Recent Publications

Exploration of an intrinsic low-collisionality, high performance, grassy ELM regime in the DIII-D tokamak

Zeyu Li, H Q Wang, Xi Chen, Xueqiao Xu, Rongjie Hong, Filipp Khabanov, V. S. Chan, Patrick H Diamond, Brian S Victor, Nami Li, et al.

Recent DIII-D tokamak experiments have demonstrated the integration of intrinsic low-collisionality grassy edge-localized mode (ELM) regime with high-performance hybrid core scenario, possibly offering a core-edge compatible solution for ITER and future fusion reactors. This regime features grassy ELMs (with ELM energy loss over pedestal stored energy <2%) at ITER-relevant pedestal top collisionality (ν_e^*~0.1), ITER similar shape, while maintaining high core confinement (H_98y2~1.5) in non-inductive hybrid scenarios. A small ELM-focused database is constructed to investigate the parametric dependence of the small/grassy ELM regimes. Access and sustainment of this regime appear to be favored by high poloidal beta (β_p>1.5), higher ratio of separatrix-to-pedestal density (n_(e,sep)⁄n_(e,ped) >0.4) and low pedestal top collisionality (υ_(e,ped)^*=0.1-0.4). In this scenario, the pedestal width exceeds the prediction from EPED–KBM scaling, consistent with the expectation of a turbulence-limited pedestal. Linear modeling using ELITE indicates that this grassy ELM regime is along the peeling boundary. Relative to a large-ELM phase, the grassy-ELM phase exhibits a broader inner-target heat-flux width and a substantially reduced ELM-induced transient heat-flux increment. These results motivate further evaluation of low-collisionality grassy ELMs as a potentially reactor-relevant operating regime, while full-duration sustainment and compatibility with divertor detachment remain unresolved.

DLS-Extended: a reduced model to assess the impact of impurity radiation location on optimal magnetic geometry choices for the STEP divertor

Mike Kryjak, Cyd Cowley, David Moulton, Ryoko Tatsumi Osawa, Stuart Scott Henderson, Omkar Myatra, Benjamin Dudson, Peter Alec Hill, Liam Pattinson, Christopher Paul P Ridgers

Alternative Divertor Configurations (ADCs) often make use of high total flux expansion (fR) and connection length (L∥) to improve the access to and the controllability of detachment, a regime vitally important for reactor-class tokamaks such as the Spherical Tokamak for Energy Production (STEP). Achieving detachment on such high-power devices is enabled through radiation from seeded impurity species such as argon and neon, which can lead to radiative losses throughout the flux tube, altering the impact of magnetic geometry on detachment. The Detachment Location Sensitivity (DLS) model analytically predicts detachment access and sensitivity based on upstream conditions, the magnetic geometry and seeded impurity radiation. It assumes the radiation region to have no spatial extent. In this work, we relax this assumption and introduce DLS-Extended, a new reduced 1D model which can capture the effects of the spatial distribution of impurity radiation along the field line. The prediction of radiation extent was verified against SOLPS-ITER simulations of an initial STEP design with a good match. DLS-Extended predicts that the radiation extent found in STEP weakens the detachment access benefit of fR and strengthens the benefit of L∥, while the detachment stability of the inner and outer legs is increased and decreased, respectively. This is due to a number of novel radiaton-driven effects and has implications for the optimum strike point position in reactor scale devices. These findings highlight the need for more experimental studies in the presence of broad-radiating impurities. DLS-Extended is distributed under the LGPL-3 open source license and is publicly available on GitHub.

Modeling hohlraum wall expansion with higher-moment multi-species fluid model

C. D. Decker, C. J. Bruulsema, W. A. Farmer, J. Harte, D. P. Higginson, A. J. Kemp, W. Riedel, J. S. Ross, D. J. Strozzi, G. Zimmerman

We examine the validity of a multi-species 13-moment fluid approach to modeling hohlraum wall expansion occurring in inertial confinement fusion targets used at the National Ignition Facility. We compare our simulation to experiments conducted at the Omega laser facility [Pape et al., Phys. Rev. Lett. 124, 025003 (2020)], which produced counter-propagating gold–carbon plasmas, a phenomenon arising during hohlraum wall expansion. The two experiments we use to benchmark this fluid model produced counter-propagating gold–carbon plasmas in (1) vacuum and (2) a helium atmosphere. We find that simulations using this advanced fluid model replicate certain aspects of the vacuum experiment, such as material interpenetration, and distinct anisotropic gold and carbon ion temperatures are not captured with single-species lower-moment fluid models. However, our simulations underpredict the temperature of scarce gold ions that penetrate deep into the carbon—a phenomenon potentially requiring a kinetic treatment to model. When applied to the helium gas filled experiment, this advanced fluid model matches the amount of material mixing and gold stagnation point but overpredicts ion temperatures in the gold. Finally, we examine the applicability of single fluid models (as well as lower-order multi-species models) by comparing simulated plasma quantities vs the helium gas-fill density. As expected, we find the models converge as the helium gas density is increased. We find that for gas-fill densities larger than 0.6 mg/cm3 the single fluid model gives similar bulk plasma densities and temperatures as the multi-fluid 13-moment model. However, for gas-fill densities lower than 1.0 mg/cm3, simulations show substantial low-z material mixing in the gold—requiring multi-species modeling. Moreover, accurately modeling the low-z mixing to within 30% requires the full multi-species 13-moment fluid model.

Compact Experimental Negative TriAngUlarity Reactor (CENTAUR): A design study for a compact, affordable breakeven tokamak

The CENTAUR Collaboration, Samuel W. Freiberger, Evan Bursch, Javier Chiriboga, Hiro J. Farre-Kaga, Eliot Felske, Sophia Guizzo, John Labbate, Shreyas Seethalla, Frederick Sheehan, et al.

This work presents the compact experimental negative triangularity reactor (CENTAUR), a low overnight cost, high-field tokamak, breakeven reactor design, achieving a predicted total fusion power of 40MW and scientific energy gain of 1.3. Ballooning stability calculations confirm that the device's pedestal is within the first stability regime, which is consistent with the expected ELM-free operation associated with negative triangularity (NT) plasmas. The geometry of the NT divertor allows for high fraction of radiated power (13.5$\%$) between the separatrix and plasma facing components. Heat transport modeling based on simulations of the edge region show heat loads into plasma facing components well below material limits. The magnet system employs rare-earth barium copper oxide (REBCO) high-temperature superconductors in 18 toroidal field coils, an hourglass-shaped central solenoid, and six poloidal field coils to support high-field ($B_0=10.9$ T) plasma confinement, shaping, and current drive. Neutronics analysis shows that a 12 cm $B_4C$ shield keeps superconducting magnet heating below the 33~K quench limit during 10 s, 40 MW DT pulses. With this shielding, the modeled fluence indicates HTS components can survive more than ten times the 3000-pulse design lifetime. Iteration of economic analysis in tandem with the technical design process allows CENTAUR to achieve its overnight cost goal of $\$$2B determined using a custom costing model that predicts a total overnight cost of $1.6$B$\pm0.2$B.

Neutronic Analyses for the Design of the Chamber of Xcimer’s Athena Fusion Pilot Plant

Francisco Ogando, Robert Earley, Wayne Meier, Rene Raffray, Patrick Sauvan, Jaime Marian, Susana Reyes, Conner Galloway

Modeling of divertor heat flux limits and lithium vapor shielding in NSTX-U using UEDGE code

MD Shahinul Islam, Maxim V Umansky, Vlad Soukhanovskii

In this work, UEDGE simulations coupled self-consistently with a two-dimensional wall transport solver (Wall-Li) are used to investigate two closely related issues important for NSTX-U operation: (1) plasma-surface interactions of graphite plasma-facing components (PFCs) under increasing input power for standard and snowflake divertors and (2) lithium PFC vapor shielding and its dependence on upstream plasma conditions for a standard lower single null divertor with a 5~mm layer of lithium on the graphite tiles. Simulations with graphite target PFCs show a monotonic increase in surface temperature with increasing heat flux incident on the target plates. For a broad range of operating conditions, surface temperatures exceed the graphite sublimation temperature (>1200°C, when graphite starts massively sublimating) once peak heat fluxes exceed 7~MW/m2. This occurs despite enhanced carbon radiation. A snowflake-minus divertor configuration is analyzed to assess divertor heat handling on the secondary strike point (SP\#2) under conditions where both magnetic field-line incidence and cross-field transport are uncertain. Simulation results indicate that the heat flux to SP\#2 is dominated by enhanced CM transport rather than parallel conduction, leading to increased deposited heat flux with increasing CM strength and incidence angle. Lithium vapor shielding is investigated using a self-consistent model in which lithium sourcing depends on local plasma and surface conditions. The model demonstrates that lithium vapor shielding provides passive thermal cooling, maintaining surface temperatures below 700°C even as the core power increases. However, lithium accumulation upstream increases once surface temperatures exceed 600°C. Increasing the core density augments the drag force on impurity ions due to the parallel plasma flow in the scrape-off layer, confining lithium closer to the target and thereby extending the operational window for effective vapor shielding. Overall, these results highlight the necessity of (1) thick lithium layers to prevent strike-point depletion, (2) active cooling mechanisms (such as fast-flowing liquid lithium) to suppress excessive surface temperatures and evaporation, as lithium operation is constrained by surface temperature, and/or (3) deuterium gas puffing near the divertor to further increase SOL flow and density

Optical diagnostic design for measuring the radiation front with a mid-leg pumped divertor on DIII-D

Gilson Ronchi, Robert S. Wilcox, Jonathan H. Yu, Andreas Holm, Filippo Scotti, Adam McLean, Morgan Shafer, Joshua Hicok, Tyler Elsey

Integration of X-point radiator divertor operation with high beta hybrid core plasmas in DIII-D

H Q Wang, Xinxing Ma, Zeyu Li, Roberto Maurizio, Anthony W Leonard, Filippo Scotti, Qiming Hu, Dan M Thomas, Andrea M Garofalo, Siye Ding, et al.

Recent DIII-D experiments have demonstrated the compatibility of complete divertor detachment with a high-beta core using the high beta hybrid scenario plasmas with an ITER-similar shape and nitrogen impurity seeding. With complete divertor detachment (near-zero divertor particle flux and temperature), the radiation peaks inside the X-point indicating the achievement of the X-Point Radiator (XPR) regime, which is a highly dissipative divertor operation scenario that may be attractive for future reactors. SOLPS-ITER modelling with full drifts is able to qualitatively reproduce the experimental measured boundary plasma conditions and radiation patterns from attached to XPR detached divertor state for these high-beta hybrid plasmas. Experiments found that when the radiation peak is inside but close to the X-point, as noted ‘shallow XPR’, complete divertor detachment and high-beta high-confinement core (βN ~3.0, H98~1.25) could be simultaneously achieved. However, this plasma remains ELMing with giant ELMs (W/W ~ 3-4%). With stronger N2 impurity injection, the plasma enters a ‘deep XPR’ regime where the radiation peak is close to inboard side of the pedestal and core radiation is about a factor of 2 higher. With deep XPR, the ELMs are strongly mitigated. However, the confinement is significantly reduced to H98<1.0, which is attributed to the 50% lower pedestal pressure and 30% colder pedestal temperature. SOLPS-ITER simulations highlight the key role of impurity radiation, neutrals and divertor closure in the formation of an XPR and the effects of drifts on the distribution of plasma and radiation near the X-point, all of which are important for the divertor design and operation in future reactors. The modelling also exhibits qualitatively good agreement with experimental observations on the pedestal performance responding to the radiation dynamics, which provides physics insight on the core-edge integration between divertor dissipation and high-performance core that is critically important for future tokamak fusion reactors.

SOLPS-ITER modeling of a dedicated divertor for negative triangularity operation on DIII-D

J.D. Lore, F. Scotti, A. Marinoni, K.E. Thome, C. Lasnier, M. Shafer, D. Truong, H.Q. Wang

Achievement of a high-density, high-confinement, and high-beta tokamak plasma regime in DIII-D, and implications for a lower-current path for ITER and FPP

Andrea M Garofalo, Bart Van Compernolle, Siye Ding, Jeremy M Hanson, Christopher Thomas Holcomb, Tomas Odstrcil, Nathan Jordan Richner, Shengyu Shi, H Q Wang, Torrin Bechtel, et al.

Experiments on DIII-D have demonstrated a density-confinement synergy that enables sustainment of high performance in a previously unattained parameter regime of simultaneous very high energy confinement quality (H 98y2 ≥ 1.5), very high line-average density Greenwald fraction (ƒ Gr = πa 2 <n>/I P ≥ 1.4), and high toroidal beta (β T ≥ 3%). Tokamak operation in this regime is essential for a compact steady-state FPP, as well as for Q=10 with 500 MW of fusion power in ITER at I P << 15 MA. These experiments leveraged the knowledge that, in the high-poloidal-beta (β P ) regime, impurity and density gradients can enhance turbulence stabilization caused by high α MHD (α MHD ~(dβ P )⁄dr). This was described by theoretical predictions and gyrokinetic transport simulations [M.T. Kotschenreuther et al, 2024 Nucl. Fusion, 64 076033], and later confirmed by experiments on DIII-D [S. Ding et al, 2024 Nature 629 555]. To increase both β P and β T , the new experiments increased the ideal-wall stability β N -limit by using a smaller plasma-outer wall distance and higher triangularity in the plasma cross section (top/bottom average δ~0.9), enabled by the recent “shape & volume rise” (SVR) modification to the DIII-D divertor. The higher triangularity also contributed to achieving higher ƒ Gr by enabling higher pedestal density. At high density, the pedestal is ballooning limited and exhibits small and frequent ELMs, while the divertor is near detachment even without any impurity seeding. High plasma performance was attained and sustained reproducibly, with the eventual terminations brought about by an MHD mode destabilized as the current profile slowly continued to evolve. A path to stationary fully noninductive operation might include ECH injection to reduce both core impurity accumulation and the electron collisionality, thus increasing the bootstrap current. These experiments provide the first experimental demonstration of the ƒ Gr , H 98y2 , and β T values required simultaneously for ITER Q = 10 at I P < 10 MA, pointing to practical ways to improve the energy confinement in a fusion reactor.

Modeling stimulated Brillouin backscatter from the inner laser cones during indirect-drive inertial confinement fusion experiments at the National Ignition Facility

A. J. Kemp, T. Chapman, L. Divol, D. P. Higginson, E. Kur, N. Lemos, S. MacLaren, P. Michel, D. J. Strozzi, G. B. Zimmerman

We report progress modeling stimulated Brillouin scatter (SBS) at the National Ignition Facility (NIF). For indirect-drive, ignition-relevant hohlraum experiments, backward SBS light is a long-standing concern due to its potential for damaging laser optics as well as affecting the symmetry of the x-ray field that drives capsule implosions. To model SBS, we use maps of underdense plasma conditions, i.e., temperatures, densities, materials, charge states, and flows, from hydrodynamics simulations of the hohlraum to run backscatter simulations with the code pF3D [Berger et al., Phys. Plasmas 26, 012709 (2019)]; the latter calculates propagation of incident- and backscattered light of a NIF quad in the paraxial approximation. For the inner cone quads, in designs that utilize significant wavelength detuning, i.e., those that use cross-beam energy transfer to control implosion symmetry, this approach typically has overestimated both the peak power and the duration of SBS measured in experiments. In this work, we discuss how multi-species hydrodynamics simulations can lead to better agreement with experiments through changes to the simulated plasma conditions and the resulting SBS growth rates. Specifically, we discuss SBS reflectivity in the 23° and 30° inner cone quads, compare simulated spectra to FABS measured ones, and discuss how the time history of the backscattered light is related to the absorption/refraction of the incoming light off density features in the hohlraum plasma.

X-ray diagnostics, analysis, verification, and exploration (xDAVE) code for the prediction and interpretation of x-ray Thomson scattering experiments

Hannah M. Bellenbaum, Dave A. Chapman, Maximilian P. Böhme, Thomas Gawne, Sebastian Schwalbe, Willow M. Martin, Michael Bussmann, Dirk O. Gericke, Uwe Hernandez Acosta, Jan Vorberger, et al.

X-ray Thomson scattering (XRTS) is a common diagnostic used in the warm dense matter (WDM) regime to estimate plasma parameters like density, temperature, and charge state. Experimental analysis typically relies on a forward model to obtain estimates for these parameters, as the measured spectrum is a convolution of the dynamic structure factor (DSF) and the source-instrument function. The Chihara decomposition, where the spectrum is separated into contributions from bound and free electrons, is commonly used to estimate DSFs in the WDM regime, as it allows for the fast calculation of DSFs and therefore can easily be applied in a large-scale parameter optimization. Due to the limited availability of XRTS codes, we present “x-ray diagnostics, analysis, verification, and exploration”, a code designed to quickly estimate DSFs using the Chihara decomposition and analyze experimental spectra. The code is validated by re-analyzing an experiment with isochorically heated beryllium at the OMEGA Laser Facility. In addition, we demonstrate the applicability of the code to plan experiments and predict scattering spectra through the coupling to a ray-tracing code. Finally, the importance of accounting for the energy-dependence of spectrometer instrument functions is demonstrated by comparing ray-tracing simulations to the standard convolution for strongly compressed beryllium experiments at the National Ignition Facility, similar to previously published results.

Analytic formula for the interface temperature when a Marshak wave propagates from one medium to the next

Mordecai D. Rosen

There is current interest in designing and experimentally testing a thin “window” in the gold wall of a laser-heated hohlraum that has produced an igniting capsule. This window can ultimately shine on an external physics package to extend the field of high energy density, taking advantage of the enhanced temperature of the hohlraum following ignition. The window must still have some amount of gold to ensure good drive symmetry for the implosion of the ignition capsule. This thin gold must be backed by a lower-Z material for structural integrity. Simulations of such a two-medium structure show the usual non-linear, x-radiation-driven, conduction-heating Marshak wave (MW) propagating through the thin gold. They also show the MW propagating further through the low-Z backing. The radiation-hydrodynamic simulations show a particular transition temperature at the boundary of the two media. We present here an analytic theory for this value, as a function of the low-Z material, which matches the simulations well. This work may also be of interest to the general problem of transport transitions between two media.

The role of momentum transfer in the detachment front response to power transients for reactor scale tokamaks

Matthew Khan, Mike Kryjak, Istvan Cziegler, Benjamin Dudson, Stuart Scott Henderson, David Moulton, Ryoko Tatsumi Osawa, Christopher Paul P Ridgers

We have investigated the response of detached plasmas to multi-ms timescale power transients using 1D simulations of the scrape-off layer on a scale and parameter range relevant for reactor-scale tokamaks, such as the Spherical Tokamak for Energy Production (STEP). Simulations were performed using the fluid code Hermes-3, employing the solver CVODE for robust time integration. Including or disregarding the momentum received by the neutral gas via charge exchange reactions in simulations is found to dramatically affect the detachment front's response. In the analysis, motion of the ionisation front of a detached plasma is determined by two distinct mechanisms; a fast initial pressure-dominated response as the neutral cloud is rapidly compressed, and a slow burn-through dominated one as the cloud is ionised. The former is only observed if the momentum transfer to neutrals is kept. With transfer of momentum to the neutrals disabled, the plasma only interacts with the neutrals through ionisation and recombination. Thus, the front speed during the transient is determined by the rate of burn-through of the neutrals only. Simulations are compared to a recent semi-empirical reattachment model [S.S. Henderson et al. 2024 Nucl. Fusion 64 066006], where the best qualitative agreement is seen for cases without neutral momentum and for power rises of $\sim2\times$ their baseline power. The difference in these agreements are encompassed by the range of potential fits provided by model parameter choice.

Simulations of SMBI on High-Field and Low-Field Sides of the HL-2A Tokamak

GuanFeng Wu, Jingchun Li, Xueqiao Xu, kai xuan fan, Guoliang Xiao, Yiren Zhu

Enhancing plasma injection efficiency and penetration depth is crucial for achieving high-performance steady-state fusion confinement in ITER, the next generation magnetic confinement fusion device. Based on the trans-neut module within the BOUT++ boundary plasma turbulence program framework, this study conducts two-dimensional simulations and comparative analyses of supersonic molecular beam injection (SMBI) on the high-field side (HFS) and the low-field side (LFS), using the actual divertor geometry of the HL-2A tokamak. The physical model encompasses plasma density, heat, and momentum transport equations, as well as neutral particle density and momentum transport equations. The results show that HFS SMBI produces deeper neutral penetration in normalized poloidal-flux space than LFS SMBI. The ion density inside the LCFS in the HFS case reaches about 1.5-2 times that in the LFS case, accompanied by a stronger reduction of the edge plasma temperature. The deeper HFS penetration is associated with the smaller local field-line slope on the HFS path, which causes a finite-width SMBI source to be mapped differently along the magnetic field compared with the LFS case. These results indicate that HFS SMBI can enhance both the penetration depth and the fueling efficiency in the present HL-2A geometry.

Effect of outer divertor leg detachment on the high field side scrape-off layer in DIII-D and ASDEX Upgrade

R. Gerru, D. Hachmeister, M.G. Burke, L. Horvath, T.M. Wilks, A. Bortolon, J.W. Hughes, Q. Pratt, F. Scotti, C. Tsui, et al.

In this work, evidence is presented that detachment of the outer divertor leg leads to a reduction of electron density and neutral pressure in the high-field side (HFS) scrape-off layer (SOL) of the ASDEX Upgrade (AUG) and DIII-D tokamaks with ion B × ∇ B drift directed toward the X-point (favorable configuration). These results are observed across multiple diagnostics and without the use of impurity seeding to reach detachment. In AUG, outer divertor leg detachment correlates with a decrease in electron density near the separatrix at the inner midplane, measured with HFS reflectometry. A concurrent reduction in inner divertor density and neutral pressure at the inner target is observed using divertor Thomson scattering and neutral pressure gauges. These effects are present in both L- and H-mode plasmas. In DIII-D, a similar reduction is detected through analysis of line-integrated hydrogenic emission measured by multiple diagnostics in the HFS SOL close to the separatrix. The consistent trends in both devices indicate that high electron density and strong hydrogenic emissivity in the HFS SOL are common features of H-mode plasmas in the favorable configuration, independent of wall material and divertor geometry. In L-mode plasmas, the reduction in electron density and neutral pressure is not observed in DIII-D, possibly due to differences in wall material. These results emphasize the importance of the divertor state in determining the two-dimensional neutral distribution and edge fueling.

Observation of Self-Similarity in the Magnetic Fields Generated by the Ablative Nonlinear Rayleigh-Taylor Instability

L. Gao, P. M. Nilson, I. V. Igumenschev, G. Fiksel, R. Yan, J. R. Davies, D. Martinez, V. Smalyuk, M. G. Haines, E. G. Blackman, et al.

Magnetic fields generated by the nonlinear Rayleigh-Taylor growth of laser-seeded three-dimensional broadband perturbations were measured in laser-accelerated planar targets using ultrafast proton radiography. The experimental data show self-similar behavior in the growing cellular magnetic field structures. These observations are consistent with a bubble competition and merger model that predicts the time evolution of the number and size of the bubbles, linking the cellular magnetic field structures with the Rayleigh-Taylor bubble and spike growth.

Hot Spot Evolution Measured by High-Resolution X-Ray Spectroscopy at the National Ignition Facility

Lan Gao, B. F. Kraus, K. W. Hill, M. B. Schneider, A. Christopherson, B. Bachmann, M. Bitter, P. Efthimion, N. Pablant, R. Betti, et al.

Evolution of the hot spot plasma conditions was measured using high-resolution x-ray spectroscopy at the National Ignition Facility (NIF). The capsules were filled with DD gas with trace levels of Kr, and had either a high-density-carbon (HDC) ablator or a tungsten (W)-doped HDC ablator. Time-resolved measurement of the Kr He$β$ spectra, absolutely calibrated by a simultaneous time-integrated measurement, allows inference of the electron density and temperature through observing Stark broadening and the relative intensities of dielectronic satellites. By matching the calculated hot spot emission using a collisional-radiative code to experimental observations, the hot spot size and areal density are determined. These advanced spectroscopy techniques further reveal the effect of W dopant in the ablator on the hot spot parameters for their improved implosion performance.

Absolute Calibration of a Time-Resolved High Resolution X-ray Spectrometer for the National Ignition Facility (invited)

Lan Gao, B. F. Kraus, K. W. Hill, M. Bitter, P. Efthimion, M. B. Schneider, A. G. MacPhee, D. B. Thorn, J. Kilkenny, J. Ayers, et al.

A high resolution, Diagnostic Instrument Manipulator (DIM)-based x-ray Bragg crystal spectrometer has been calibrated for and deployed at the National Ignition Facility (NIF) to diagnose plasma conditions in ignition capsules near stagnation times. The spectrometer has two conical crystals in the Hall geometry focusing rays from the Kr He$α$, Ly$α$, and He$β$ complexes onto a streak camera, with the physics objectives of measuring time-resolved electron density and temperature through observing Stark broadening and the relative intensities of dielectronic satellites. A third von Hámos crystal that time-integrates the Kr He$α$, He$β$ and intervening energy range provides in-situ calibration for the streak camera signals. The spectrometer has been absolutely calibrated using a microfocus x-ray source, an array of CCD and single-photon-counting detectors, and multiple K- and L-absorption edge filters at the Princeton Plasma Physics Laboratory (PPPL) x-ray laboratory. Measurements of the integrated reflectivity, energy range, and energy resolution for each crystal are discussed. These calibration data provide absolute x-ray signal levels for NIF measurements, enabling precise filter selection and comparisons to simulations.

Study of shock wave dynamics in strong magnetic fields

Z. J. Minaker, V. V. Ivanov, R. C. Mancini, L. S. Leal, A. V. Maximov

Shock waves in magnetized plasmas are central in many astrophysical systems, yet laboratory measurements have remained limited by 10–20 T. This work studies the dynamics of shock waves without and with magnetic fields as strong as 100–150 T generated by the 1 MA pulsed power machine. A shock wave in hydrogen was produced by a 0.8 ns laser pulse with intensity of 5 × 1014 W/cm2. Laser imaging diagnostics including multi-frame and two-color interferometry, shadowgraphy, and schlieren imaging providing sub-nanosecond temporal and 10 μm spatial resolution of shock wave evolution. The density of hydrogen gas was in the range of (2–10) × 1018 cm−3. Results show that without an applied magnetic field, shock waves expand symmetrically with velocities up to 1600 km/s. For shock waves in a magnetic field there is suppressed radial expansion perpendicular to the B-field and enhanced azimuthal expansion parallel to the B-field. In hydrogen, a laser-initiated ionization wave formed ahead of the shock waves. Plasma instability was observed on the front of the shock wave in the magnetic field. MHD simulations confirm that magnetic fields modify the shock wave conditions, with velocity anisotropy scaling with field strength, establishing a laboratory platform to study magnetically mediated and collisionless shock wave physics.