Performance of beryllium targets with full-scale capsules in low-ll 6.72-mm hohlraums on the National Ignition Facility

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1 Performance of beryllium targets with full-scale capsules in low-ll 6.72-mm hohlraums on the National Ignition Facility A. N. Simakov, 1,a) D. C. Wilson, 1 S. A. Yi, 1 E. N. Loomis, 1 J. L. Kline, 1 G. A. Kyrala, 1 A. B. Zylstra, 1 E. L. Dewald, 2 R. Tommasini, 2 J. E. Ralph, 2 D. J. Strozzi, 2 A. G. MacPhee, 2 J. L. Milovich, 2 J. R. Rygg, 2 S. F. Khan, 2 T. Ma, 2 L. C. Jarrott, 2 S. W. Haan, 2 P. M. Celliers, 2 M. M. Marinak, 2 H. G. Rinderknecht, 2 H. F. Robey, 2 J. D. Salmonson, 2 M. Stadermann, 2 S. Baxamusa, 2 C. Alford, 2 Y. Wang, 2 A. Nikroo, 2 N. Rice, 3 C. Kong, 3 J. Jaquez, 3 M. Mauldin, 3 K. P. Youngblood, 3 H. Xu, 3 H. Huang, 3 and H. Sio 4 1 Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 2 Lawrence Livermore National Laboratory, Livermore, California 94551, USA 3 General Atomics, San Diego, California 92186, USA and 4 Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA (Dated: April 21, 2017) When used with 1.06-mm beryllium (Be) capsules on the National Ignition Facility, gold hohlraums with the inner diameter of 5.75 mm and helium gas ll density of 1.6 mg/cm 3 exhibit signicant drive degradation due to laser energy backscatter (of order 1417%) and missing X-ray drive energy (about 32% during the main pulse). Also, hard to simulate cross-beam energy transfer (CBET) must be used to control the implosion symmetry. Larger, 6.72-mm hohlraums with ll densities 0.6 mg/cm 3 generally oer improved drive eciency, reduced hot-electron preheat, and better control of the implosion symmetry without CBET. Recently, we carried out an exploratory campaign to evaluate performance of 1.06-mm Be capsules in such hohlraums and determine optimal hohlraum parameters. Specically, we performed a hohlraum ll-density scan with a three-shock, 9.5-ns laser pulse and found that an appropriate axial laser repointing and azimuthal outer-quad splitting resulted in signicantly improved hohlraum energetics at ll densities 0.3 mg/cm 3 (with backscattered and missing energies being of about 5% and 23% of the total laser energy, respectively). The capsule shape at stagnation was slightly oblate and improved with lowering the ll density. We also performed an implosion with a lower-picket, 12.6-ns pulse at the hohlraum ll density of 0.15 mg/cm 3 to observe comparable hohlraum energetics (about 3% of backscattered and 27% of missing energy) but an even more oblate implosion shape. Thus, achieving symmetric implosions of 1.06-mm Be capsules in low-ll, 6.72-mm gold hohlraums with reasonably low-adiabat pulses may not be feasible. However, symmetric implosions have recently been successfully demonstrated in such hohlraums with 0.8-mm Be capsules. a Electronic mail: simakov@lanl.gov

2 2 I. INTRODUCTION Beryllium (Be) ablators possess a number of important advantages [14] over carbon based ablators [e.g., plastic [5] (CH) or high-density carbon [6] (HDC)] for indirectly driven Inertial Con- nement Fusion (ICF) capsules elded on the National Ignition Facility (NIF) [7]. In particular, the low opacity and relatively low density of Be result in its enhanced mass ablation rate, ablation pressure, and ablation front velocity. This signicantly improves Be ablation front stability [3, 8]; and also allows Be ablators to operate at lower hohlraum radiation temperatures. As a result, such ablators can be used to directly address the two currently regarded as the most important ICF capsule implosion degradation mechanisms: hydrodynamic perturbations of the capsule seeded by engineering features, such as the capsule tent and the fuel ll tube; and low-mode radiation drive asymmetries resulting from moderate length scale separation between the outer capsule and the inner hohlraum radii (typically, about a factor of three). So, Be capsules are relatively insensitive to the perturbations. And larger hohlraums can be used to improve the radiation drive symmetry at the same capsule size (e.g., Fig. 61 of Ref. [9]), albeit at a price of reduced radiation temperature for the same laser drive parameters. Incidentally, the reduction in the radiation temperature can be counteracted by replacing cylindrical hohlraums with so-called rugby hohlraums [1012], since they have smaller wall area at the same maximum outer radius. The rst Be targets were elded on NIF in 2014, with the rst layered deuterium-tritium (DT) capsule imploded in June of 2015 (the NIF shot N150617) [13, 14]. These experiments established that previously hypothesized potential Be target performance degradation mechanisms were not limiting the performance. Examples include the internal Be structure seeding hydrodynamic instabilities [15]; or a large amount of ablated Be in the hohlraum preventing laser beams from properly depositing their energy at the hohlraum wall. At the same time, performance of the DT Be capsule was comparable with, but overall no better than performance of CH capsules in similar early high-foot targets [16]. It is believed [17], that performance of both the Be and CH capsules in such targets was limited by a lack of implosion symmetry caused by low-mode radiation drive asymmetries in the employed gold hohlraums with the inner diameter of 5.75 mm and the helium gas ll density of 1.6 mg/cm 3 (herein, referred to as conventional hohlraums). Nonlinear plasma physics eects play important roles in such hohlraums, making them extremely hard or even impossible to accurately simulate using existing radiation-hydrodynamics (or rad-hydro) codes. Some most important examples are the laser energy

3 3 scattering and losses via the Stimulated Raman (SRS) and Brillouin (SBS) Scattering processes; and the missing drive energy [18], which has to be accounted for through ad hoc time dependent laser power multipliers m(t) 1 to reconcile rad-hydro simulations and experimental observations [19]. When combined, the two mechanisms can reduce the hohlraum drive by up to a factor of two [20], e.g., about 16% of backscattered (BS) and 32% of missing [21] drive energy for the DT ice layered Be shot N To tune implosion symmetry in such hohlraums one typically has to rely upon another nonlinear, time-dependent plasma process, the so-called Cross-Beam Energy Transfer (CBET). By changing wavelengths of dierent NIF laser cones before a shot by a few angstroms it is possible to transfer energy from some cones into others via an ion acoustic wave [22]. Typically, it is necessary to transfer energy from outer to inner cones. The reasons are a signicant amount of energy BS from the inner cones (predominantly via SRS); and a relatively small gap between the capsule and the hohlraum wall that quickly lls with ablated plasma, which impedes the inner cone propagation. The magnitude and direction of the transfer depend upon the cones wavelengths dierence λ and is normally limited in simulations via an ad hoc saturation parameter applied during the main laser pulse. Recently, NIF experiments with Be [23, 24], CH [25, 26], and HDC [12, 27, 28] capsules started employing larger, lower-ll ( 0.6 mg/cm 3 ) hohlraums with the inner diameter of the largest hohlraums of 6.72 mm. Such hohlraums typically exhibit signicantly reduced laser BS (a few percent) and miss less drive energy (between 10 and 25%), thereby reducing the hohlraum drive degradation [20, 26, 27]. For high-foot plastic targets, the improved drive eciency was found to compensate the increase in the hohlraum wall area in 6.72-mm vs mm hohlraums, resulting in radiation temperatures similar to those of conventional hohlraums [26]. Hot electron generation is suppressed by a factor of about 100 as compared with conventional hohlraums, reducing the capsule preheat and improving its compression [26]. Despite the lower hohlraum ll gas density allowing the inner hohlraum wall to expand inwards faster, a larger gap between the capsule and the wall leaves more room for the inner laser cones to properly propagate and deposit their energy. This mitigates the need for employing CBET and improves the capsule implosion symmetry control. All-in-all, using the low-ll 6.72-mm hohlraums for high-foot plastic targets resulted, at xed laser drive energy, in approximately 40% increase in the capsule stagnation pressure, 50% increase in the neutron yield, and 20 to 70% improvement in the capsule compression uniformity [26]. The NIF Be campaign has so far carried out four shots with full-scale capsules [23], as well as four shots with sub-scale capsules (with the outer radius of approximately 0.8 mm) [24] in low-ll

4 mm gold hohlraums. The main goal of the shots was to evaluate performance of Be targets in such hohlraums, including the drive degradation and the implosion symmetry, for dierent hohlraum ll-gas densities, laser pulse lengths, and capsule sizes. Herein, we will report the results for the full-scale capsule implosions; the sub-scale capsule results will be presented elsewhere. The paper is organized as follows. Section II describes the NIF targets used in the experiments, while Sec. III describes and discusses the results obtained, including the BS, missing energy, and the implosion shape. The latter section also provides comparisons with post-shot simulations' results. Finally, we summarize our fundings in Sec. IV. II. THE TARGETS For the experiments discussed herein, we employed the standard 6.72-mm gold hohlraum [26], which is mm long, has the inner diameter of 6.72 mm and the laser entrance hole (LEH) diameter of 3.9 mm. The hohlraum was lled with the 4 He gas of 0.15, 0.3, or 0.6 mg/cm 3 density. We used the same capsule as in the rst NIF Be campaign [13, 14] (see Fig. 1), with the outer radius of approximately 1.06 mm, the overall Be ablator thickness of approximately 160 µm, and a pyramid of copper doped ablator layers with the maximum dopant amount of approximately one atomic percent. As will be discussed in detail later on, we elded a keyhole target [29] (NIF shot N151006), two 2DConA targets [30] (NIF shots N and N160831), and a 1DConA target [31] (NIF shot N160327). The keyhole target used the capsule as shown in Fig. 1, but without the DT ice layer and lled with liquid deuterium instead. The ConA targets used capsules with the ice layer replaced with an undoped Be layer of equal mass and lled with deuterium-helium mixture (70 atomic % 3 He + 30 atomic % D) of approximately 6 mg/cm 3 density. For the purposes of determining the target behavior versus the laser pulse length two dierent laser pulses were employed: a 9.5-ns and a 12.6-ns long. The total laser power and the cone fractions, as given by the ratio of the inner cones power to the total power, for the two pulses are shown versus time in Figs. 2a and 2b, respectively. These are three-shock, high fuel adiabat [5] (α = 2.9 and 2.2, respectively, for a DT layered capsule) pulses with the maximum power of 360 TW and the total energy of approximately 1.3 MJ. Following the CH and HDC campaigns, we employed λ = 0. Both pulses feature 0.7-ns long pre-pulses, often referred to as toes, with powers of approximately 3.9 and 2.0 TW, respectively. The purpose of a toe [32] is to burn through a window closing the LEH at low power to avoid any signicant laser BS during the subsequent higher-power

5 µm Tube 10 µm OD SiO2 Hole 5 µm 936 (Total thickness ) DT ice (70 µm) Be+Cu 0% 0.4% 1.0% 0.4% 0% DT gas Figure 1. Pie diagram of the Be capsule. D E Figure 2. Laser power (a) and inner cone fractions (b) versus time for the 9.5-ns (black) and the 12.6-ns (red) pulses. portion of the pulse, the so-called picket. During the toe only the inner laser cones are engaged. And their power is determined by the implosion symmetry requirements for the laser cone fraction during the picket since, due to the NIF laser operational constraints, the inner cone power during both the toe and the picket must remain unchanged. The toe duration is chosen to be su ciently long for the high-density LEH window plasma to clear the LEH aperture before the picket

6 6 power turns on. The picket power and energy determine the rst shock strength (and velocity), which, in turn, is the main parameter setting the fuel adiabat and thus compressibility. The 9.5-ns pulse used the picket power of 43 TW, whereas the 12.6-ns pulse employed 22.6 TW. Since the laser pulses timing is tuned to make all the three shocks break out of the DT ice at approximately the same time, the weaker the rst shock, the longer the low-power portion of the pulse immediately following the picket, the so-called trough; and thus the longer the entire pulse. Time histories of the total power after the trough are identical for the two pulses. Finally, as is clear from Fig. 2b, we employed piece-wise constant laser cone fractions to tune the implosion symmetry. Each individual portion of the pulse, except for the one when the main power is on (i.e., the main pulse), was tuned independently using the LLNL radiation-hydrodynamics code HYDRA [33]. Borrowing from the CH campaign experience [25, 26], we chose the main-pulse cone fraction to be During the rst NIF Be campaign [13, 14], the laser energy BS was dominated by the innercone SRS, which scattered away between 33 and 45% of the inner-cone energy, depending on the target type. Hence, the need for CBET from the outer into the inner cones to ensure a symmetric implosion. A rationale for replacing the high-ll hohlraum with a low-ll one stems from the fact that SRS decreases with the hohlraum ll density [34]. At the same time, while SBS scattered much less energy than SRS, and then primarily from the outer cones, SBS is much more dangerous from the laser optics damage perspective; and thus it should be controlled very carefully. One way of reducing the outer-cone SBS is via decreasing the corresponding laser intensity at the hohlraum wall. We accomplished this for the campaign under discussion in two dierent ways. First, we modied the standard pointing of the laser at the inner wall of the 6.72-mm hohlraum by displacing the 44.5 /50 cones along the hohlraum axis towards/away from the hohlraum waist by additional 320/180 µm, reducing their overlap at the wall. Second, we modied focusing of the laser quads belonging to the outer cones via displacing or splitting their four beams azimuthally [35] by 500 µm at the wall as compared with the standard case. This also reduced overlapping of the beams and the laser power (and intensity) at the wall, as is shown in Fig. 3.

7 7 300 Power (TW/cm 2 ) Azimuthal Angle (degrees) without azimuthal splitting with azimuthal splitting Axial Position (cm) Figure 3. A map of the laser power density deposited at the inner hohlraum wall by the outer cones as a function of the azimuthal angle and the position along the hohlraum axis without (left) and with (right) the azimuthal quad splitting. III. THE RESULTS Next, we will discuss the results obtained in the NIF shots N151006, N151229, N160327, and N We will concentrate on the hohlraum performance and the hohlraum-capsule coupling, and discuss the amount of the laser BS, missing X-ray drive energy, and the implosion symmetry. For this experimental campaign, we started with a short, 9.5-ns pulse, basing this decision upon the fact that short-pulse HDC targets demonstrate very low backscattered (of order few percent) and low missing [of order (1015)%] energies [27]. Having explored this pulse we proceeded to a longer, 12.6-ns pulse. A. Laser Backscatter We start by discussing the BS energy measurements. To mitigate the risk of the NIF laser damage due to SBS, target types new to the facility are commonly required to initially employ truncated in time laser pulses. Since keyhole or VISAR (Velocity Interferometer System for Any Reector) targets [29] are intended to measure the shock velocities, timing, and symmetry, they normally do not require the full pulse and are thus often used for the rst shot with a new target. Our rst shot, N151006, used such a target and truncated the main pulse duration by about a nanosecond. Some other pulse modications were also made for piggy-back CBET studies. We

8 8 employed a rather large hohlraum ll gas density of 0.6 mg/cm 3 hoping for more predictive HYDRA simulations. Moreover, such a density has been used with 6.72-mm hohlraums by the NIF CH [26] and HDC [12] campaigns; although the HDC campaign typically employed so-called near-vacuum lls of mg/cm 3 [27]. The measured overall BS was about 8.6% of the laser energy and came primarily from inner- and outer-cone SRS (see Table I). This is about a factor of two lower than with conventional hohlraums [13, 14]. Table I. Backscattered laser energy by the cone and mechanism. The coupling corresponds to (1 - total BS energy/total laser energy) 100%. Shot N N N N Hohlraum ll density (mg/cm 3 ) Pulse length (ns) 9.5 (truncated) SRS (kj) SBS (kj) SRS (kj) SBS (kj) SRS (kj) SBS (kj) SRS (kj) SBS (kj) Total BS (kj) Total laser energy (kj) Coupling (%) 91.4 ± ± ± ± 0.5 To further reduce the laser BS we next reduced the hohlraum ll gas density. The shots N151229, which used a 2DConA target [30] intended to give time gated images of X-rays from a copper backlighter transmitted through the capsule, and N160327, which employed a 1DConA target [31] to obtain a streaked capsule radius versus time, used ll densities of 0.15 and 0.3 mg/cm 3, respectively. Both shots used the same 9.5-ns pulse. As follows from Table I and is shown in Fig. 4a, going from the 0.6 mg/cm 3 to the 0.3 mg/cm 3 ll reduced both the inner- and outer-cone SRS signicantly and resulted in the overall BS reduction from 8.6% to about 5%. With the further reduction of the ll density to 0.15 mg/cm 3 the outer-cone SRS decreased even more, while the inner-cone SRS remained approximately unchanged. At the same time, the outer-cone SBS increased as the ll

9 9 [%] [ / ] [%] [ ] Figure 4. Laser backscatter versus the hohlraum ll density for the 9.5-ns pulse (a), and versus the pulse length for the 0.15 mg/cm 3 hohlraum ll (b). The red/blue/green lines correspond to the inner/outer/total laser cones. The dashed/dotted/solid lines correspond to SRS/SBS/total BS. density decreased from 0.6 mg/cm 3 to 0.3 mg/cm 3 to 0.15 mg/cm 3 in such a way as to keep the overall outer-cone BS approximately the same. Thus, no overall reduction in the laser BS occurred as the ll density decreased from 0.3 mg/cm 3 to 0.15 mg/cm 3. One important point to keep in mind, however, is that while the BS comparison between the 0.3 mg/cm 3 and 0.15 mg/cm 3 ll densities is quite fair, the keyhole target with the 0.6 mg/cm 3 ll used a truncated and then further modied 9.5-ns pulse, so that its inclusion in this comparison is somewhat less justied. Finally, the shot N160831, which used a 2DConA target and employed a 0.15 mg/cm 3 hohlraum ll density, lengthened the laser pulse from 9.5 ns to 12.6 ns by reducing the picket power (and energy). It produced about 3% of the overall BS (see Table I and Fig. 4b), primarily due to reduction of the outer-cone SBS as compared with the 9.5-ns pulse of the shot N The outer-cone SRS remained at a very low level; while both the inner-cone SRS and SBS, and therefore the total inner-cone BS, remained approximately the same as in N To summarize, reduction in the hohlraum ll density at the xed pulse length of 9.5 ns resulted in the inner-cone SRS reduction, rst leading to the overall BS reduction and then saturation at about 5% level for ll densities below about 0.15 mg/cm 3, when the inner-cone SRS became small compared with the total outer-cone BS. Lengthening of the pulse via reducing the picket power and energy at the xed hohlraum ll density of 0.15 mg/cm 3 resulted in the outer-cone SBS reduction, leading to the overall BS reduction and likely saturation at about (1.52)% level for pulse lengths above about 14 ns, when the outer-cone SBS extrapolates to become small compared with the total inner-cone BS. The few-percent BS values are consistent with those for the HDC targets in nearvacuum hohlraums [36] and are better than the 10.4% BS values (about 5.4% on the inner and 5.0%

10 10 on the outer cones) for the CH targets in hohlraums with 0.6 mg/cm 3 lls [26]. B. Missing X-ray Drive Energy and Laser Power Multipliers Next, we will discuss the missing X-ray drive energy observed in the experiments; or, more accurately, the over-prediction by HYDRA of the hohlraum X-ray drive generation and radiation temperature [37]. Until the code model is improved, it is necessary to introduce into HYDRA simulations ad hoc time dependent laser power (or drive) multipliers m(t) 1 [19] to match basic experimental observables, such as shock velocities, the capsule implosion velocity, and the bang time (i.e., the time of the peak neutron production or X-ray emission). Various choices of the function m(t) are possible when only the most basic observables are used. However, the more observables are matched the better such functions are constrained. Reference [13] presents a detailed discussion on the topic and employs results from the eighteen-channel soft X-ray spectrometer DANTE [3840] to come up with the most constrained choice of m(t) to date. For comparison purposes, herein we will employ a simple two-step model for m(t), as shown with a blue line in Fig Total Laser Power (TW) Drive Multiplier Power (TW) Power Multiplier Time (ns) 0.75 Figure 5. A delivered laser power for the 9.5-ns pulse (dashed red line) and a corresponding simple timedependent power multiplier model (solid blue line). Keyhole targets measure shock velocities and thus can provide information on the laser power multipliers during various early portions of the laser pulse, e.g., the picket and the trough. It follows from the measurements of the rst shock velocity in NIF shot N that the picket power multiplier for our 9.5-ns pulse is close to 1.0. Unfortunately, we were unable to measure velocities

11 11 of the second and third shocks in this experiment and thus determine the power multipliers for the later portions of the pulse's foot. Here, the foot refers to the low-power portion of the pulse. For comparison, the picket and trough multipliers for the Be target in the conventional hohlraum were determined to be close to 0.93 and 1.0, respectively [13]. This motivates our choice for the value of the initial at portion of the power multiplier curve to be 1.0 for the 9.5-ns pulse. Since no data exists for the foot multipliers for the 12.6-ns pulse, herein, we will use for them the same value of 1.0. We obtained the value of the second at portion of the power multiplier curve by matching the X-ray bang time from the experiments and the simulations. We found that for the 9.5-ns pulse this value is about for both hohlraum ll-gas density values of 0.15 and 0.3 mg/cm 3. For the 12.6-ns pulse and the ll density of 0.15 mg/cm 3 we obtained the power multiplier value of approximately Thus, while the amount of the BS energy slightly decreased as we went from the 9.5-ns to the 12.6-ns pulse (at the same ll-gas density), the amount of the missing energy slightly increased at the same time, resulting in an approximately the same or slightly more degraded drive. To put things into perspective, the HDC campaign obtained for (68)-ns laser pulses and the near-vacuum hohlraum ll density of mg/cm 3 the values of the main-pulse power multiplier between 0.85 and 0.9, depending on the employed HDC equation of state [27, 41]. And the CH campaign obtained for (1315)-ns pulses and the ll density of 0.6 mg/cm 3 the power multiplier of about 0.89, albeit at larger levels of the laser BS (about 10%) [26]. However, one needs to keep in mind that each of the campaigns employed a unique function m(t) [41] and thus a direct comparison of these main-pulse power multiplier values is not necessarily very meaningful [21]. C. Implosion Symmetry As has been already mentioned, our NIF Be shots N151229, N160327, and N resulted in somewhat oblate implosions. Figure 6 shows a time-resolved equatorial X-ray image of the Be capsule obtained approximately 130 ps after the X-ray bang time t bang ns of the 2DConA target N When symmetry of the 17%-level contour of the X-ray self-emission image at bang time is analyzed in terms of Legendre polynomials, we obtain for their amplitudes P 0 96 µm, P 2 /P 0 16%, and P 4 /P 0 3%. The P 2 and P 4 asymmetries exhibit moderate time swings of about d/dt(p 2 /P 0 ) 23%/ns and d/dt(p 4 /P 0 ) 13%/ns. Figure 7 shows a time integrated equatorial X-ray image for the 1DConA shot N Symmetry analysis of the 17% self-emission

12 12 contour gives P0 61 µm, P2 /P0 25%, and P4 /P0 5%. A signi cant di erence in P0 sizes between the two shots, coupled with unexpectedly large and rather time insensitive values of for N may indicate that the self-emission signal for that shot was saturated. P0 If true, the self-emission symmetry information for N may not be reliable. Figure 6. Time resolved equatorial X-ray image of the capsule soon after the X-ray bang time tbang ns : MN for the 2DConA shot N Figure 7. Time integrated equatorial X-ray image for the 1DConA shot N Given this caveat, taking the X-ray symmetry information for the two aforementioned Be shots at a face value indicates that for high-adiabat Be capsule implosions in low- ll 6.72-mm hohlraums, which are driven by high-picket, 9.5 ns long laser pulses, the implosion shape gets less oblate with

13 13 lower hohlraum ll-gas density. This situation is shown in Fig. 8.This trend appears to be opposite to the trend observed for CH capsule implosions in 6.72-mm hohlraums driven by lower-adiabat, 13 ns pulses. So, 2DConA CH target shots N (the ll density of 0.6 mg/cm 3 ) and N (the ll density of 0.3 mg/cm 3 ) obtained for the time-integrated equatorial X-ray self-emission images P 2 /P 0 24% and 46%, respectively. A possible caveat for such a conclusion is, however, the fact that the main power duration in the laser pulse employed in the latter shot was truncated by about 0.5 ns as compared with the former shot; and a variation of the main-power duration is quite capable of inuencing the implosion shape. - - / [%] - - N N [/ ] Figure 8. Relative amplitudes of P 2 distortions of equatorial X-ray self-emission images for the Be shots N and N NIF shot N lengthened the pulse (i.e., its foot) by about 3.1 ns to about 12.6 ns. Figure 9 shows a time-resolved equatorial X-ray image of the Be capsule obtained approximately 40 ps after the X-ray bang time t bang ns. Symmetry analysis of the 17%-level contour of the X-ray self-emission image at bang time gives P 0 73 µm, P 2 /P 0 45%, and P 4 /P 0 9%. The P 2 and P 4 asymmetries exhibit time swings of about d/dt(p 2 /P 0 ) 23%/ns and d/dt(p 4 /P 0 ) 32%/ns. Thus, lengthening the pulse resulted for our Be targets in a more oblate implosion. In fact, comparing Be shots N and N gives about 9%/ns increase in the implosion oblateness due to the pulse lengthening. This trend is also opposite to that obtained by the CH campaign. Indeed, CH target shots N and N employed nominally identical (or very similar) capsules, hohlraum ll-gas densities (0.6 mg/cm 3 ), and the main-power portions of the laser pulses. The main dierence was in the picket power and the corresponding pulse's foot duration, since N employed an ultra-high-picket, 10.3-ns pulse. It follows from the two shots that the oblateness of the time-

14 14 integrated equatorial X-ray self-emission images decreases for such CH targets with the pulse's foot lenghtening at a rate of approximately 5%/ns. At the same time, the observed Be symmetry change with lengthening the pulse is consistent with that for HDC capsules in 5.75-mm hohlraums with near-vacuum lls [42]. It is currently unclear whether the dierence has primarily to do with the dierent hohlraum ll-gas densities, dierences between the ablators, or both. One caveat to this conclusion is a possible saturation of the X-ray signal for N shot and the associated uncertainty in the corresponding implosion symmetry results. Figure 9. Time resolved equatorial X-ray image of the capsule soon after the X-ray bang time t bang ns for the 2DConA Be shot N Finally, we would like to comment on post-shot simulations for the NIF Be shots discussed herein. HYDRA, which was used in this work, has limited predictive capabilities in terms of implosion shape, and various ad hoc techniques are often used to improve agreement between observations and simulations. For example, to reproduce implosion shapes of HDC capsules in near-vacuum hohlraums one often needs to employ the so-called enhanced beam-propagation model, which articially modies laser frequencies of various NIF cones [27]. In our case, the implosion shape agreement can be signicantly improved by articially modifying in the simulations the hohlraum ll-gas density. Specically, by increasing it for shot N from 0.15 to 0.2 mg/cm 3 produces an equatorial X-ray self-emission image at bang time (see Fig. 10) that is very similar to the observed image of Fig. 9. In particular, we obtain P 0 73 µm, P 2 /P 0 46%, and P 4 /P 0 14%. Moreover, as is shown in Fig. 11, while not in ideal agreement, time evolution of the quantities P 2 /P 0 and P 4 /P 0 in simulations and experimental observations are also rather similar. With the matched implosion shape, we obtain for the simulated DD neutron yield about , which is about

15 15 (11 16)% larger than the observed yield. Figure 10. Equatorial X-ray self-emission image just before the X-ray bang time for the 2DConA Be shot 3 N from HYDRA post-shot simulations with 0.2 mg/cm hohlraum ll-gas density. IV. CONCLUSIONS We carried out four NIF shots to experimentally study implosions of 1.06-mm Be capsules in low- ll, 6.72-mm gold hohlraums. The implosions were driven by high-adiabat, 9.5-ns and 12.6-ns laser pulses. The main goal of the experiments was to understand how the hohlraum behavior in terms of observed backscattered and missing drive energy, as well as the capsule implosion symmetry, depended on the hohlraum ll-gas density and the pulse length. Use of the short, 9.5-ns pulse was motivated by favorable hohlraum and capsule implosion symmetry properties observed for short pulses in HDC implosions. To reduce the outer-cone SBS we axially separated the 44.5 cones and split azimuthally laser quads from these cones to reduce the hohlraum wall laser and 50 intensity. We observed that the backscattered laser energy was of order 5% for the shorter pulse 3 at ll densities of 0.15 and 0.3 mg/cm, but reduced to about 3% for the longer pulse at the ll 3 density of 0.15 mg/cm. The low amount of the BS energy is comparable with that observed in the HDC target implosions, but is several times lower than that in the CH implosions. It appears that the inner-cone BS was dominated by SRS that could be reduced by reducing the ll density, 3 with saturation at about 0.3 mg/cm. At the same time, the outer-cone BS was dominated by

16 16 D Figure 11. Time evolution of the quantities P2 /P0 E (a) and P4 /P0 (b) around the X-ray bang time tbang ns for the equatorial X-ray self-emission image for Be shot N The red lines are from HYDRA 3 post-shot simulations with the arti cially increased from 0.15 to 0.2 mg/cm hohlraum ll-gas density; the blue lines are from the experiment. the SBS, which reduced with lengthening the pulse and would likely saturate for pulses over 14 ns long. We also concluded that about 23% and 27% of the laser energy for the 9.5-ns and 12.6-ns pulses, respectively, were missing from the HYDRA simulations and were not getting converted into the hohlraum X-ray drive. This amount of the missing energy is higher than that in both HDC and CH target implosions. Finally, all our Be implosions exhibited somewhat oblate shape, with oblateness increasing with the ll density and the pulse length. Thus, it appears that achieving symmetric implosions of full-scale Be capsules in low- ll, 6.72-mm gold hohlraums with reasonably low-adiabat (or relatively long) pulses may not be feasible. This is consistent with results obtained so far by the CH campaign. However, subsequent experiments with smaller, 0.8-mm Be capsules in such hohlraums have successfully produced symmetric implosions. These results will be reported elsewhere. ACKNOWLEDGMENTS We thank the LLNL colleagues D. A. Callahan, D. E. Hinkel, and O. A. Hurricane for valuable discussions. Execution of NIF experiments requires participation and collaboration of large teams of individuals. We thank all those who have been making the NIF laser facility a success, in particular

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