Bremsstrahlung and Kα fluorescence measurements for inferring conversion efficiencies into fast ignition relevant hot electrons

Chen, C. D.; Patel, P. K.; Hey, D.; Mackinnon, A. J.; Key, M.H.; Akli, K. U.; Bartal, T.; Beg, F. N.; Chawla, S.; Chen, H.; Freeman, R. R.; Higginson, D. P.; Link, A.; Y., T.; MacPhee, A. G.; Stephens, R. B.; Woerkom, L. D. Van; Westover, B.; Porkoláb, M.

doi:10.1063/1.3183693

Cited by 87 publications

(57 citation statements)

References 34 publications

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“…The igniter laser pulse requirements for fast ignition depend on the conversion efficiency from laser energy to hot electrons [7], the electron energy spectrum [8], the electron transport efficiency to the ignition hot spot [9,10], and the electron energy deposition efficiency in the hot spot [10]. The required laser energy has been estimated at approximately 100 kJ in a 20 ps pulse [1,11].…”

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confidence: 99%

“…Cone-guided fast-ignition inertial confinement fusion (FI) depends on the efficient transfer of laser energy to a forward directed beam of $2 MeV electrons at the tip of a hollow cone embedded in the side of an inertialconfinement fusion fuel capsule [1]. This scheme is particularly susceptible to laser prepulse [2,3] as the cone wall confines the expanding preformed plasma [4,5] increasing both density scale lengths and laser beam filamentation [6].The igniter laser pulse requirements for fast ignition depend on the conversion efficiency from laser energy to hot electrons [7], the electron energy spectrum [8], the electron transport efficiency to the ignition hot spot [9,10], and the electron energy deposition efficiency in the hot spot [10]. The required laser energy has been estimated at approximately 100 kJ in a 20 ps pulse [1,11].…”

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confidence: 99%

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Limitation on Prepulse Level for Cone-Guided Fast-Ignition Inertial Confinement Fusion

MacPhee¹,

Divol²,

Kemp³

et al. 2010

Phys. Rev. Lett.

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108

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The viability of fast-ignition (FI) inertial confinement fusion hinges on the efficient transfer of laser energy to the compressed fuel via multi-MeV electrons. Preformed plasma due to the laser prepulse strongly influences ultraintense laser plasma interactions and hot electron generation in the hollow cone of an FI target. We induced a prepulse and consequent preplasma in copper cone targets and measured the energy deposition zone of the main pulse by imaging the emitted K radiation. Simulation of the radiation hydrodynamics of the preplasma and particle in cell modeling of the main pulse interaction agree well with the measured deposition zones and provide an insight into the energy deposition mechanism and electron distribution. It was demonstrated that a under these conditions a 100 mJ prepulse eliminates the forward going component of $2-4 MeV electrons. Cone-guided fast-ignition inertial confinement fusion (FI) depends on the efficient transfer of laser energy to a forward directed beam of $2 MeV electrons at the tip of a hollow cone embedded in the side of an inertialconfinement fusion fuel capsule [1]. This scheme is particularly susceptible to laser prepulse [2,3] as the cone wall confines the expanding preformed plasma [4,5] increasing both density scale lengths and laser beam filamentation [6].The igniter laser pulse requirements for fast ignition depend on the conversion efficiency from laser energy to hot electrons [7], the electron energy spectrum [8], the electron transport efficiency to the ignition hot spot [9,10], and the electron energy deposition efficiency in the hot spot [10]. The required laser energy has been estimated at approximately 100 kJ in a 20 ps pulse [1,11]. Since the ignition hot spot diameter is $40 m, the cone tip must be similar in diameter and the laser intensity $4 Â 10 20 W=cm 2 . Existing petawatt class laser systems deliver up to 1 kJ with typical energy contrast $1 Â 10 À5 and with nonlinear devices this ratio can be improved by a further order of magnitude [12]. Contrast due to amplified superfluorescence and spontaneous emission is independent of the final laser energy; hence, for an ignition pulse of 100 kJ the prepulse energy on target could range from 100 mJ to 1 J. Recent work by Baton et al. [5] has shown that some amount of prepulse can strongly affect coupling to cones; however, a detailed understanding of this limit has not been reported.In this Letter we report recent studies of laser interactions with hollow cone targets comparing simulations and experiments in conditions approaching full fast ignition (FI) using prepulse up to 100 mJ with main pulse irradiance $10 20 W cm À2 for picosecond durations. These parameters were accessible using the Titan laser at LLNL, which delivers ð150 AE 10Þ J in ð0:7 þ = À 0:2Þ ps at 1 m with $10% of the energy deposited above an intensity of $10 20 W cm À2 at best focus, as described in [13].We compare coupling for two well-characterized prepulse conditions: (1) an intrinsic Titan laser prepulse with ð7:5 AE 3Þ mJ in 1.7 n...

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confidence: 99%

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confidence: 99%

Limitation on Prepulse Level for Cone-Guided Fast-Ignition Inertial Confinement Fusion

MacPhee¹,

Divol²,

Kemp³

et al. 2010

Phys. Rev. Lett.

Self Cite

108

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“…This estimation uses the assumption of a single temperature distribution; however it has been shown that the internal electrons can have a dualtemperature distribution by monitoring the x-ray spectra (Chen et al 2009;Zulick et al 2013). Knowing this, future experiments using this diagnostic are planned in conjunction with simultaneous x-ray and electron spectrometer measurement which will provide a more accurate temperature diagnostic.…”

Section: Experimental Datamentioning

confidence: 99%

“…The dominant process has been shown to depend on the scale length of the pre-plasma: Brunel heating for shorter scale lengths and the J × B mechanism for intermediate scale lengths (L s ≈ 5λ) (Santala et al 2000). Angular measurements of these absorption processes have been made (Norreys et al 1999) and different angular distributions have been proposed/observed due to longer pre-plasma influences on the front surface (Courtois et al 2009;Pérez et al 2014). The transport of electrons through the target is significantly influenced by the internal magnetic fields created by the electron beam and also the background resistivity MacLellan et al 2013).…”

Section: Introductionmentioning

confidence: 99%

Measurement of the angle, temperature and flux of fast electrons emitted from intense laser–solid interactions

et al. 2015

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High-intensity laser-solid interactions generate relativistic electrons, as well as high-energy (multi-MeV) ions and x-rays. The directionality, spectra and total number of electrons that escape a target-foil is dependent on the absorption, transport and rear-side sheath conditions. Measuring the electrons escaping the target will aid in improving our understanding of these absorption processes and the rear-surface sheath fields that retard the escaping electrons and accelerate ions via the target normal sheath acceleration (TNSA) mechanism. A comprehensive Geant4 study was performed to help analyse measurements made with a wrap-around diagnostic that surrounds the target and uses differential filtering with a FUJI-film image plate detector. The contribution of secondary sources such as x-rays and protons to the measured signal have been taken into account to aid in the retrieval of the electron signal. Angular and spectral data from a high-intensity laser-solid interaction are presented and accompanied by simulations. The total number of emitted electrons has been measured as 2.6 × 10 13 with an estimated total energy of 12 ± 1 J from a 100 µm Cu target with 140 J of incident laser energy during a 4 × 10 20 W cm −2 interaction.

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“…Filter stack technique uses differential filtering to discriminate the γ-ray spectrum [10,11]. The spectrometer consists of thirteen filters of increasing Z, five 100 µm thick filters Al, Ti, Fe, Cu and Mo, followed by 150 µm Ag, 500 µm Sn and Ta filters, and one 1.58 mm Au filter.…”

Section: Inroductionmentioning

confidence: 99%

Development of Compton X-Ray Spectrometer for Fast Ignition Experiment<sup> </sup>

Kojima

Arikawa

Ikenouchi

et al. 2014

Plasma and Fusion Research

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In the context of Fast Ignition, the fast electron temperature is a key parameter to determine the laserto-fast electron energy conversion efficiency. Bremsstrahlung X-ray (γ-ray) emission represents an attractive alternative to measure this fundamental parameter. In this study, a single-shot high-energy γ-ray spectrometer with sensitivity ranging from 0.5 MeV to 7 MeV was developed, allowing to estimate the γ-ray spectrum in the fast ignition. Keywords: fast ignition, inertial confinement fusion, fast electron, x-ray spectrometer, filter stack spectrometer DOI: 10.1585/pfr.9.4405109 InroductionFast Ignition (FI) is an alternative approach to Inertial Confinement Fusion, consisting in the separation of the compression and heating stages. In FI, the heating of the compressed Deuterium-Tritium (DT) fuel is produced by a ultra-high intensity laser-generated fast electron beam depositing its energy in the high density plasma by Coulomb interaction. The coupling efficiency is strongly dependent on the fast electron energy distribution, therefore representing a crucial parameter to be diagnosed [1]. From simulations, in order to ignite a compressed DT pellet having density of 300 g/cm 3 , 18 kJ of fast electron energy has to be deposited within a diameter of 40 µm in the compressed fuel. Fast electrons in the 1-3 MeV energy range are responsible for the energy deposition in such a small volume, allowing for the creation of a lateral hot spot. Thus high conversion efficiency into 1-3 MeV energy range fast electrons is fundamental for the achievement of Fast Ignition [2][3][4]. However, it is difficult to measure initial fast electron temperature from the outside of plasma because of the strong sheath potential around the target. The bremsstrahlung X-ray is an attractive alternative diagnostic for this aim. In previous works, many kind of γ-ray detectors with different sensitivities have been developed. Various techniques have been adopted to measure the fast electron energy spectrum, such as vacuum electron specauthor's e-mail: kojima-s@ile.osaka-u.ac.jp * ) This article is based on the presentation at the Conference on Laser and Accelerator Neutron Source and Applications (LANSA '13).troscopy, nuclear activation, Bremsstrahlung, buried fluorescent foils, proton emission, and coherent transition radiation [5][6][7][8][9]. Filter stack technique uses differential filtering to discriminate the γ-ray spectrum [10,11]. The spectrometer consists of thirteen filters of increasing Z, five 100 µm thick filters Al, Ti, Fe, Cu and Mo, followed by 150 µm Ag, 500 µm Sn and Ta filters, and one 1.58 mm Au filter. Finally four Pb filters with thickness varying from 1 mm to 4 mm for differential filtering are positioned in the stack. Nuclear activation technique uses photo nuclear activation to discriminate the γ-ray spectrum. In this method, γ-ray spectrum is measured from the activation ratio of a pseudo- . However, observable energy range of these two detectors are respectively up to 0.5 keV and above 7 MeV, respectively. Thus, ...

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Bremsstrahlung and Kα fluorescence measurements for inferring conversion efficiencies into fast ignition relevant hot electrons

Cited by 87 publications

References 34 publications

Limitation on Prepulse Level for Cone-Guided Fast-Ignition Inertial Confinement Fusion

Limitation on Prepulse Level for Cone-Guided Fast-Ignition Inertial Confinement Fusion

Measurement of the angle, temperature and flux of fast electrons emitted from intense laser–solid interactions

Development of Compton X-Ray Spectrometer for Fast Ignition Experiment<sup> </sup>

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