Joint Amplitude-Phase Optimization of Broadband Lasers Raises Absolute Two-Plasmon-Decay Thresholds Above Coherence-Time Predictions
Abstract
Broadband lasers suppress the absolute two-plasmon-decay instability that preheats inertial-fusion targets. The usual scaling relates this suppression to the drive's coherence time, a power-spectrum statistic insensitive to spectral phase. By gradient descent through a differentiable enveloped wave solver, we show that at fixed power spectrum phase optimization raises the threshold from the random-phase median of 2.8 Imono to 4.1 Imono, but produces a transform-limited (TL) pulse train with peak-to-average ratio R=32. Joint amplitude--phase optimization reaches 5.6 Imono at R=3.6, above the best tested TL line-count result of 4.5 Imono at R=16; relaxing the peak constraint raises the joint threshold to 6.4 Imono. An exact growth-rate budget evaluated inside the simulation separates the two mechanisms. TL suppresses coupling mainly by concentrating the pump field into short bursts, thereby lowering its time-averaged magnitude at fixed average intensity. The joint optima retain near-random values of this amplitude measure while suppressing both the available coupling and the fraction realized through pump--daughter alignment. The advantage of joint optimization persists from 1% to 4% bandwidth and from OMEGA-scale to ignition-scale conditions.
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BibTeX
@misc{joglekar2026joint,
title = {{Joint Amplitude-Phase Optimization of Broadband Lasers Raises Absolute Two-Plasmon-Decay Thresholds Above Coherence-Time Predictions}},
author = {A. S. Joglekar and R. K. Follett and D. H. Froula and J. P. Palastro},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.31942},
eprint = {2609.31942},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.31942},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Joint Amplitude-Phase Optimization of Broadband Lasers Raises Absolute Two-Plasmon-Decay Thresholds Above Coherence-Time Predictions
AU - A. S. Joglekar
AU - R. K. Follett
AU - D. H. Froula
AU - J. P. Palastro
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2609.31942
UR - https://arxiv.org/abs/2609.31942
AB - Broadband lasers suppress the absolute two-plasmon-decay instability that preheats inertial-fusion targets. The usual scaling relates this suppression to the drive's coherence time, a power-spectrum statistic insensitive to spectral phase. By gradient descent through a differentiable enveloped wave solver, we show that at fixed power spectrum phase optimization raises the threshold from the random-phase median of 2.8 I_mono to 4.1 I_mono, but produces a transform-limited (TL) pulse train with peak-to-average ratio R=32. Joint amplitude--phase optimization reaches 5.6 I_mono at R=3.6, above the best tested TL line-count result of 4.5 I_mono at R=16; relaxing the peak constraint raises the joint threshold to 6.4 I_mono. An exact growth-rate budget evaluated inside the simulation separates the two mechanisms. TL suppresses coupling mainly by concentrating the pump field into short bursts, thereby lowering its time-averaged magnitude at fixed average intensity. The joint optima retain near-random values of this amplitude measure while suppressing both the available coupling and the fraction realized through pump--daughter alignment. The advantage of joint optimization persists from 1% to 4% bandwidth and from OMEGA-scale to ignition-scale conditions.
ER -