# Driver at 10 MJ and 1 shot$/$30min for inertial confinement fusion at high
gain: efficient, compact, low-cost, low laser-plasma instabilities, beam-color
selectable from 2$\omega$$/$3$\omega$$/$4$\omega$$/$, applicable to multiple
laser fusion schemes

Zhan Sui ( չ<EMAIL_ADDRESS>Shanghai Institute of Laser Plasma, China
Academy of Engineering Physics, Shanghai 201800, China  Ke Lan ( )  Institute
of Applied Physics and Computational Mathematics, Beijing 100094, China

###### Abstract

The ignition at the National Ignition Facility (NIF) set off a global wave of
research on the inertial fusion energy (IFE). However, IFE requires a
necessary target gain G of 30-100, while it is hard to achieve the fusions at
such high gain with the energy, configuration, and technical route of the NIF.
We will present a conceptual design for the next generation laser driver of 10
MJ, 2 $\sim$ 3 PW at 3$\omega$ (or 2$\omega$, then the energy and power can be
higher), and 1 shot$/$30min, which is efficient, compact, low-cost, low laser-
plasma instabilities, applicable to multiple laser fusion schemes, and aiming
for G $>$ 30.

##  I Introduction

Laser driven inertial fusion is a highly promising approach toward fusion
energy, which has been a quest of human beings for more than a half century.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory
achieved target gain G $>$ 1 in the end of 2022 Abu-Shawareb2024PRL  and set a
new fusion yield record of 5.2 MJ in February, 2024 5dot2MJyield , which
successfully demonstrated the feasibility of laboratory laser fusion. However,
it still far below the target gain G of 30 to 100 required by the inertial
fusion energy (IFE) MTV . In fact, limited by its configuration Lan2022MRE ,
energy NNSA2015 , and the technical route identified in the 1980s Haynam2007 ;
Manes2016 , it is hard for the NIF to reach such high gain. Hence, a driver
with novel laser technologies is mandatory in the path toward IFE. In this
paper, we will give a conceptual design for an efficient and low-cost laser
driver of 1 shot/30min, aiming for G $>$ 30\. A schematic is shown in Fig. 1,
and the main technical specifications are shown in Table 1. In the following,
we will address the novel technologies to improve the energy storage
efficiency, extraction efficiency, flux, volume, etc. for this driver.

Figure 1: (Color online) Schematic of the general layout of the laser driver with its target chamber. Insets: (a) the arrangement of a laser beam, which is injected at FE with an initial energy of 5 J, reflected at PC with an amplified energy of 1.5 kJ, then amplified in the main amplifier and filtered by LAFs, and finally delivered at SF with an energy of 20 kJ and a beam size expansion from 260 mm to 530 mm; (b) a beamlet of 4 beams at 3$\omega$; (c) octahedral spherical hohlraum and its ideal laser arrangement with 16 beamlets per end. Here, FE is the frond end, LAF is the laser filtering, PC is the Pockels cell, and SF is the Spatial Filter. The figure on the left of inset (b) shows the scale of the laser driver. Table 1: Main technical specifications. Laser energy |  $>$ 10 MJ
---|---
Laser power | 2 $\sim$ 3 PW
Laser wavelength | beam-color selectable
| from 2$\omega$$/$3$\omega$$/$4$\omega$
Waist diameter | 1000 $\mu$m
Main pulse width | 3 $\sim$ 5 ns
Repetition rate | 1 shot/30min
F Number | 10 $\sim$ 15
End number | 6
Beamlet number per end |  $N_{Q}$ = 16
Beam number per beamlet | 4
Driving mode | octahedral configuration
Shooting angle | all beamlets have the same
for details: | incidence angle $\theta_{L}$
Refs. [10; 4] | ranging from 50∘ to 60∘,
| and the same initial azimuthal
| angle $\phi_{L0}$ ranging
| from $0$ to $2\pi/{4N_{Q}}$.
Cost |  $\leq$ 20 billion RMB

##  II Key technologies

1\. Multi-front-end technology

Laser-plasma instabilities (LPIs) is one of the main obstacles of the NIF to
achieve high gain. In our novel design, each laser beam has an independent
front-end, i.e. one distributed feedback (DFB) oscillator per beam line. Then,
the regulation freedom can be improved because each laser beam has no fixed
phase relationship, and therefore, the incoherent superposition can be
realized at the target point. This is conducive to the beam smoothing and can
effectively suppress LPIs. Moreover, a super light spring of incoherence in
all dimensions of time, space, and angle can be used to further suppress LPIs
incoherent2023MRE . Thus, it is possible for the driver to work also at a
frequency lower than 3$\omega$, for a higher energy conversion efficiency and
a higher damage threshold for the optic components in the final optic
assembly.

2\. Near-field spatial separation amplification of pre- and main pulse

In the ignition target designs, the laser pulse consists of pre-pulse and main
pulse, and the power of pre-pulse is usually much lower than that of the main
pulse. This makes the conversion efficiency of the pre-pulse part very low
during the frequency conversion process, leading to an overall frequency
conversion efficiency of the ignition pulse about 30$\%$ lower than that of
the square-wave pulse Manes2016 . In our novel design, the pre- pulse and main
pulse are spatially separated in the amplifier by adopting the spatiotemporal
shaping. Furthermore, the pre-pulse corresponds to a smaller aperture, while
the main pulse corresponds to the rest. This can be used to adjust the power
densities of the pre-pulse and the main pulse to be equivalent, so as to
improve the overall conversion efficiency. The superposition effect of pre-
pulses and main pulses at the target point can be the same as that of the
traditional amplification method.

3\. High fluence amplification employed laser material with low emission cross
section, long fluorescence lifetime, and high energy storage

Most of existing laser drivers use neodymium glass as the gain medium, which
has a large radiative transition probability and a high gain due to its short
fluorescence lifetime and large emission cross section, however, it has a
relatively small energy storage. In our novel design, the driver uses the new
sensitizing doped laser material, which has a fluorescence lifetime (
$\sim$650 $\mu$s), low emission cross section ($\sim$ 2 $\times$ 10 -20 cm2)
and high stored energy (0.5 J/cm3) as the main amplifier gain material. In
addition, a high-flux and multi-pass amplification is used to effectively
extract the stored energy. This type of amplifier is expected to achieve a
high flux output of 30 J/cm2.

4\. Water-cooled xenon lamp with annular section and fluorescent conversion
separator material

As known, the operational efficiency of high-power laser devices is relatively
low, only about 3 - 4 shots per day. In the novel design, we use a xenon lamp
with an annular section as the pump, which fluorescence can be converted into
an absorption band and absorbed by the laser medium by using a fluorescence
conversion baffle material. Hence, the pumping light-laser conversion
efficiency can be remarkably improved. Meanwhile, the xenon lamp is subjected
to water cooling, so that the heat can be taken away in time and the laser
medium can be prevented from heating up continuously. As a result, the
emission interval can be effectively shortened, and a shooting capability of
30 minutes per emission can be expected. This technology can improve the
radiation efficiency of the large diameter xenon lamp and the absorption
efficiency of gain medium, leading to a much higher energy storage efficiency
of disk amplifier and a much shorter emission interval.

5\. Near-field multi-pass split-plate amplifier based on angle-sensitive film

The energy extraction efficiency is crucial important for a driver. In order
to improve the extraction efficiency, we design a near-field multi-pass
amplifier. The amplifiers have a certain splitting angle, and the film layers
of surfaces are designed to be angle-sensitive. By this way, the pulse has a
high reflectivity on the surfaces in the first two paths of transmission, and
the angle corresponding to the third path has a high transmissivity Spaeth2016
. Thus, the actual transmission distance of the pulse passing through the
amplification medium is three times the thickness of the medium. In other
words, it is a three-pass amplification, in which the pulse has nearly equal
fluence in one amplifier medium while the integral fluence is three times
enhanced. This amplification mode has a simple system structure but a high
extraction efficiency.

6\. Two-pass amplification configuration combined with near-field three-pass
amplification

Combined with a near-field three-pass amplification, the main amplifier is
equipped with a two-pass amplification configuration, and the amplification of
48 equivalent pieces can be realized by adopting 8 pieces of gain media. This
system has a compact structure (with a length about 20 m), a large integral
flux, a balanced flux of each piece of gain medium, and a high extraction
efficiency, which can meet the image transmission of the system when combined
with the traditional spatial filter.

7\. Spatial filtering technology based on angular spectrum sensitive nonlinear
crystal

In current laser technology, it takes the low-gain high-fluency amplification.
In this case, the B integral of the system is severe, and a spatial filtering
is required to suppress the increase of B integral Haynam2007 . However, the
traditional spatial filters generally have a long size and require a vacuum
assembly, which cost is expensive. In our design, we use the nonlinear near-
field filtering method in the designed single or cascaded nonlinear crystals,
then the spatial high-frequency components of pulses can be converted to the
second-harmonic wave and filtered in the following amplification processes. In
addition, the low-frequency components of pulses have a low conversion
efficiency, so the spatial filtering can be realized. Especially, the
nonlinear spatial filtering can be conveniently inserted into the amplifier,
resulting in a more compact structure of the laser driver with significant
space and cost savings.

8\. Beam combination system based on the non-collinear nonlinear frequency
conversion

As known, the beam number of laser driver is restricted because the solid
angle of the target chamber is fixed. In our design, the two fundamental-
frequency beams of light are combined into one beam at third-harmonic
frequency, through non-collinear second-harmonic and sum-frequency conversion.
Furthermore, the beam number can be doubled by using the non-collinear beam
combination scheme, and the output pulse can be ensured to have sufficient
brightness with a small F number under the condition that the load capacity of
the device is constant.

9\. New Measurement and Control Technology

Compared with the current advanced control technologies, the existing control
system used in the laser drivers lags far behind. In our design, it will
employ the centralized control system based on global wireless Internet of
Things (IOT), together with a high-resolution and high-stability time
synchronization system based on high- speed communication technology, and a
full-system control based on artificial intelligence (AI) and miniaturized
measurement package.

10\. General layout: semi-underground centered on the target chamber

According to our above technical schemes, the space size of the driver can be
greatly reduced, and also the underground layout becomes possible. The general
layout is semi- underground centered on the target chamber, which has six
laser shooting ends Lan2014POP , with 16 beamlets per end and 4 beams per
beamlet, guided into the target chamber, as shown in Fig. 1. It can reduce the
cost of construction and environmental control.

##  III Summary

We have presented a conceptual design for the next generation laser driver of
10 MJ, 2 $\sim$ 3 PW and 1 shot/30min, with the ideal laser arrangement of the
octahedral spherical hohlraum approach Lan2014POP ; Craxton2020DPP ;
Marangola2021 , which is applicable to multiple schemes for IFE route choice
and feasibility demonstration. The highly symmetric radiation drive at all
times and for all spectra naturally generated inside an octahedral spherical
hohlraum is mandatory in achieving a fusion of high gain with the delicate
capsule designs Haan2011POP ; Ren2018POP ; Qiao2021PRL . Under a 10 MJ drive,
it can be expected to achieve G $\sim$ 30 by assuming a burn depletion MTV
$\Phi$ $\sim$ 45$\%$ of a 2 mg DT fuel by indirect-drive, and G $\sim$ 100 by
assuming $\Phi$ $\sim$ 30$\%$ of a 10 mg DT fuel by direct-drive. Here, we
consider a lower $\Phi$ for direct-drive due to its worse asymmetry aroused by
the laser imprinting and beam crossing. Detail designs of such targets are to
be presented in our forthcoming publications. As in Table I, the beam-colors
of the novel driver are selectable from 2$\omega$$/$3$\omega$$/$4$\omega$$/$
among beamlets, which means that each laser beam can simultaneously and
independently work at either 2$\omega$, 3$\omega$, or 4$\omega$ for various
physics purposes Lan2017POP ; CYH2018POP . This can be realized via nonlinear
frequency conversion process by removing or changing the crystals. Compared
with the prior art, our novel laser driver design can remarkably improve the
energy storage efficiency, extraction efficiency, flux, repetition, and
meanwhile, greatly reduce its volume, metal components and cost.

ACKNOWLEDGMENTS The authors appreciate Dr. Xiaohui Zhao of Shanghai Institute
of Laser Plasma, Dr. Yongsheng Li, Dr. Hui Cao, Dr. Yaohua Chen, and Dr.
Xiumei Qiao of Institute of Applied Physics and Computational Mathematics in
Beijing for their beneficial helps. This work is supported by the National
Natural Science Foundation of China (Grant No. 12035002).

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