Time-resolved study of the extreme-ultraviolet emission and plasma dynamics of a sub-joule, fast capillary discharge

Transcription

1 Time-resolved study of the extreme-ultraviolet emission and plasma dynamics of a sub-joule, fast capillary discharge J. C. Valenzuela, E. S. Wyndham, and M. Favre Citation: Physics of Plasmas 22, (2015); doi: / View online: View Table of Contents: Published by the AIP Publishing Articles you may be interested in Observations of soft X-ray emission and plasma dynamics of a compact capillary discharge operated in xenon Phys. Plasmas 20, (2013); / Time and space resolved visible spectroscopic imaging CO2 laser produced extreme ultraviolet emitting tin plasmas J. Appl. Phys. 111, (2012); / Dynamics of reactive high-power impulse magnetron sputtering discharge studied by time- and space-resolved optical emission spectroscopy and fast imaging J. Appl. Phys. 107, (2010); / Time resolved studies of plasma evolution in a laser initiated capillary discharge AIP Conf. Proc. 563, 294 (2001); / Time and space resolved VUV-spectroscopy of a pulsed hollow cathode capillary discharge AIP Conf. Proc. 563, 282 (2001); /

2 PHYSICS OF PLASMAS 22, (2015) Time-resolved study of the extreme-ultraviolet emission and plasma dynamics of a sub-joule, fast capillary discharge J. C. Valenzuela, 1,2,a) E. S. Wyndham, 2 and M. Favre 2 1 Instituto de Fısca, Pontificia Universidad Catolica de Chile, Santiago, Chile 2 Instituto de Fısica, Pontificia Universidad Catolica de Chile, Av. Vicu~na Mackenna 4860, Macul, Santiago, Chile (Received 4 June 2015; accepted 10 July 2015; published online 3 August 2015) In this work, we discuss experimental observations on the dynamics of a fast, low energy capillary discharge when operated in argon and its properties as an intense source of extreme-ultraviolet (EUV) radiation. The discharge pre-ionization and self-triggering were accomplished by the use of the hollow cathode effect. This allowed a compact size and low inductance discharge with multi-ka current level and a quarter-period of 10 ns at sub-joule energy level. We used the novel moire and schlieren diagnostics with a 12 ps laser to obtain the time evolution of the line electron density and to study the plasma dynamics. EUV spectroscopy and filtered diodes were also implemented to estimate the plasma temperature and density throughout the evolution of the discharge. EUV source size was measured by using a filtered slit-wire camera. We observed that EUV emission starts from a compressed plasma on axis during the second quarter-period of the current and continues until the fifth quarter-period. Ionization levels from Ar VII to X were observed. By comparing the EUV emission spectra with synthetic spectra, we found that at the onset of emission (7 ns), the plasma is well fitted by a single Maxwellian electron distribution function with T e 12 ev and n e cm 3. Close to peak emission (13 ns), plasma temperature and density increase to 20 ev and n e cm 3, respectively. However, in order to successfully match the experimental data, a two component electron distribution function was necessary. Later in time, a smaller fraction in the high energy component and higher temperature suggests homogenization of the plasma. The moire and schlieren diagnostics showed multiple radial compression-waves merging on axis throughout the discharge; they are an important heating mechanism that leads to a period of severe turbulence at peak EUV emission. It was also observed that emission ceases when the axial maximum of the electron density collapses. VC 2015 AIP Publishing LLC.[ I. INTRODUCTION The capillary discharge has proved to be of great interest over recent years because of its intense emission properties in the Extreme-Ultraviolet (EUV) and soft X-ray spectral range. At the lower end of the Z-pinch energy spectrum, nevertheless, it is not devoid of the MHD and radiation physics of its more energetic cousins. Originally researched as a non-coherent radiation source, 1 an important advance was the observation and development in large aspect ratio capillary discharges of lasing at 469 Å in Ne-like argon at quite modest currents. 2,3 Other works where possible lasing conditions are established have used nitrogen 4 or material ablated from the wall, such as carbon 5 and sulfur. 6 These discharges have mostly been performed in capillaries with a diameter of approximately 5 mm, maximum currents of the order of 40 ka, and with quarter periods of 50 ns or more. Intense, short-pulses of non-coherent EUV emission may also be obtained in much lower energy discharges (0.1 1 J) by exploiting the Transient Hollow Cathode Mechanism (THCM) in conjunction with a marked pressure a) Present address: Center for Energy Research, University of California at San Diego, La Jolla, California 93093, USA. Electronic mail: jcval@ucsd.edu gradient along the capillary tube. Peak currents are much more modest (2 8 ka) with a current quarter period of between 6 and 10 ns These devices were originally operated in a single shot spark-gap triggered mode using capillaries with an internal diameter (ID) between 0.8 and 3 mm and length between 10 and 40 mm. 12 Later, it was realized that this configuration is ideal for direct pulsed power charging at a high repetition rate. 13 A preliminary discussion of the plasma parameters has been presented in Ref. 13 for discharges in argon. In this work, we further the understanding of the plasma behavior with a detailed attempt to follow the temporal evolution of the plasma. This is achieved by implementing two new diagnostics. In addition, we are able to correct a number of line assignations given in Ref. 13 as a consequence of using the synthetic spectrum generator, PrismSpect. 14 The implementation of this high repetition rate capillary discharge has been reported using other gases. When operated in nitrogen, He-like nitrogen emission in the waterwindow band was observed. 15 Recently, observations of the plasma dynamics and emission for discharges in xenon gas has been published, 16 motivated by the potential development of a EUV source for metrology and lithography in industrial applications. 17,18 In that work, moire and schlieren 19 optical X/2015/22(8)/083501/9/$ , VC 2015 AIP Publishing LLC

3 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) measurements revealed details of the plasma dynamics and allowed a measurement of the electron density on axis at the onset of the EUV emission. The time-integrated and timeresolved EUV spectra were observed, finding ionization levels as high as Xe VII-XI with a strong component in the unresolved transition array around 110 Å. However, due to the extreme complexity of the emission spectrum of xenon, an estimate of the electron temperature was not possible. In a previous work, performed in different gases, it has been found that, in order to match successfully the experimental spectra, a second high energy component in the electron distribution function has to be invoked. These electrons shift the ionization level of the plasma and reach high ionization states that otherwise would require excessively high temperatures for these devices. The new diagnostics implemented here allow to temporally resolve the plasma dynamics. The first one is the refractive-optical, moire and schlieren technique, performed with a pico-second kind laser, which permits measurement of the electron line density and to derive the plasma motion. The second new diagnostic is time-resolved EUV spectroscopy. Together, these diagnostics allow us to unveil the role of the fast electrons on the spectra. These occur at two instances: first, as a collimated beam originating from the hollow cathode and which leads to breakdown and second, as a tail in the electron distribution function at the time of the EUV emission. Peak EUV emission also coincides with the onset of intense localized density gradients within a welldefined volume centered on axis. At a later time, we are able to relate the end of the EUV emission with a collapse of the centrally peaked electron density on axis. II. EXPERIMENTAL DETAILS The results presented in this work were obtained using capillaries with an internal diameter of 1.6 mm and length of 21 mm. The filling pressure was 400 mtorr at the cathode region and 40 mtorr at the exit (anode). These values were used in order to optimize the THC performance and EUV emission. We used a cathode aperture of 0.8 mm for optimum EUV emission, found in a previous work. 13 In order to implement the moire diagnostic, the aperture was enlarged to 1.6 mm to obtain a complete view of the capillary tube. The initial gas conditions are adjusted, so that self-breakdown occurs at 24 kv, except when using the optical diagnostic where the self-breakdown occurs at 20 kv, corresponding to an initial stored energy of 0.5 J. In all cases, the anode aperture was 1.6 mm. The capillary discharge has been operated to frequencies up to 500 Hz, but in this work we used a reduced repetition rate to avoid aging effects of the capillary wall. The implementation of the capillary discharge has been discussed elsewhere. 9,13 moire and schlieren deflectometry permits observation of the evolution of the line electron density and is the subject of a recent report. 19 The time-resolved EUV spectrum was obtained by using a 1 m Rowland circle grazing incident spectrometer 20,21 in its scanning monochromator mode and a wide-band silicon diode with a well specified response 22 behind the exit slit. The wavelength spectrum was swept by rotating the diffraction grating in single steps using a stepping motor. The spectral resolving power is mainly defined by the entrance and exit slit, and a compromise between available signal on the diode and spectral resolution had to be made. By integrating over 16 shots at each position, sufficient signal to noise ratio was obtained with an overall system rise-time of 2 ns. The resultant resolving power, k/dk, was a slow monotonic function over the recorded spectrum varying from 20 at 30 Å to 80 at 210 Å. At the short wavelength end of the spectrum (50 Å), it was found that the second order is substantially enhanced, where the incident and diffracted angles are close to specular reflection. The use of time integrated measurement of the spectrum obtained using a 4096 element CCD array coated with a phosphor layer allows an improved spectral resolution by a factor of 4. The size of the emitting source is obtained using a slitwire imaging 23 system, in which the characteristic size of the source is inferred from the shadow casted by a thin wire. In this implementation, the slit-wire camera is formed using wires of diameters between 125 and 500 lm and filtered with 1 lm Ti, 2.5 lm polycarbonate, and 3 lm Al filters in order to measure the source size in three spectral bands. The dimensions of the cathode aperture are known to affect the behavior of the hollow cathode-produced electron beam and the pre-ionization volume and in consequence the dynamics of the main discharge. As mentioned above, the cathode aperture was enlarged to 1.6 mm in order to obtain a full view of the capillary during the optical diagnostic. We compared the temporal evolution of the soft X-ray/EUV emission by using filtered detectors for cathode apertures of 0.8 and 1.6 mm. To this end, we used a diamond detector 24 filtered with 2 lm polycarbonate and a wideband planar Si diode 22 filtered with 1.5 lm Al. III. EXPERIMENTAL RESULTS A. Filtered Si diode and diamond signals In Fig. 1, we show the filtered Si diode and the diamond detector signals, together with the current, for cathode apertures of 0.8 and 1.6 mm, as discussed above. Note that to obtain traces of similar magnitude, the polycarbonate filtered signal is amplified by 40 in Fig. 1(a) and 3 in Fig. 1(b). In addition, the circles on the upper figure indicate the times of the moire and schlieren frames presented in Section III C. We may relate the Al and the polycarbonate filters to ionization states that correspond to the spectra discussed below (see Figs. 3 and 4). The relevant transmission edge of polycarbonate at 43.8 Å gives information principally of the Ar IX resonance and intercombination lines at 49 Å. In contrast, the Al filter has an extensive wavelength pass band ( Å), corresponding to Ar V to Ar VIII ionization stages. For both apertures the principal features are the same. Emission starts close to maximum current, first, in the Alfiltered signal and, after 2 ns, the polycarbonate-filtered signal. Both signals reach their maximum well after di/dt has changed sign and continue well into the third quarter-cycle of

4 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) TABLE I. XUV source dimensions measured at different spectral bands. Filter material-thickness Ti Polycarbonate Al 1 lm 2lm 1.5 lm Diameter (mm) Standard deviation (mm) FIG. 1. Filtered detector signals and discharge current showing a comparison for two cathode apertures: 1.6 mm (upper) and 0.8 mm (lower). The circles on the current trace (upper) indicate the times when the optical frames were taken (see Section III C). the current. The Al-filtered signal continues into many quarter-cycles, whereas the polycarbonate-filtered signal is much reduced. The Al-filtered signal is in proportion to the observed volume (4 times larger for the 1.6 mm cathode than 0.8 mm), whereas the amplitude of the polycarbonatefiltered signal for the larger aperture is approximately one third of the value for the 0.8 mm aperture. We infer that the 1.6 mm cathode aperture plasma may be slightly cooler, but that the dynamic evolution as shown by the temporal emission behavior is closely similar. A similar trend was presented in Ref. 16 for xenon discharges, where the line density was compared for both apertures using the optical diagnostics. B. Time-integrated filtered slit-wire camera images The measurement of the characteristic EUV source dimension, obtained from the slit-wire camera, is presented in Table I. The Ti filter L-edge at 27 Å allowed the shortest wavelength lines seen in the spectrometer to be imaged (see Fig. 3). These are Ar IX and X transitions in the Å range. The Ti filter is significantly less transparent to the Ar IX transitions at 49 Å and its satellite lines. The relevant edge of the polycarbonate filter is at 43.8 Å; hence, it is mainly transparent to the Ar IX line at 49 Å, excluding many of the shorter wavelengths seen by the Ti filter. On the other hand, the Al filter has an extensively long wavelength pass band ( Å); in this range, we expect principally Ar V to VIII transitions. Table I shows a typical source diameter of 180 lm of the cooler plasma and a clear tendency to a smaller source diameter as the progression is made to shorter wavelengths. C. Line electron density measurement using optical probing The line electron density was obtained using moire and schlieren deflectometry. 19 The filling pressure was adjusted to maintain the same ratio of pressures between the anode and cathode (10:1), while maintaining the same gas load. In Fig. 2, we present a series of moire and schlieren images, which allow measurement of the temporal evolution of the electron line density. The time of each image is also indicated on the current trace in Fig. 1 (upper). In Fig. 2, the upper images are actually 2 pairs taken on two separate shots. On each image, we plot (in yellow) the derived FIG. 2. Moire-schlieren deflectometry shows the line electron density at the filling pressure gradient of 10:1. The yellow traces show line electron line density derived from the fringe shift. The light blue lines with arrows follow one of the principal compression waves. The schlieren effect is seen as dark rings on the images.

5 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) electron line density as a function of the radius, with an estimate of the error. This is found by averaging multiple fringes shift of the same shot. The zero value of the density must be estimated; we choose a value that corresponds to singly ionized plasma at the capillary wall interface at the initial neutral density, this is supported also by spectroscopy data shown in Sections III D and III E. The schlieren effect seen as dark rings on the Moire images (see Fig. 2) is an accidental but useful product of the spatial filtering required to prevent mixing of the diffraction orders of the Ronchi gratings. 19 It gives a useful visual idea of the strong transverse density gradients along the path of integration, which become especially severe on axis after 14 ns. The salient observations are as follows: the first detected fringe shift occurs at approximately 9 ns, which coincides with the rising edge of the EUV emission (see Figs. 1 and 4). Before peak emission, two incoming compression waves are clearly identified at two distinct radial positions (0.1 and 0.45 mm), the inner wave converges quickly on axis between 10.4 and 11.4 ns, resulting in a peak of the density profile, whereas the speed of the outer wave is slower. This wave is indicated by the light blue lines in Fig. 2. Its mean radial velocity is approximately cm/s. This may be compared with the ion sound speed estimated at cm/s. By taking into account the integration path length, the average on-axis electron density at 14.4 ns is cm 3. The width of the density feature is approximately twice the value found with the Al-filtered slit wire camera measurement (see Table I). The dark region, where the schlieren effect is strongly observed, is of the same size. After peak emission between 15 and 28 ns so much light is refracted out of the central volume that it is impossible to follow the fringes and no images are shown. At later times, between 29.5 and 40.5 ns, another slower-incoming compression wave is observed also indicated by blue lines. Its radial velocity is approximately cm/s. Notice that by this time Ar IX and X ionization stages are absent (see Fig. 4(a)). It is noteworthy to mention that the line density inside this central volume gradually diminishes from a central peak to a plateau. At the time when the plateau has formed, the EUV emission ceases. D. Time-integrated EUV spectroscopy results The resolution of the spectrometer is increased by operating in the time-integrated mode as compared to the timeresolved mode. This allows a more precise line identification of the emission spectrum. In conjunction with the synthetic spectrum generator (PrismSpect 14 ), we are able to estimate bounds on the temperature, density, and electron distribution function of the plasma. Fig. 3 shows a typical Ar spectrum together with pertinent synthetic spectra, the results combine two gratings: 600 lines/mm and 1200 lines/mm between 30 and 220 Å, corresponding to the red and black curves in Fig. 3(a), respectively. The two synthetic spectra in Figs. 3(b) and 3(c) allow the identification of emission lines and plasma parameters. It may be seen that the greater part of the observed lines coincide with the synthetic spectra, whose conditions are n e ¼ cm 3 and T e ¼ 16 and 22 ev, respectively. Some other observed species principally Ar VII and VIII transitions are associated with lower electron temperatures (for example, the Ar VII transition at Å is predicted to be prominent at 14 ev). The time-resolved spectrum in Section III E will clarify their occurrence. Three characteristic features of the spectrum are especially significant in establishing the bounds of the plasma parameters. The first consideration arises from both the relative and absolute strengths of the Ar IX transitions at 49 Å, taken together with the Ar VIII satellite lines between 50 and 55 Å. Second, the overall intensity of this extensive group of lines relative to the spectrum beyond 100 Å. Third, the relative strengths of the Ar VII to X transitions observed between 100 and 220 Å. The effect of a fast electron component explains the Ar IX and X transitions at these comparatively low temperatures. For example, on simulating a Maxwellian plasma (not shown) at 18 ev, Ar IX and X transitions are absent. Indeed, only at 40 ev, we do see the key indicator transitions at 49 and 165 Å. In effect, the simulations show that the relative intensity of Ar VIII satellite lines, apparent in Fig. 3(c), are in proportion to the fraction of fast electrons with a characteristic energy above a threshold value of 100 ev. Furthermore, the relative intensity of the Ar IX transition at 49 Å to these Ar VIII satellites depends strongly on the lower Maxwellian temperature. For example, we point out that at 16 ev (Fig. 3(c)), the simulation shows the Ar VIII satellites to be too, intense. This indicates that the plasma is hotter than 16 ev. Further support of temperatures above 16 ev comes from the observation of the Ar X transitions at 165 and 171 Å. These are absent in Fig. 3(c), but are clearly present in Fig. 3(b). Estimates of the bounds on the electron density may be estimated in a similar way on comparing simulations at 22 ev. At a density of cm 3, we find the Ar VIII satellite lines between 50 and 55 Å to be significantly enhanced. On the other hand, at a density of cm 3, we do not find conditions where the two Ar IX transitions at 49 Å are sufficiently strong; while at the same time, the Ar X lines are present and the Ar VIII satellites are suppressed. These constraints lead us to propose a density of cm 3 as an optimal compromise. A discrepancy between the observed and the synthetic spectra concerns the Ar IX lines at 49 Å. It can be seen from the 1200 l/mm grating spectrum of Fig. 3(a) that the resonance 1 S 0 1 P 1 transition is less intense than the 1 S 0 3 P 1 intercombination counterpart. This is in disagreement with PrismSpect; however, the same effect has been discussed recently, 25 where it is attributed to plasma opacity. Plasma opacity may be modelled in PrismSpect, but our observation is not reproduced. E. Time-resolved EUV spectroscopy The time evolution of the EUV spectrum is the subject of Fig. 4. The time-resolved spectrum allows a detailed description of the sub-millimeter EUV emitting region. The synthetic spectrum generator PrismSpect offers a means of resolving, at least approximately, the temporal evolution of the plasma conditions. The relation between the observed

6 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) FIG. 3. Experimental and synthetic time-integrated spectra. (a) Observed spectra using 600 and 1200 lines/mm gratings, with line assignations. The transitions that differ significantly from the synthetic spectrum are indicated in blue (14 ev) and green (16 ev). ((b) and (c)) Relevant simulated spectra as referred to in the main text. and synthetic spectra is now discussed and then resumed in Table II. It must be emphasized that these are tentative estimates only and more rigorous results will require a full 3D treatment. The upper figure, Fig. 4(a), is a contour map obtained from the spectrometer operated as a sweptwavelength monochromator. On the right hand side of Fig. 4(a), we also display the current. Dashed lines at different times refer to specific times discussed in the following. Figs. 4(b) 4(e) are synthetic spectra at conditions corresponding to different instants of the discharge. The synthetic spectra are no more than approximations, as there is no reason to expect the plasma to be homogeneous nor is a timeindependent synthesized spectrum reliable. The threshold of the detected emission is reached at 7.0 ns, and a promising fit with a single Maxwellian electron distribution with a temperature of 12 ev is found at 7.5 ns, which is shown in Fig. 4(b). The principal transitions observed here are Ar VII at 167 and 169 Å, Ar VIII at 180 Å, and Ar VII at 193 Å. By 9 ns, a group of persistent Ar VIII transitions (4p 1 3s 1 and 6f 1 3d 1 ) at 159 Å have grown in relation to the transitions at 7.5 ns. This is consistent with an increase in temperature to 14 ev, also shown in Fig. 4(b). An important change in the spectrum is observed at 10 ns, when the Ar IX emission at 49 Å and the Ar VIII satellite lines start to grow. It should be noted that the limited finesse of the spectrometer operated in this mode does not allow distinction between the Ar IX and Ar VIII lines. However, the two Ar IX transitions (barely resolved using the 1200 l/mm grating in Fig. 3(a)) dominate by a factor of approximately 4:1 over the satellite transitions. These features are easily seen in the second order of diffraction marked with an S in Fig. 4(a). By this time, a single Maxwellian plasma is no longer consistent with the observed spectrum and a fast electron component has to be invoked. The peak intensity of these features occurs at 14 ns, just before current reversal. They disappear at approximately 21 ns, 3.5 ns before inverse current peak. In Fig. 4(c), we

7 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) FIG. 4. Time-resolved EUV observations and simulated spectra for discussion. (a) Upper observations related to discharge current. Indications of principal transitions and satellites (S). (b) (e) Simulations at densities and electron temperatures indicated, with and without small fraction of a second energetic Maxwellian. present two synthetic spectra for an electronic density of cm 3 at two values of T e (18 and 24 ev), both with a fast electron component of 3% at 200 ev. These spectra are useful in identifying the conditions at ns. The highest of the two temperatures, 24 ev, shows enhanced Ar X lines (166 and 171 Å) and a suppression of the Ar VIII satellite lines with respect to the strong Ar IX transitions at 49 Å. The distribution of Ar VIII satellites, Ar IX and X, between 30 TABLE II. Summary of the plasma conditions in Fig. 4 between 7.5 and 33 ns using the synthetic spectrum generator PrismSpect. Time [ns] Te (ev) Ne (cm 3 ) Plasma properties Maxwellian Second energetic electron component evident: 50 ev at 1% Compression. Two-component plasma: (i) T e ev, n e , energetic electrons: ev at 1%. (ii) T e 20 ev, n e , energetic electrons: 2% 3% at ev Maximum compression. Predominantly one-component plasma: energetic electrons: 2% 3% at ev Hot, one-component plasma, reduced energetic electron fraction, decreasing density, see Fig. 4(e) No significant energetic electron component. Minimal effects of compression in the spectrum. Minimal deviation from a single Maxwellian distribution.

8 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) and 100 Å is shown to help clarify the large number of predicted transitions. At 18 ev, the satellite lines are prominent and the Ar X lines are weaker. Careful inspection of the experimental spectrum at this time leads us to assign a temperature of ev. A particularly sensitive indicator of even a weak fast electron component, found to be relevant at the start of emission (9 10 ns) where the density and temperature are still comparatively low, is found in two effects. The first is the suppression of the Ar VIII transition at 138 Å (5 d(1) 3 p(1)), and the second is the enhancement of the Ar VIII transitions at 180 and 184 Å. We show an example of this effect in Fig. 4(d), where a comparison of spectra at a density of and 16 ev with and without a small component of fast electrons (1.5% at 50 ev). The experimental spectrum at this time is in agreement with this suggestion. As we have observed above, after 10 ns the overall spectrum increases greatly in intensity and is no longer compatible with a single Maxwellian electron distribution. While many transitions change strongly in their relative strength, the Ar VIII transition at 159 Å dominates until almost the end of emission. The simulations of synthetic spectra show that this transition is comparatively insensitive to both variations in the temperature above 16 ev and to the fast electron component. Hence, we may take its intensity as a measure of the increase of the plasma density. We estimate a three-fold increase in the characteristic electron density between 9 and 14 ns. The optical diagnostic reveals converging radial ionization or compression waves and the confinement of the plasma leads to an axial density gradient as well; rapidly varying heterogeneous plasma is expected. The spectrum between 10 ns and 13 ns is not susceptible to satisfactory interpretation while supposing time-independent or single component plasma. This may be seen in the contradiction that the electron temperature for a prominent Ar IX transition at 49 Å, with only a small Ar VIII satellite component requires ev (see Figs. 4(b) and 4(c), while a prominent Ar VII transition at 193 Å requires less than 16 ev (see Fig. 4(d)). There is also a density effect in that the relative intensity of the Ar VII transition, compared with the Ar VIII and Ar X transitions between 160 and 180 Å, diminishes rapidly above cm 3. A further complication is that the observed spectrum always shows at least a 5:1 predominance of the Ar IX transition at 49 Å over other Ar VIII, IX, and X transitions between 32 and 44 Å; this is in contrast to the simulations, see, for example, Fig. 4(c). The simplest picture is to assume the observed spectrum is the superposition of emission from more than one zone. If two zones are supposed, a plausible fit to the data may be obtained. We present an estimate of the plasma parameters of these two zones in Table II. By 13 ns, the Ar VII emission is decreasing rapidly, while other transitions remain similar in intensity. A consistent argument here proposes the homogenization of the plasma with an electronic density of approximately cm 3 and temperature of ev. A qualification to the foregoing is the intensity of the Ar X (2s 1 2p 6 2s 2 2p 5 ) transition at Å. This line is as intense as the Ar VIII transition at 159 Å from 13 ns until 18 ns. This observation is indicative of even hotter plasma with a significant fast electron component, as we will argue as follows. From 15 ns onwards, corresponding to the start of the third current quarter cycle, there is a gradual but marked decrease in overall intensity, which may be ascribed to decreasing density. The continuing presence of the Ar IX transitions at 49 Å, without an increase in the neighboring Ar VIII satellite component, is consistent with maintaining a temperature of at least 18 ev, as are the Ar X transitions at 166 Å and 171 Å. The prominence of these last lines leads us to propose a temperature of at least 26 ev. In Fig. 4(e), we show a synthetic spectrum at 28 ev and cm 3 with a somewhat reduced fast electron component (1% at 200 ev). The fit at 28 ev is satisfactory in relation to the intensity of the principle Ar VIII, IX, and X transitions, but overestimates the Ar IX 2p 5 3d 1 2p 6 transition at 41.5 Å. It is possible that this discrepancy is due at least in part to the decrease of the sensitivity of the spectrometer below 40 Å. However, no sensitivity curves are available. Finally, from 22 ns onwards, the emission driven by the fast electron component all but disappears. Ar VII emission reappears at approximately 30 ns, and the spectrum has more in common with the Maxwellian plasma at 14 ev of Fig. 4(b). However, there are differences between the idealized steady state plasma of the simulation and the evidence of sharp local density gradients and an incoming compressional wave of the moire and schlieren diagnostic (Fig. 2 at 33 ns). In Table II, we summarize the preceding estimates of the plasma parameters between 7.5 and 35 ns. IV. DISCUSSION The titanium filtered image of the slit-wire camera shows that the most energetic transitions come from plasma whose diameter is approximately 100 lm, whereas the Alfiltered image indicates a diameter almost twice as large. Similarly, the Ti filter transmits only those parts of the spectrum, where a fast electron component is important. The Al filter cut-off is just above the Ar X referred to above. Both the polycarbonate-filtered diamond signal of Fig. 1 and the time-resolved spectrum of Fig. 4 show that emission from Ar IX and X stages diminishes very substantially at approximately 22 ns, that is, approximately 4 ns after current reversal. In contrast, radiation transmitted by the Al filter extends for a further 20 ns. The electron line density profile shows an on-axis maximum with a characteristic diameter of approximately lm at 14 ns. At later times, for example, at 33 ns, the axial feature is of the order of mm in diameter. Remarkably, the Al-filtered slit-wire images show that the emitting plasma remains well confined. This disagreement could be explained by supposing that the emitting plasma is approximately spherical in form rather than extending along the axis. In this way, its contribution to the deviation of light along the much longer 2 cm integration path length of the optical diagnostic is minimal. The axial confinement has been inferred from empirical observation of severe damage to the capillary wall at a distance of 2 3 mm from the anode. 26 A second explanation of the discrepancy could be due to the use of a larger cathode aperture for the

9 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) optical diagnostic. This causes the preionization of a larger portion of the gas inside the capillary. This may lead to plasma of greater diameter and colder, as well. This argument is consistent with the amplitude of the polycarbonate filtered diamond signal (Fig. 1), where we observe 4 times more signal from discharges using the smaller cathode aperture, whereas the Al-filtered signal is in proportion to the aperture area. Source size measurements using the slit-wire camera on the 1.6 mm cathode should unravel the disagreement. The first useful fringe modulation obtained from the moire and schlieren diagnostic occurs at 9 ns. From the first two frames in Fig. 2 (taken from the same shot), we can clearly identify two incoming waves centered at 0.1 and 0.45 mm. The first one, quickly collapses on axis with a speed of 0.01 cm/1ns ¼ 10 7 cm/s, increasing the density by a factor of 3. While the EUV spectrum (see Fig. 4(a)) indicates an average ionization increase from 7 to 9, allowing us to conclude that plasma compression on axis occurs just as the shorter wavelength emission switches on. Between 13.4 and 14.4 ns, we still observe the second incoming feature at the outer radius indicated by the light blue paired lines in Fig. 2 with an average speed of cm/s. From Fig. 4(a), we do not see a significant change in ionization level at this time; while from Fig. 2, we do observe that the line electron density doubles. This leads us argue that this second feature is also a compression wave and not an ionization wave. From the time-resolved spectroscopy results, we observed that between 10 and 13 ns, at least two plasma zones are needed to fit the observed spectrum. Given the strong imposed axial density filling profile, it is a simple step to suppose that plasma closer to the anode will be compressed first and then progressively towards the cathode as time advances, giving rise to a non-homogeneous plasma column with axial as well as radial plasma velocity components. It is at this time that we have found the fast component of the electron distribution function to be more pronounced. After 14.4 ns, the temperature rises to its maximum value and the fast electron component diminishes as does the electronic density (see Table II). Coincident with the formation of the hot and EUV emitting plasma is the growing volume of extreme density gradients. In Section III C, we have mentioned that between 15 and 26 ns, we were unable to follow any fringe shift in the central feature, even with the short pulse laser used in the optical diagnostic (12 ps). We suspect this to be due to fine-scale density gradients caused by instabilities of turbulent plasma. In common with discharges in nitrogen 15 and xenon, 16 peak EUV emission intensity always occurs after di/dt reversal and extending through current reversal. This suggests that in addition to Joule heating and conversion of the compression waves kinetic energy into heat, fine-scale plasma dynamics could also play an essential role on heating the plasma, mainly where the magnetic field is reversing polarity after peak current. This is well known in coronal plasmas 27 but is less explicitly referred to in high energy density and non-maxwellian plasmas. The EUV emission ceases at 35 ns (Fig. 4(a)). We may identify this with the ending of the centrally peaked line density structure, even though there is clearly a modest radial velocity wave between 29.5 and 40.5 ns. We have found that the ability of the PrismSpect code to model an arbitrary electron distribution function is especially useful. From 10 ns onwards, we apply a second Maxwellian of arbitrary energy and percentage to fit the time evolving spectrum with some success. Discrepancies do, however, remain as remarked above. For approximately 2 ns after peak current (9 ns), the spectrum is compatible with a Maxwellian distribution, heated from 12 to 14 ev (Fig. 4(b)). The emission spectrum starting at 10 ns requires a significant fraction of energetic electrons in order to explain the Ar IX - X transitions and their respective satellite lines. The former are favored at higher temperatures (>20 ev), while the latter grow at lower temperatures (<18 ev). From 10 to 13 ns, the spectrum is compatible with the superposition of two characteristic plasmas: one compressed and hot, with significant energetic electron content; the other, of lower density and temperature, with a less significant energetic electron component, again see Table II. Inhomogeneous plasma in the axial coordinate is to be expected as a respond to the strong initial filling density gradient, where the lower density plasma near the anode is compressed by the J B force before the higher density plasma closer to the cathode (zippering effect). By 15 ns, when the current reverses, all the plasma has the same temperature of ev and a density of approximately cm 3. During the third quarter current cycle, the density falls, but the spectrum corresponds to even hotter plasma of approximately 26 ev but with a reduced fast electron component. The THCM is important only at the beginning of the discharge when there is sufficient potential on the electrodes to accelerate the electron beam. But, at the time of EUV emission, the plasma is highly conductive and the THCM has ceased; hence, the high energy component on the electron distribution has to be related with a different mechanism, e.g., instabilities. At present the electron acceleration mechanism is unknown and in a future work, we will perform measurement on the electron energy distribution to better understand the heating process. Due to the complexity of the plasma dynamics and EUV emission spectrum, numerical simulations would be useful to better understand the physics of these devices and optimize the EUV emission. Unfortunately, not much work has been reported. There is only one work that the authors are aware of that tries to reproduce experimental results of a capillary discharge with similar electrical and geometrical characteristics to ours. By comparing our experimental results with these simulations, 28 we find several similarities: the simulation shows the zippering effect on the plasma compression and a radial density profile development between 7 and 12 ns, which, on inspection, is close to the observations reported here. The pre-ionization dynamics shows the role of the THCM generated electrons in establishing the axial conduction channel during the first 4 ns of the discharge. The dimensions of the central feature observed using moire and schlieren for a 10:1 filling pressure gradient, as well as the

10 Valenzuela, Wyndham, and Favre Phys. Plasmas 22, (2015) EUV source size presented in Table I, are consistent with the simulation, however there are discrepancies. The main difference is that the timing of peak EUV emission in the simulation occurs before peak current at 5 ns. Another point of disagreement is that, given the sensitivity of the moire and schlieren diagnostic, we would expect to observe an axial electron density feature at 4 ns and evidence of radial compression at 7 ns. The simulation is performed in a He:Kr mixture, so differences must be expected. However, the EUV emission timing of our work presented here, together with previous work in N:He admixtures 15 and Xe, 16 have in common that no emission is observed until the second quarter cycle. This indicates that some important piece of the plasma physics is missing in the simulation and it shows the importance of time dependent diagnostics to correctly benchmark the modelling. V. CONCLUSIONS Capillary discharges have been researched extensively as compact radiation sources for the past 20 years comprising diverse energy scales and initiation mechanisms. On this work, we have studied the dynamics and EUV emission of a sub-joule capillary discharge as a potential high rep-rate radiation source. The THCM and the initial axial density gradient play the key role on the discharge initiation and also on reducing the physical size of the discharge. Nevertheless, diagnosing such a small and fast device is always a challenge and new methods had to be developed. The initial preionizing electron beam and the non-uniform initial axial density impose another difficulty to the problem compared to standard capillary discharges. The use of the moire-deflectometry diagnostic has allowed observation of multiple compression waves at different stages in the discharge. These, together with Ohm effect, are the fundamental plasma heating mechanisms. Strong density gradients, both radially and axially, could also play a key role heating the plasma where they could lead to turbulent behavior. But more work, experimental and theoretical, is needed to better understand the dominant heating mechanism. Time integrated spectroscopy is of high value in identifying the multiple ionization states of the emitting plasma as it is of higher resolution. Meanwhile, by using the novel swept-wavelength monochromator method, we were able to follow the EUV spectrum in time. Plasma was observed to evolve from a single Maxwellian distribution early in time (7 9 ns) of 12 ev to a two component distribution around peak emission: a low temperature, Maxwellian plasma plus a high energy component. Later on, a smaller fraction in the high energy component and higher temperature suggest homogenization of the plasma. All emission is observed to cease when the centrally localized density peak disappeared at 40 ns. 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