Recent advances in inertial confinement fusion (ICF) at the National Ignition Facility (NIF), including ignition and energy gain, are enabled by a close coupling between experiments and high-fidelity simulations. Neither simulations nor experiments can fully constrain the behavior of ICF implosions on their own, meaning pre- and postshot simulation studies must incorporate experimental data to be reliable. Linking past data with simulations to make predictions for upcoming designs and quantifying the uncertainty in those predictions has been an ongoing challenge in ICF research. We have developed a data-driven approach to prediction and uncertainty quantification that combines large ensembles of simulations with Bayesian inference and deep learning. The approach builds a predictive model for the statistical distribution of key performance parameters, which is jointly informed by past experiments and physics simulations. The prediction distribution captures the impact of experimental uncertainty, expert priors, design changes, and shot-to-shot variations. We have used this new capability to predict a increase in ignition probability between Hybrid-E shots driven with 2.05 MJ compared to 1.9 MJ, and validated our predictions against subsequent experiments. We describe our new Bayesian postshot and prediction capabilities, discuss their application to NIF ignition and validate the results, and finally investigate the impact of data sparsity on our prediction results.
I. INTRODUCTION
Throughout the 50-year history of inertial confinement fusion (ICF) experiments1–3 researchers have faced a significant challenge: how can data from complex and expensive experimental facilities inform predictions of future experiments? What are the uncertainties in those predictions? Making progress on these questions has been challenging since ICF datasets are sparse and difficult to diagnose, making pure data-driven approaches infeasible, while physics-based simulations often fail to predict experimental outcomes based on pre-shot information alone. We present a hybrid statistical approach that aims to combine information from high-yield experiments at the National Ignition Facility (NIF),4–6 large-scale radiation hydrodynamics simulation studies,7 and statistical learning into a probabilistic prediction for the outcome of proposed designs.
Inertial confinement fusion research relies on large-scale radiation hydrodynamics simulations8–14 for the design,15–18 optimization,19,20 and interpretation21–24 of experiments. These simulations integrate a full suite of physics models25,26 relevant to the evolution of ICF and high energy density physics (HEDP) experiments, allowing a complete prediction of the assembled plasma and diagnostic signals from the initial target design and drive parameters. Nevertheless, state-of-the art simulations are not truly predictive of individual ICF experiments: simulations will typically overpredict performance even with all the information available before a shot is fired. As a result, pre-shot predictions usually rely on expert-driven, ad hoc modifications that degrade the performance to reflect the expectations of the experimental team. This reliance on expert priors introduces biases that are difficult to quantify and remove from the design process, and is inherently limited to a handful of performance degradation mechanisms. Scaling to high-dimensional design studies, and integrating existing experimental data in a formal manner, remain significant challenges in ICF research.
We take a Bayesian model calibration approach that provides a principled framework to combine data from past experiments into a single statistical model that predicts the outcome of future shots. The approach infers probability distributions over unknown simulation inputs for each shot in the series and links them together into a common, jointly informed “variability” model that describes the statistical variations between shots at the NIF. By introducing simulations into the analysis, we explicitly include known physics in the analysis, allowing us to make efficient use of the available experimental data and to ensure that predictions are always physically correct and consistent. Taking a Bayesian approach ensures that all sources of uncertainty can be accounted for and propagated to quantities of interest in order to place rigorous uncertainty bounds on predictions.
Data-driven approaches like the one reported here are a potentially powerful tool in ICF research, high energy density, and plasma physics and are being investigated by several groups.27,28 Statistical and machine learning29 models, trained only on experimental data, have been used to predict the performance and sensitivity of ICF implosions30–34 and to propose new experiments.35 Bayesian methods have been used with relatively simple physics models to infer parameters in implosion experiments,36–41 laser-driven opacity42 and equation-of-state43,44 experiments, and others. For an introduction and review of current work, see a recent review by Knapp and Lewis.45 Transfer learning46 has been used to combine simulations and experiments into a single model which reliably describes data47–49 but compromises the interpretation of the model inputs. In contrast to those works, our ICF models are designed to allow the use of very sparse NIF data and the best fidelity simulations possible while retaining the interpretability of the physics at work.24,50,51
In this work, we describe our statistical approach and its application to the prediction of fusion ignition in recent NIF “Hybrid-E” experiments. We have used the model to perform a joint Bayesian analysis of 9 sub-ignition NIF shots, driven with 1.9 MJ of laser energy, and then to predict the onset of ignition in larger 2.05 MJ drive shots. We performed this analysis prior to any 2.05 MJ experiments, and can therefore use subsequent ignition shots to validate our predictions. We will describe our model calibration approach and the design decisions that motivated it, discuss the Bayesian postshot analysis of performance and variability in 1.9 MJ shots, and then present our predictive model for 2.05 MJ shots. Finally, we will discuss the uncertainties in our analysis and their impact on the predicted probability of ignition.
II. A BAYESIAN APPROACH TO EXTRACTING INFORMATION FROM NIF EXPERIMENTS
The Bayesian approach provides a framework to extract information from NIF shots, in the form of posterior distributions over uncertain parameters, and use it as prior information to make predictions for future shots. This process formalizes the ad hoc institutional knowledge currently used in pre-shot design and simulation work. The fundamental building block of the approach is the Bayesian inference of posterior distributions based on experimental data, which can be approached with standard methods but nevertheless faces practical challenges due to the nature of ICF data.24
Constraining variations in performance across NIF shots is challenging due to the highly nonlinear onset of fusion reactions and their impact on the overall dynamics of the implosion, the large number of design and physics parameters, and the expense of experimental shots. As a result of these challenges typical NIF datasets are too sparse to constrain performance variability on their own (Fig. 1). We have solved this problem by framing our Bayesian inferences as a model calibration, allowing us to introduce a second constraint in the form of high-fidelity simulations. The simulations provide significant information on the nonlinearities of the response, allowing the experimental data to be used more efficiently to constrain only the most uncertain aspects of the implosion physics rather than known physical scaling results and correlations.
NIF data are too sparse to fully constrain the nonlinear onset of high performance. Theory and simulations predict a complex relationship between yield and downscatter ratio (blue), while NIF experiments (black) only resolve a linear correlation. As a result, a data only model (orange) gives incorrect behavior on extrapolation, which motivates our hybrid model calibration-based approach.
NIF data are too sparse to fully constrain the nonlinear onset of high performance. Theory and simulations predict a complex relationship between yield and downscatter ratio (blue), while NIF experiments (black) only resolve a linear correlation. As a result, a data only model (orange) gives incorrect behavior on extrapolation, which motivates our hybrid model calibration-based approach.
We select input calibration Eq. (4) for three main reasons. First, the calibration parameters have a clear meaning (in the context of the simulation code), meaning that the calibration provides an interpretable explanation for the observed performance. Second, predictions from the calibrated model are always valid outputs of the simulation and therefore they will automatically obey known physical laws and correlations between observations. Third, the interpolation and extrapolation behavior of the calibrated model (i.e., as a function of x) is determined by the known physics of the problem, potentially modified by , meaning that nonlinear behavior emerges in predictions even when experimental data are not sufficient to show it. The joint advantages of interpretability, consistency, and physics-based extrapolation are not available from the output calibration, making input calibration the clear choice for our purpose.
III. BAYESIAN PREDICTIVE MODELING OF ICF EXPERIMENTS
Our predictive model uses a series of Bayesian postshot procedures to independently calibrate capsule simulation inputs for each shot in an NIF experimental campaign. Each shot has its own inferred distribution of simulation inputs, which varies across the experimental campaign; we use these variations to constrain a second statistical model for the shot-to-shot variability for a given design. The combined variability and postshot models form a hierarchical model, which we represent as a Bayesian network in Fig. 2. The inferred shot-to-shot variations, which we call our variability model, can then be used to aid in the prediction of new shots by exposing simulations of the new design to the same (or suitably modified, see Sec. IV C) variations. In this manner, our first stage of analysis extracts information from existing shots through model calibration, which is then used to make informed predictions or extrapolations. The stages of analysis are described in more detail in Secs. III A–III C.
Our data-driven model uses a shared variability model to link latent input parameter distributions across existing and future NIF shots. The parameters , , and are learned by Bayesian inference by matching the simulated outputs to observed data. The transformations and represent simulations of the existing and new capsule design, respectively, and captures prior information.
Our data-driven model uses a shared variability model to link latent input parameter distributions across existing and future NIF shots. The parameters , , and are learned by Bayesian inference by matching the simulated outputs to observed data. The transformations and represent simulations of the existing and new capsule design, respectively, and captures prior information.
A. The Bayesian postshot
The posterior distribution Eq. (6) is the distribution of simulation inputs, which are consistent with the observed data. The analysis proceeds by stochastically sampling values of from the posterior using Markov chain Monte Carlo (MCMC).52,59 In principal, the MCMC algorithm can sample from arbitrary posterior distributions and can therefore support complex correlations between elements of , as well as multiple peaks in probability, which arise when multiple values of can produce a match to the data. These features of the MCMC approach are significant advantages when dealing with ICF data since the simulated outputs are highly correlated by the physics at work and the calibration problem tends to be underdetermined by the available diagnostic quantities.
The final practical challenge in applying our model calibration approach is the computational expense of the MCMC algorithm. MCMC generates samples iteratively by proposing values of and accepting or rejecting according to Eq. (6), which in our case requires a radiation-hydrodynamics simulation. High-probability regions are repeatedly sampled by simulations with almost the same , while low-probability regions require very long MCMC runs to reach. As a result, converged MCMC samples require millions of simulation runs performed in serial, a completely infeasible task with any realistic ICF simulation.
We solve this problem by pre-running a large number of simulations in parallel on a large high performance computing (HPC) resource7,60–62 and using the resulting ensemble of input–output pairs to build a rapid interpolator or surrogate. The surrogate can then be used during MCMC inference to predict the simulation output very quickly, and by choosing a differentiable surrogate model, we can further improve performance using gradient accelerated MCMC sampling techniques.63 Together, these two improvements allow a Bayesian calibration, which would be impossible without a surrogate, to be completed in a matter of minutes on a desktop computer. Our chosen method of surrogate modeling is through deep neural network (DNN) interpolation, which has several advantages over more traditional methods. We will describe this approach in detail in Sec. IV.
B. Modeling shot-to-shot variability
Sampling from the postshot and variability posteriors, Eqs. (6) and (8), is performed simultaneously by implementing the full Bayesian network shown in Fig. 2 using the Bayesian modeling library pymc.65 We obtain samples of the variability parameters and along with simulation inputs for each shot in the experimental campaign . We have found that rapid convergence is possible if short, independent Bayesian postshot inferences are performed before attempting to tune the full problem.
Finally, we will comment on the approximations introduced to our shot-to-shot variability model. Although we have introduced a normal approximation for the variability distribution in Eq. (7), the inferred shot-to-shot input variations—obtained by integrating over MCMC samples and —are not constrained to be normal. The inferred distribution can in fact be quite complex; however, it should be noted that the normal approximation does have a smoothing effect on the variability posterior. This smoothing is important in the application to ICF since it makes the problem more numerically tractable when there are very few experiments; however, the normal approximation may need to be revisited for other problems where more data are available.
C. Predicting future shots
The Bayesian postshot procedure we have described generates a data-driven model for the shot-to-shot variations seen over a series of NIF experiments. Using this model, we may make a prediction for a future shot, for example, one at a new value of the independent variables , by propagating the shot-to-shot variation distribution through new simulations. This is achieved by evaluating the new simulations at a set of samples from the variability distribution generated during MCMC inference. This evaluation would be trivial if the desired was included in the original simulation study since the previous surrogate can be used; however, this is rarely the case in practice.
To make the propagation of variability samples through new simulations tractable, we require a new surrogate that emulates simulations of the proposed design. This second surrogate may be produced in the same way as the first, that is, by using millions of CPU-hours (and months of wall time) to run thousands of new simulations. While computationally feasible, we have found this approach to be incompatible with experimental timescales; the details of a shot are often confirmed only a week or less before a prediction is required, meaning that new surrogate models must be trained in a few days. To be useful for experimental planning we must reduce the number of simulations needed to create a new surrogate by several orders of magnitude.
We have shown that new surrogates, suitable for generating predictions for new designs, can be reliably trained on only 10–100 new simulations using a deep learning technique known as transfer learning.46 Transfer learning uses the structure of a deep neural network to identify subsets of an existing model that can be updated, or retrained, using a new (but related) dataset. Applications in computer vision provide an intuitive example: a neural network can be trained to “understand” images on open-source data, extracting information that can be re-used to understand more specialized datasets like medical images. In an ICF context, these ideas have been used to update a simulation surrogate using small experimental47,49 and synthetic datasets.48 Here, we transfer learn from the previous design to the new one using a small set of new simulations.
While the transfer-learned surrogate typically has much lower accuracy than the source surrogate it is still sufficient for our prediction tasks. In the postshot analysis, where the surrogate is used to match observational data, we require a surrogate accuracy and precision comparable to the experimental uncertainties. For prediction, the surrogate is used to propagate the shot-to-shot variability model, which is very broad and quite uncertain (see Sec. IV C for a discussion), and so the surrogate accuracy requirements are correspondingly relaxed.
IV. APPLICATION TO NIF HYBRID-E EXPERIMENTS
Recent high-yield experiments at the NIF have demonstrated significant heating by fusion alphas and fusion energy gain using the so-called Hybrid-E implosion design. Hybrid-E implosions combine advances in target design18 and capsule manufacturing quality with tuned laser drives to provide robust megajoule energy yields, often exceeding the laser energy required to drive the implosion.66
Our data-driven predictive model for high-yield, 2.05 MJ drive Hybrid-E shots is informed by a series of sub-ignition experiments performed with 1.9 MJ of laser energy. The series consists of shot N210808, the first experiment to demonstrate a Lawson criteria greater than one, and a subsequent set of near repeats that aimed to test performance variability and sensitivity to key degradation mechanisms. These experiments, which we referred to as the “210808+R” campaign, are the largest run of experiments of similar design and provide a high-quality dataset for our data-driven variability model. We give a short summary of the shots in the 210808+R set in Table I
NIF Hybrid-E experiment N210808 and subsequent near-repeat shots provide a high-quality dataset to constrain performance variability in ignition-class ICF implosions. The shots show significant performance variability driven by a combination of factors, both known and unknown pre-shot.
Shot ID . | Primary neutron yield ( ) . | Energy yield (MJ) . |
---|---|---|
![]() | 1.35 | |
![]() | 0.420 | |
![]() | 0.689 | |
![]() | 0.463 | |
![]() | 0.241 | |
![]() | 0.159 | |
![]() | 0.114 | |
![]() | 0.131 | |
![]() | 0.266 |
Shot ID . | Primary neutron yield ( ) . | Energy yield (MJ) . |
---|---|---|
![]() | 1.35 | |
![]() | 0.420 | |
![]() | 0.689 | |
![]() | 0.463 | |
![]() | 0.241 | |
![]() | 0.159 | |
![]() | 0.114 | |
![]() | 0.131 | |
![]() | 0.266 |
We aim to calibrate a single capsule-only simulation model, performed with the radiation-hydrodynamics code HYDRA,8 across the 210808+R dataset. This requires that the calibrated inputs span both the known design changes and the unknown drivers of variability. To define this parameter set we start with a nominal N210808 HYDRA simulation, derived from NIF drive tuning data and integrated hohlraum simulations, which are then modified with a set of parameters that define . We have developed a set of nine input parameters that capture the expected variations and allow high-quality fits to all shots in the 210808+R series (discussed at length in Sec. III A). These parameters include modifications to the timing, energy, and peak power of the x-ray drive on the capsule (three parameters), a multiplier on the nominal fraction of x-ray energy in the gold emission M-band, and time-dependent drive asymmetries (three parameters), and compression degradation through a fall-line mix model and preheat of the DT fuel layer (two parameters). They define a 9-dimensional simulation input space which we sampled using an ensemble of 30 000 HYDRA simulations performed using approximately 20 × 106 cpu-hours on the Trinity supercomputer at Los Alamos National Laboratory.67
The HYDRA simulation ensemble consists of a set of simulation input–output pairs in which the input values are selected through a combination of space-filling latin hypercube sampling and an adaptive scheme that aims to optimally sample the posterior distributions for shot N210808.68, We use the resulting dataset to construct a fast emulator of the full simulated response . We have found that deep neural networks (DNNs) provide a fast and reliable approach to emulate our simulations with the added benefit of differentiability and the opportunity to leverage new approaches like transfer learning.46 In this work, we use the DJINN architecture,69 which allows a high-quality surrogate model to be found by tuning a single hyperparameter. Our final DNN has six hidden layers with widths [11, 15, 23, 39, 71, 135], ReLU activations, and no dropout regularization, and predicts ten scalar output quantities. The average performance on both train and test datasets is excellent for all output features (Table II), and we have further confirmed that the prediction uncertainty (the standard deviation in the differences between predictions and ground truth) is comparable to or smaller than experimental uncertainties. This second check ensures that surrogate uncertainties do not overwhelm the experimental data during the Bayesian inference.
Our DJINN70 DNN surrogate reliably reproduces a range of simulated outputs on both the training data and held-out test data.
Output feature . | Training . | Testing . |
---|---|---|
Log-yield | 0.99 | 0.99 |
Equatorial | 0.98 | 0.95 |
Downscatter ratio | 0.97 | 0.96 |
Nuclear bang time | 0.99 | 0.99 |
Nuclear burn width | 0.99 | 0.99 |
Primary neutron image | 0.97 | 0.96 |
Primary neutron image | 0.99 | 0.99 |
Neutron velocity | 0.98 | 0.98 |
Implosion velocity | 0.99 | 0.99 |
Fuel adiabat | 0.98 | 0.98 |
Output feature . | Training . | Testing . |
---|---|---|
Log-yield | 0.99 | 0.99 |
Equatorial | 0.98 | 0.95 |
Downscatter ratio | 0.97 | 0.96 |
Nuclear bang time | 0.99 | 0.99 |
Nuclear burn width | 0.99 | 0.99 |
Primary neutron image | 0.97 | 0.96 |
Primary neutron image | 0.99 | 0.99 |
Neutron velocity | 0.98 | 0.98 |
Implosion velocity | 0.99 | 0.99 |
Fuel adiabat | 0.98 | 0.98 |
With a complete and verified set of MCMC samples, we can validate the physics in our Bayesian postshot analyses and variability models and also make extrapolated predictions for ignition-scale shots. These applications are discussed in Secs. IV A–IV C.
A. Bayesian postshot for Hybrid-E shots
The Bayesian postshot attempts to match multiple experimental observables with strong physics-driven correlations between them. As a result of these complex target data, it is not guaranteed that a given simulation and choice of calibrated inputs is capable of simultaneously matching the full set of observables for a given shot. We, therefore, begin our analysis by validating that the HYDRA simulations, combined with input parameters described previously, can successfully match the experimental data. We then move on to a discussion of the quality of the inferred shot-to-shot variability distribution.
The fit to experimental data is excellent across all shots in the 210808+R dataset for all of the output features of interest. Figure 3 compares experimental and posterior-predictive distributions and shows that they overlap within error bars with high likelihood. This level of high-quality match to multiple observables is far better than can be achieved by hand-tuning a small number of parameters.
The Bayesian postshot process calibrates HYDRA simulations to match multiple highly correlated observed quantities across all of the shots in the N210808+R dataset. We plot posterior-predictive fitted quantities against their experimental values, which agree with high statistical significance. As a further validation, we include features that were not explicitly fit during MCMC sampling (marked with a ), which also show a close match to data.
The Bayesian postshot process calibrates HYDRA simulations to match multiple highly correlated observed quantities across all of the shots in the N210808+R dataset. We plot posterior-predictive fitted quantities against their experimental values, which agree with high statistical significance. As a further validation, we include features that were not explicitly fit during MCMC sampling (marked with a ), which also show a close match to data.
The present Bayesian postshot also performs well on observables that were not explicitly fit by the MCMC algorithm. Figure 3 includes hold-out features (marked with a symbol in plot titles), which serve as an after-the-fact check against the MCMC procedure finding the “right answer for the wrong reason.” In some previous analyses, this check has found fits that reproduce target data well but failed to match or have very large posterior uncertainties on kept-out features. In the present analysis, we see encouraging agreement across all of the output features considered.
B. Shot-to-shot variability
The variability model accurately reproduces the experimental shot-to-shot distributions. Figure 4(a) shows the predicted joint distribution of DSR and , compared with the N210808+R dataset. The agreement is qualitatively very good; the calibrated simulations reproduce the strong linear correlation in the data and predict a strong peak in yield close to the experimental median. We further validate the distribution through a detailed comparison of the cumulative probabilities from the model and experiment (Fig. 5), which demonstrates that our model correctly reproduces the bulk of the empirical data distribution. The main discrepancy is in the predicted long tail at low performance, which is a consequence of our assumed normal distribution over simulation inputs and the very sparse nature of the NIF data.
Our data-driven model reproduces shot-to-shot variations in the 1.9 MJ laser drive “N210808+R” dataset [panel (a)] and makes physics-based extrapolations to the proposed 2.05 MJ design [panel (b)]. The model predicts a significantly increase probability mass above the ignition criteria for the higher drive case. (a) The inferred shot-to-shot variability distribution (contours) and associated N210808+R experimental dataset (points). (b) Predicted shot-to-shot distribution for the higher drive case.
Our data-driven model reproduces shot-to-shot variations in the 1.9 MJ laser drive “N210808+R” dataset [panel (a)] and makes physics-based extrapolations to the proposed 2.05 MJ design [panel (b)]. The model predicts a significantly increase probability mass above the ignition criteria for the higher drive case. (a) The inferred shot-to-shot variability distribution (contours) and associated N210808+R experimental dataset (points). (b) Predicted shot-to-shot distribution for the higher drive case.
The calibrated shot-to-shot distribution accurately matches the data distribution, in particular, in the high-probability center of the distribution. We compare the cumulative distribution function of the energy yield from the trained model (blue) and the 210808+R data (orange). The agreement is very good with the exception of the low-probability, very low performance tail.
The calibrated shot-to-shot distribution accurately matches the data distribution, in particular, in the high-probability center of the distribution. We compare the cumulative distribution function of the energy yield from the trained model (blue) and the 210808+R data (orange). The agreement is very good with the exception of the low-probability, very low performance tail.
The shot-to-shot variability model provides information about the drivers of performance variations through the inferred covariance over simulation inputs . The diagonal elements of the covariance describe the degree to which different inputs vary from shot-to-shot, and the off diagonal elements describe physics-driven correlations. The absolute values of these entries are determined by the sensitivity of simulated outputs to each input, which complicates the interpretation of the covariance itself. Instead, we identify the most important contributions to the yield variation by “switching off” variations of specific inputs and quantifying the change in the predicted yield distribution. This is achieved by fixing the values of certain inputs at the distribution mean (for example) and conditioning the variability model on the fixed values; successively applying this approach to each set of simulation inputs allows us to compare the impact of variations over the experimentally relevant scales.
We find that the mix, which we define as the combination of fall-line mix and preheat, is the only degradation that, when removed, results in a noticeable reduction in shot-to-shot yield variability (Fig. 6). Other inputs are sufficiently correlated that removing one set in isolation is not enough to significantly change the predicted yield. Further analysis of the high-dimensional variability model is possible through dimensional reduction techniques or by linear projection; however, this detailed interpretation of the postshot variability model is beyond the scope of this work. Instead, we will proceed to discuss the use of the model to predict the outcome of future experiments.
Mix (fall-line and preheat) plays an important role in determining performance variability, while other degradation sources have a smaller impact. We plot the distribution of energy yield for Hybrid-E shots, driven with 1.9 MJ of laser energy, from our statistical model. Different yield distributions (colors) represent predictions with subsets of input variations “switched off”—set to the distribution mean—which measures their impact on overall variability.
Mix (fall-line and preheat) plays an important role in determining performance variability, while other degradation sources have a smaller impact. We plot the distribution of energy yield for Hybrid-E shots, driven with 1.9 MJ of laser energy, from our statistical model. Different yield distributions (colors) represent predictions with subsets of input variations “switched off”—set to the distribution mean—which measures their impact on overall variability.
C. Predicting performance of ignition-scale experiments
Our approach predicts the outcome of future shots by propagating samples from the trained shot-to-shot variability model through simulations of the new design to predict the distributions of observables before experiments are performed. We have used this approach to extrapolate from the 1.9 MJ drive Hybrid-E experiments used to train the model to new 2.05 MJ drive experiments. In this section, we will present the predicted performance based on the 210808+R dataset, then use experiments performed with a 2.05 MJ drive to validate the model. Finally, we will quantify the uncertainty in our predictions due to the sparsity of both experimental and simulation datasets.
The first step of our predictive workflow is to generate a DNN surrogate of HYDRA simulations for the new shot design. To make this computationally tractable on a timescale useful to experiments we use transfer learning to re-train the initial DNN to the new task. In transfer learning, an existing DNN is updated for a new (but related) task by re-training a subset of the network layers using a small amount of new data. In this work, we have found that effective transfer learning can be achieved with only new simulations, resulting in a new surrogate in just a few hours of HPC time. With this very small number of simulations the transfer-learned 2.05 MJ surrogate model has lower performance than the original one (test ); however, we have confirmed that prediction uncertainties are small compared to the variability model we aim to propagate. With this capability we can respond quickly to the demands of the experimental facility, potentially producing new predictions as critical decisions are made in the days prior to a high-value experiment.
The shot-to-shot variability distribution, propagated through our transfer-learned surrogate model for the 2.05 MJ design, predicts a significantly higher performance than with a 1.9 MJ drive and that 2.05 MJ drive experiments begin to access the robust compressive ignition regime [Fig. 4(b)]. The predicted distribution of yield and DSR shifts upward along the ignition cliff (the negatively correlated region around 100 kJ to 1 MJ of yield) and begins to turn the corner into the positively correlated robust ignition regime.
We can quantify the increase in performance by integrating the predicted distributions into a probability of ignition for each design. For the analysis shown in Figs. 4(a) and 4(b), the ignition probability increases from to (we discuss the uncertainties in these probabilities later in this section). We present these metrics, along with several others, in Table III.
Performance metrics from the data-driven models developed in this paper compare well with experimental data. We compare percentiles from the postshot variability model for the 1.9 MJ drive NIF data and the pre-shot predictive model for the 2.05 MJ drive data. Although the experimental percentiles are quite uncertain due to data sparsity, the agreement is good.
. | 1.9 MJ Model . | 1.9 MJ Experiment . | 2.05 MJ Model . | 2.05 MJ Experiment . |
---|---|---|---|---|
Median yield | 270 KJ | 240 KJ | 2 MJ | 1.9 MJ |
interval | (140–500) KJ | (140–420) KJ | (0.6–4.9) MJ | (1.5–2.8) MJ |
. | 1.9 MJ Model . | 1.9 MJ Experiment . | 2.05 MJ Model . | 2.05 MJ Experiment . |
---|---|---|---|---|
Median yield | 270 KJ | 240 KJ | 2 MJ | 1.9 MJ |
interval | (140–500) KJ | (140–420) KJ | (0.6–4.9) MJ | (1.5–2.8) MJ |
We may validate our predictions [Fig. 4(b)] by comparing them with subsequent NIF experiments performed with a 2.05 MJ drive (recall that no data with this drive were used to produce the prediction). Comparison with five NIF shots performed with the 2.05 MJ drive to date, including the first fusion experiments to ignite, shows excellent agreement with the prediction with three shots igniting and a median yield in excellent agreement with our predictions (Table III). While the comparison between 50% intervals from the prediction and model is less good, the estimate of percentiles from only five NIF shots is very uncertain, and so more NIF data will be required to properly validate the tails of our predicted distribution.
Finally, we can quantify the uncertainty in our predictions that arises due to the small datasets available. We investigate the impact of two uncertainty sources: first, surrogate prediction uncertainty arising due to the limited number of simulations available and, second, the sparsity of NIF data used to train the variability model. In both cases, we take an ensemble approach by training multiple models on different data realizations and treating the distribution of models as a measure of uncertainty. We have produced 50 different predictions with different DNN surrogates and experimental datasets (described below) and use the range of predictions to measure the uncertainty in our performance model.
We measure the impact of surrogate uncertainty by training 50 DNN surrogates using different test/train data splits, initial weights and biases, and seeds for random number generators. Since the DNN training process is typically non-convex, with many local minima, this procedure produces 50 distinct DNN surrogates with comparable performance on test and train data but different functional forms. It should be noted that while this procedure is commonly used for this purpose, the distribution of predictions is not a true uncertainty (for example, the standard deviation may not be properly calibrated); however, the procedure still serves to measure the sensitivity of our predictions to this uncertainty source.
The impact of the sparsity of experimental data is investigated by re-sampling from the shot-to-shot variability model described previously to produce 50 synthetic datasets. Effectively, we use the result of a single inference as a reference solution and measure the sensitivity of that solution to the sparsity of a typical NIF dataset. Similar to the surrogate sensitivity, this procedure is not a true measurement of uncertainty since the ground-truth experimental distribution is not known; however, we can still measure sensitivity and compare it with the shot-to-shot and surrogate distributions.
Data sparsity introduces significant uncertainty in the details of our predictions; however, we have found that the fluctuations in predictions for 1.9 and 2.05 MJ shots are correlated (Fig. 7). As a result, while the precise ignition probability for a given design has an absolute uncertainty of 10%–20%, the relative performance gain between designs is quite robust. This is an encouraging result since it suggests that even sparse datasets can be meaningfully used to design new high performance and robust implosions.
The predicted ignition probability is sensitive to changes in training data, but the predicted increase in probability between designs is robust. We plot histograms of the predicted ignition probability (%) for the 1.9 MJ (horizontal axis) and 2.05 MJ (vertical axis) designs from a series of 50 independent prediction models trained on different simulation and experimental datasets. (a) Impact of simulation data sparsity and surrogate model uncertainty. (b) Impact of experimental data sparsity.
The predicted ignition probability is sensitive to changes in training data, but the predicted increase in probability between designs is robust. We plot histograms of the predicted ignition probability (%) for the 1.9 MJ (horizontal axis) and 2.05 MJ (vertical axis) designs from a series of 50 independent prediction models trained on different simulation and experimental datasets. (a) Impact of simulation data sparsity and surrogate model uncertainty. (b) Impact of experimental data sparsity.
V. DISCUSSION AND CONCLUSIONS
We have described a novel data-driven predictive model for the performance and variability of inertial confinement fusion experiments. The model combines past experimental data with large-scale ensemble simulation studies to produce a calibrated distribution for shot-to-shot variations posed in terms of simulation inputs. Our Bayesian model calibration approach affords several advantages over other methods by jointly informing the model with experiments and simulations and allowing for a complete quantification of uncertainty in predictions. Our model is interpretable, physically consistent, makes physics-based extrapolations to new designs, and captures uncertainty from multiple sources.
Each stage of the Bayesian analysis and prediction approach has been validated through detailed comparison with the available experimental data. When applied to nine recent “Hybrid-E” ICF experiments at the National Ignition Facility with 1.9 MJ of laser drive, the model accurately matches multiple observed quantities across all shots as well as the full performance distribution across the whole campaign. When used to extrapolate and predict new experiments with a 2.05 MJ drive, the calibrated model predicts a significant ( ) increase in ignition probability, which has been confirmed by subsequent experiments. While the higher drive data are insufficient to validate the predicted distribution in detail, the central distribution moments are in excellent agreement with NIF data.
The sparsity of simulation and experimental data, which arises due to the expense of collecting new samples, has an important impact on the details of the predicted distributions for individual designs but not on the expected increase in ignition probability between designs. We have quantified the impact of data sparsity through an ensemble technique where multiple models are trained using different realizations of the data, and the variations in ignition probability are used as a proxy for sensitivity. We find that while individual ignition probabilities vary across ensemble members, predictions for different designs are correlated such that comparisons between designs are consistent. This is an encouraging result for future data-driven design studies using our techniques.
Deep learning plays an essential role in making the present approach computationally tractable and in enabling predictions on a timescale relevant to experiments. Deep neural network surrogate models allow expensive radiation-hydrodynamic simulations to be decoupled from Bayesian inference steps and to be performed in a massively parallel manner, enabling our analysis to include state-of-the-art physics. The use of transfer learning to update existing surrogate models to new implosion designs allows us to make new predictions in a matter of days in order to respond to on-the-ground experimental changes as they occur.
The methods and results presented here are a prototype for several new types of data-driven design and analysis studies for ICF. The Bayesian postshot, which forms the first step of the prediction model, can be used to calibrate and place uncertainties on unobserved parameters like x-ray drives, implosion velocities, etc. The calibrated shot-to-shot variability model, combined with transfer learning or other multi-fidelity approaches, can be used to evaluate performance variability during automated design optimization studies. These “design-for-robustness” studies would enable new implosion designs that can hit yield targets with a low probability of failure with many applications in ICF and high energy density physics. Finally, our use of deep learning in the framework provides an exciting potential for the inclusion of non-scalar data like capsule metrology, x-ray images, and neutron spectra.
ACKNOWLEDGMENTS
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. Lawrence Livermore National Security, LLC. LLNL-JRNL-862855.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Jim A. Gaffney: Conceptualization (equal); Data curation (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal). Kelli Humbird: Conceptualization (equal); Investigation (equal); Project administration (equal); Writing – review & editing (equal). Andrea Kritcher: Investigation (equal); Writing – review & editing (equal). Michael Kruse: Conceptualization (equal); Investigation (equal); Writing – review & editing (equal). Eugene Kur: Conceptualization (equal); Data curation (equal); Investigation (equal); Writing – review & editing (equal). Bogdan Kustowski: Conceptualization (equal); Data curation (equal); Investigation (equal); Software (equal); Writing – review & editing (equal). Ryan Nora: Conceptualization (equal); Data curation (equal); Investigation (equal); Software (equal); Writing – review & editing (equal). Brian Spears: Conceptualization (equal); Investigation (equal); Writing – review & editing (equal).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.