high-energy-density physics

CHARGED-PARTICLE SPECTROSCOPY ON OMEGA

The MIT High-Energy-Density Physics group, in collaboration with the Laboratory for Laser Energetics at the University of Rochester, the Lawrence Livermore National Laboratory, and with the SUNY Geneseo Nuclear Structure Laboratory, State University of New York .

(Updated 28 March 2001)

ABSTRACT

This document provides an overview of our Charged-Particle Spectroscopy program, which is designed to provide diagnostic information about ICF plasmas by measuring energy spectra of charged fusion products and ablator ions. While the program is designed with the National Ignition Facility (NIF) in mind, it currently involves experiments on the OMEGA facility at the University of Rochester Laboratory for Laser Energetics. Two magnet-based spectrometers and several small range-filter-based spectrometers are used for spectroscopic measurements of energetic charged particles on OMEGA. Measured spectra include lines of D3He protons (14.7 MeV) and alphas (3.6 MeV), DT alphas (3.5 MeV), and DD protons (3.0 MeV), tritons (1.0 MeV), and 3He (0.8 MeV), T-3He deuterons (9.5 MeV), and continuous "knock-on" spectra of deuterons, tritons, and protons ejected in elastic collisions with 14-MeV fusion neutrons. These and other measurements provide fusion yields, ion temperatures, <rR> of fuel and shell, stopping power in hot plasmas and quantification of anomalous acceleration effects. In addition, copious fluxes of energetic ablator protons have been observed from 100 keV to 1.4 MeV, sometimes including sharply-defined "lines". The maximum energy of the ablator protons strongly suggests that the capsule can be charged up to more than 1 MV while the laser is on.

TABLE OF CONTENTS

ABSTRACT

1 INTRODUCTION

2 THE OMEGA LASER EXPERIMENT

3 THE CHARGED-PARTICLE SPECTROMETERS

3.1 The CPS

3.2 The Wedge-Range Filter (WRF)

4 PRIMARY FUSION PRODUCTS

5 SECONDARY FUSION PRODUCTS

6 14-MeV NEUTRON KNOCK-ON's

7 ABLATOR IONS

8 TERTIARY PARTICLES

9 FUTURE DIRECTIONS

REFERENCES

1 INTRODUCTION

Nuclear measurements provide a direct method for studying essential parameters of ICF implosions, such as ion temperature, yields, capsule convergence, shell breakup, fuel and shell <rR>, implosion asymmetry, and anomalous acceleration effects [references 1-12]. Charged nuclear products of importance for understanding and quantifying these processes are listed below in Table I. Table II lists energetic ablator protons and ions of interest. With some notable exceptions [8-10], the emphasis in the past has been on utilizing neutrons to investigate a subset of these processes [1-7]. The work described here, based on charged-particle-spectrometers which have been developed, is focusing on charged nuclear products as a sensitive means of studying capsule implosion characteristics [11-30]. To the best of our knowledge, the high-resolution charged-particle spectra obtained in this program are the most comprehensive set of measurements obtained in the ICF program.

The OMEGA laser experiment is discussed in Section 2, while the spectrometer hardware design and interface with the OMEGA is described in Section 3. Section 4 to 8 describes how measured spectra of the different types of fusion products give information of fusion yields, ion temperatures, fuel and shell <rR>, and interesting properties of ablator ions such as anomalous acceleration effects (suggesting that the capsule can be charged up to more than 1 MV while the laser is on) and sharp lines. Published papers, regarding a certain topic, are indicated at the end of the section describing that specific topic. In section 9, the future directions are outlined.

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2 THE OMEGA LASER EXPERIMENT

At the present time, the OMEGA laser system at the Laboratory for Laser Energetics, University of Rochester, is the flagship of the U.S. inertial fusion program. OMEGA is a neodymium-doped phosphate glass laser capable of delivering up to 30 kJ of frequency-tripled, 0.35 mm light. This energy can be distributed into 60 beams to irradiate a spherical target with a root-mean-square uniformity of <5 %. The uniformity of individual beams is achieved through the implementation of two-dimensional smoothing by spectral dispersion (SSD). Each beam can be individually focused with a pointing accuracy of ±16 mm and a timing resolution of ±10 ps on target. A variety of pulse shapes are possible, with widths ranging from 115 ps to a few nanoseconds. The laser is typically operated at a repetition rate of about one shot per hour. The OMEGA target chamber is a 3.3 m diameter aluminum “soccer ball” with 60 beam ports and 32 diagnostic ports. A vacuum of about 10-5 torr is maintained during the experiments. Figure 2.1 shows a photograph taken during one shot at the OMEGA.

Figure 2.1: Photograph taken during a shot at the OMEGA laser facility. In this case 30 laser beams are used to implode a deuterium-filled cryogenic target.

3 THE CHARGED-PARTICLE SPECTROMETERS

The current set of charged-particle spectrometers on OMEGA includes:

CPS 1: A spectrometer utilizing a 7.6-kG magnet and CR-39 detectors. It resolves energetic particles from ~100 keV to 30 MeV (proton equivalent energy, or energy of a particle with the same gyroradius as a proton of this energy). The aperture is located outside the OMEGA chamber, at 235 cm from the target.

CPS 2: Uses an identical magnet to that on CPS-1, but located inside the target chamber and closer to the target position (aperture at 100 cm); the result is better counting statistics for a given aperture size, or better resolution (smaller aperture) for the same counting statistics. It also has substantial shielding for improved rejection of neutron-induced noise.

Wedge-Range Filter (WRF): Uses an aluminum wedge-shaped filter (with varying thickness of 400 mm to 1800 mm) in combination with CR-39 as the particle detector. The aluminum filter is used to range down the proton energies to the region of 100 % sensitivity in the CR-39. The advantages with the WRF are simplicity and the ability to get close to the target-chamber center (TCC). Ports TIM1 - TIM6 and KO1 - KO3 are normally allocated for the WRF (see Figure 3.1).

Figure 3.1: OMEGA port diagram, showing where spectrometers CPS 1and CPS 2 are located. Also shown are ports TIM1 - TIM6 and KO1 - KO3 in which the WRF's are normally mounted. This allow for studies of asymmetric implosion dynamics.

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3.1 The CPS

CPS 1 and CPS 2 each use a 7.6-kG neodymium-iron-boron permanent magnet with CR-39 detectors. The magnet and a sample of drawn particle trajectories are shown in Figure 3.2. Pieces of CR-39 are positioned throughout the dispersed beam, normal to the particle directions, using the mounting structure shown in Figures 3.3a and 3.3b, which allows greater than 80 % coverage between the proton-equivalent energies of 0.1 to 30 MeV. Emitted particles are selected by a slit aperture that can be varied between 1 - 10 mm in width, depending on the expected flux levels, giving solid angles between 10-6 and 10-5. The spectrometer chambers are shown in Figures 3.4 and 3.5. The more elaborate design for CPS 2 incorporates a polyethylene-lead shielding structure (see Figure 3.5) which reduces neutron noise levels on the
CR-39 by 500 %.

After every shot, the pieces of CR-39 are removed from the spectrometer and replaced by a new set. The exposed pieces are then etched in sodium hydroxide and examined under a microscope. A rapid, automated, scanning system we have developed is used to count typically between 103 – 106 tracks, or events, per shot. Accurate, and calibrated, particle trajectory calculations are used to determine the energy of particles arriving at each position on the detectors. The presence of multiple particle species is conveniently managed since, at any given detector position, the track diameters from each species are clustered into discrete diameter groups; the heavier particles, such as alphas, have relatively large stopping powers and generate tracks significantly larger than those from lighter particles such as protons (Figure 3.6). The dynamic range of the instruments allow for yields of 108 to 1016, while the energy resolution varies with energy, being about 1% at 2 MeV and 4% at 15 MeV.

Figure 3.2: One of two identical 7.6-kG magnets fabricated for the charged-particle spectrometers CPS 1 and CPS 2. The longest dimension of the magnet is 28 cm, and the pole gap is 2 cm. This magnet weighs 160 pounds, and the force between the poles is 6400 pounds. Also shown are particle trajectories from a target implosion through the magnet, which separates particles according to momentum/charge. Particle detection and identification is achieved using CR-39 detectors.

Figure 3.3: Photographs of the mounting plate assembly that is used to accurately position pieces of CR-39 in the dispersion plane of the magnet. Figure (a) shows the mounting plate assembly viewed from the perspective of the magnet. Pieces of CR-39 are positioned in each of the finger structures; these fingers are arranged in a few arcs which cover the dispersion region of the magnet. The finger at the bottom of the photo is positioned to view the target directly. X-ray film is placed at this position in order to ascertain the alignment of the spectrometer. The collimator slit is shown at the top of the photo. Figure (b) shows the loaded mounting plate assembly being lowered onto the magnet (which is obscured) inside the vacuum chamber of CPS 1. After every shot, the mounting plate must be removed, and the CR-39 unloaded. A new, freshly loaded plate must then replace it in preparation for the next shot. (This design was done by the team. Photo courtesy of Eugene Kowaluk, LLE.)

Figure 3.4: Overview of the spectrometer CPS 1 as installed on OMEGA target chamber. The collimator of CPS 1 is located at 235 cm from the target position.(Photo courtesy of Eugene Kowaluk, LLE.)

Figure 3.5: The CPS-2 spectrometer. Top) Exploded diagram, showing the enclosure, the magnet, the shielding, and the re-entrant "nose cone" containing the entry aperture. Bottom) Installation on OMEGA target chamber. The entry aperture is located 100 cm from the target position.(Photo courtesy of Eugene Kowaluk, LLE.)

Figure 3.6: (Left) Example of a digitized microscope image of CR-39, showing DD proton, DD triton and D3He alpha tracks from OMEGA Shot 20298 (D3He target). This piece of CR-39 was positioned behind a 6-mm aluminum filter, and was etched for 6 hours in NaOH at 80°C. The physical dimensions of the image are 424 x 318 mm. The fact that 3-MeV DD protons and 1-MeV DD tritons have the same gyroradii means that they appear at the same position on exiting the magnet. In addition, due to the energy spread of D3He alphas (some having energies down to 3 MeV), all three types of particle actually appear at this "triple-degeneracy" position

(Right) Intensity plot of the "triple degeneracy" point as a function of track diameter and energy. The different stopping powers of the three types of particles allows them to be distinguished easily by the diameter.

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3.2 The Wedge-Range Filter (WRF )

Another type of spectrometer, a wedge-range-filter (WRF) spectrometer, has recently been tested for the first time. In a WRF, CR-39 is again used as the particle detector, and a special wedge-shaped filter is used to range down the proton energies so they fall within the interval of sensitivity of the detector. The advantages of using range-filter measurements are simplicity and the ability to get small statistical errors by getting close to the target. The filters used here were machined from aluminum, with thicknesses varying from 400 mm to 1800 mm
(see Figure 3.7).

A highly-detailed CR-39 response calibration has been done to protons of different energies (different energies result in different track sizes), and calibrations of the transmission characteristics of various filters has been done as well. In combination, those calibrations determine a direct mapping between track diameter and incident proton energy for a given filter thickness. That mapping can be used to reconstruct the spectrum, of the indcident particles, from a histogram of track diameters.

Figure 3.7: A photograph of a wedge-range filter (WRF).The filters used are made of aluminum, and the thicknesses varies from 400 mm to 1800. The dimensions of the WRF are 45 x 45 mm.

4 PRIMARY FUSION PRODUCTS

The primary fusion products appear as discrete lines at energies predominantly determined by the nuclear energetics of each reaction (see Table I). Such lines are discrete (with a narrow width) because the thermal energy of the fuel ions, at a few keV, is small compared to the energy of the products. Detection of the the primary fusion products is an important benchmark in charged-particle spectroscopy as their discrete energies can be used to verify the absolute energy calibration of any spectrometer. In addition, the absolute yield value determined by the charged-particle spectroscopy can be tested by comparing available neutron measurements, at least for the DT and DD reactions.

An absolute yield measurement from charged fusion products is achieved by counting the number of fusion products. The DT yield can be determined by counting the number of alphas generated in the D(T,n)4He reactions, while the DD yield can be measured using the number of protons or tritons produced by the D(D,p)T reaction. The alpha yields provide direct comparison to DT neutron yields, while proton or triton yields must be related to neutron yields via the branching ratio for D(D,p)T versus D(D,n)3He reactions. In principle, a fusion yield diagnostic will also measure the implosion symmetry if the diagnostic can be placed at different positions in the target chamber (the WRF spectrometer described in section 3.2 has an important role here).

Fuel ion temperature can also be derived using the primary fusion products. This is done by either measure the yield ratios for different fusion products or measure the Doppler line widths. These are independent techniques which, used concurrently, provide a valuable consistency check.

In addition, areal density (rR) is another plasma parameter that can be deduced from the primary fusion products. This is done by directly measuring the amount of energy lost by the particle while traversing the fuel and shell. The average areal density is formulated as a line-averaged integral of the density-radius product (normally expressed as <rR>). A higher <rR> yields a larger average-energy downshift of charged particles traveling through the plasma.

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5 SECONDARY FUSION PRODUCTS

Secondary fusion particles are the result of two sequential fusion reactions. The first step is the DD reaction, producing 3He ions. The 0.8-MeV 3He ions may, while slowing down, undergo fusion reactions with thermal deuterons to produce alphas between 1.4 and 6.7 MeV and protons between 12.5 and 17.4 MeV (see Table I and Figure 5.1). It should be noted that the wide energy distribution is a result of the substantial kinematic broadening. By measuring the number of protons generated by the secondary D3He reactions, and comparing it to the number of primary DD reactions, a determination of the fuel <rR> can be done. Spectral information of the secondary protons yields, in addition, information of the total <rR> (both fuel and shell). As for primaries, a higher <rR> yields a larger avearge-energy downshift of secondary protons traveling through the plasma. In order to relate the average energy down shift to a total <rR>, modeling of some plasma parameters, such as fusion-burn profile and stopping power of a hot plasma (see project Theory of Moderately-Coupled Plasmas), is required.

Figure 5.1: Secondary fusion particles are the result of two sequential fusion reactions. The first step is the DD reaction, producing 3He ions. The 0.8-MeV 3He ions may, while slowing down, undergo fusion reactions with thermal deuterons to produce alphas between 1.4 and 6.7 MeV and protons between 12.5 and 17.4 MeV.

6 14 MeV NEUTRON KNOCK-ON's

Another method to measure fuel and shell <rR> is to measure the number of protons (from a CH-shell), deuterons and tritons (from the DT fuel) elastically scattered by 14-MeV neutrons (see Table I and Figure 6.1). The yield of the knock-on deuterons and tritons is directly proportional to the fuel <rR> and the neutron yield, and, for conditions of interest, is three to four orders of magnitude below the yield of the primary products (the yield of the knock-on protons give information of the shell <rR>). In addition, the energy-down shift of the high-energy end point of the deuteron and triton spectrum give additional information of the total <rR>. The great advantage of the knock-on measurements is that it is a simple and direct technique for determining the fuel and shell areal densities, provided that the signal-to-noise issues can be understood.

Figure 6.1: A schematic illustration of the knock-on processes in an imploded target. The central fuel part typically has a DT gas fill, while the shell is constructed of CH. Primary 14-MeV DT neutrons, generated in the fuel, elastically scatter deuterons and tritons out of the fuel and protons out of the shell. Consequently, information from the compressed core is carried out by these knock-on deuterons and tritons, and information from the compressed shell is carried out by knock-on protons. The energy downshifts of knock-on D and T spectra also contain information about the shell.

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7 ABLATOR IONS

In addition to nuclear processes that directly relate to the core and shell conditions, accelerations of ablator ions (ions from the shell) are routinely observed. This indicates the presence of hot electrons in the target corona with temperatures of approximately tens of keV which, through charge-separation, generate electric fields that accelerate the ions [20, 21]. Ablator protons, with a continuum of energies up to as high as 1.4 MeV, have been detected. The energy spectra exhibit a variety of features, displaying both relatively smooth continua as well as sharp lines. In some cases, particularly intense, discrete lines are found. Total ablator proton energy is typically estimated at 10 to 30 J, compared with 20 kJ of laser input energy. A feature common among all the spectra is a sharply-defined, maximum (endpoint) energy. This endpoint scales with average on-target laser intensity with a threshold for proton acceleration at about 1014 W/cm2. The presence of SSD (smoothing by spectral dispersion) does not have any noticeable effect on the maximum proton energy. In addition to protons, some other accelerated ablator ions have been identified, in particular deuterons and 3He, as well as what appears to be different charge states of carbon or oxygen.

8 TERTIARY PARTICLES

Tertiary reactions in fuel containing deuterium, tritium and 3He can generate the highest energy particles of interest. This process involves a sequence of three mechanisms (see Table I): initially, primary DT reactions produce 14-MeV neutrons; some of these neutrons go on to generate elastically scattered deuterons up to 12.5 MeV; and then a fraction of these high energy deuterons react with 3He fuel ions, producing a spectrum of protons up to 30.8 MeV. This spectrum is peaked near the maximum energy, mirroring the endpoint peak in the knock-on deuteron spectrum. The yield of these particles is approximately proportional to <rR>2. Owing to their low yields, tertiary fusion products have yet to be detected; however, the necessity, within the next several years, for diagnosing the high areal-densities of targets approaching ignition will mean that tertiary processes, with their associated highly penetrating particles, will become crucial diagnostic techniques.

9 FUTURE DIRECTIONS

During the next year, charged-particle studies on OMEGA will be expanded to a wider range of target and laser conditions. Special emphasis will be placed on targets that are ICF-relevant, such as thick or multi-layer shells which approach conditions in future cryogenic capsules. We will study fuel ion temperatures, shell and fuel areal densities, symmetry of implosions, implosion dynamics, and ablator proton and ion acceleration effects, comparing data with theoretical simulations wherever possible.

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