI /FRA /GRE /HEB /HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. 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