Target diagnostics for the future AWE Orion laser facility
Kevin Oades, Andrew Evans, Gary Slark, John Foster, Richard Eagleton et al.
Citation: Rev. Sci. Instrum. 75, 4222 (2004); doi: 10.1063/1.1787605
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REVIEW OF SCIENTIFIC INSTRUMENTS
VOLUME 75, NUMBER 10
OCTOBER 2004
Target diagnostics for the future AWE Orion laser facility
Kevin Oades,a) Andrew Evans, Gary Slark, John Foster,
Richard Eagleton, and Eugene Clark
Atomic Weapons Establishment (AWE), Aldermaston, Reading RG7 4PR, United Kingdom
(Presented on 22 April 2004; published 14 October 2004)
The Atomic Weapons Establishment has proposed building a new laser facility in the United
Kingdom. This will use 10 ns-class beams in conjunction with two, subpicosecond, petawatt-class
beams to access plasma conditions inaccessible to even the largest megajoule-class facilities.
Diagnostic techniques for the long pulse regime are fairly mature, whereas techniques in the short
pulse regime are still evolving. This article describes the development of a suite of target diagnostics
to exploit the high temperature, high density plasma conditions that will be achievable on the Orion
laser, and discusses some of the opportunities and problems that will be encountered in attempting
to combine the two sets of techniques.
© 2004 American Institute of Physics. [DOI: 10.1063/1.1787605]
I. INTRODUCTION
The aim of the Orion laser system is to deliver quantitative data on materials properties in the high energy density
regime, while still providing the experimental flexibility for
fundamental physics experiments. The combination of a long
pulse implosion capability and short pulse beam heating capability will allow unique access to plasma conditions that
are much more extreme than either the long pulse or the
short pulse would separately allow. Short pulse beams produce high energy density at solid (or slightly less than solid)
density, while a combination of short and long pulse beams
has the potential to create high energy density in plasmas at
much greater than solid density.
The design for the nanosecond (or “long pulse”) beams
for Orion is based on the multi-pass architecture currently
employed on the HELEN laser at the Atomic Weapons Establishment (AWE).1 Multi-pass laser designs are desirable
in that they are inherently more efficient than single-pass
designs for a given output energy. Each of the ten long pulse
beams will deliver 500 J of 0.35 ␮m light in a 1 ns square
pulse.
The two short pulse beams will use the chirped pulse
amplification technique to provide approximately 500 J in
500 fs, a power of 1 PW. The UK currently has a world lead
in this technology based at the Rutherford Appleton Laboratory (RAL), which has built and installed a single PW beam
on its VULCAN laser; 2 Orion will have two PW beamlines
closely based on the RAL design.
II. TARGET AREA FACILITIES
Plasma physics experiments will be conducted in a Target Hall whose dimensions are 20 m wide ⫻ 27 m long
⫻ 15 m high, and this space is enclosed by 1.5-m-thick
concrete walls and a 0.5-m-thick concrete roof, both dea)
Author to whom correspondence should be addressed; electronic mail:
kevin.oades@awe.co.uk
0034-6748/2004/75(10)/4222/3/$22.00
signed so as to adequately shield staff and members of the
public from the radiation, in the form of both gamma radiation and neutron emission, generated by the interaction
of the high power laser beams with the target. Using data
from experiments at RAL on both the VULCAN PW system and its predecessor 100 TW facility, the order of
magnitude of the gamma dose3 may be 100 rads, while
neutron yields are not expected to exceed 5 ⫻ 1013.
The target chamber is a 4 m internal diameter sphere
constructed of 7.5-cm-thick, type-5083 aluminium alloy. The
ten long pulse beams are arranged in two cones of five
beams, with each beam at 50° to the principal axis; this
geometry has been chosen to optimize the drive uniformity
available for a range of experiments using cylindrical Hohlraum targets. The two short pulse beams enter the chamber
above the horizontal plane in order to improve access for
diagnostic equipment close to the target, and are incident on
off-axis parabolic mirrors (OAPs) which focus the light onto
target. These OAPs have a focal length of 1800 mm, giving
an f number of around 3, and are intended to focus the light
to a spot of around 5 ␮m. Following consideration of likely
experiment geometries, one of these will be positioned on
the principal chamber axis; the other transverse to this axis.
The chamber incorporates plinths which provide support for
the OAPs and the target positioner. This approach ensures
that the OAPs and target positioner are isolated from any
movements in the target chamber, enabling Orion to meet the
goal of achieving 1021 W cm−2 on any given part of a target
with 5 ␮m pointing accuracy. Schematics of the target area
and target chamber are shown in Figs. 1 and 2, respectively.
There is provision for diagnostics to be mounted on the
chamber at positions which have been determined from a
consideration of a range of experimental geometries. Some
diagnostics will be compatible with the “ten-inch manipulator” (TIM) system4 developed for the OMEGA laser. These
TIM based diagnostics have the advantage of shot-to-shot
flexibility through the use of air locks, and therefore provision is made for TIMs on the following key lines of sight:
4222
© 2004 American Institute of Physics
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Rev. Sci. Instrum., Vol. 75, No. 10, October 2004
FIG. 1. Schematic of the Orion target area.
along the principal axis and transverse to this axis (directly
opposite the PW beams), and along two mutually orthogonal
axes at 45° to the chamber’s principal axis. A large number
of smaller diagnostic ports provide other lines of sight as
dictated by general experimental requirements.
III. TARGET DIAGNOSTICS
Orion will be equipped with an extensive suite of target
diagnostics. The diagnostics can be broken down into a number of categories, for example, optical, x-ray power, x-ray
imaging and neutron. For many of these AWE has many
years of experience, both on the HELEN laser and on lasers
such as NOVA5 and OMEGA,6 and the development of
specifications for these has been relatively straightforward.
In other areas, particularly that of neutron diagnostics, the
capabilities of Orion far exceed those of HELEN, and the
proposed experiments of interest differ from those AWE has
undertaken elsewhere, so that new systems need to be developed. As an example, the primary requirements in this case
are to provide measurements of the total neutron yield, the
ion temperature in the fuel, for standard inertial confinement
fusion (ICF)-style implosions, or the ion momentum distribution, for ultraintense laser-plasma interactions. It is envisaged that the instruments will be based around standard ICF
techniques, employing sample activation and time-of-flight
measurements, however certain aspects of the system design
FIG. 2. Schematic of the Orion target chamber.
Plasma diagnostics
4223
will have to take into account the background radiation associated with the ultraintense laser-plasma interactions. In
particular, activation techniques may be compromised by
共␥ , n兲 or 共p , n兲 interactions, while the time-of-flight detectors
need to be robust enough to yield accurate data despite the
high ␥ fluxes encountered.
Many of the issues affecting the design of the diagnostics suite are associated with this additional background due
to the short pulse laser capability. Although the long pulse
section of Orion is a very small system when compared to
large, megajoule-class facilities such as the National Facility
Ignition (NIF)7 or Laser Megajoule (LMJ),8 the fact that it
has petawatt-level short pulse beams means that the gamma
radiation dose produced will potentially be similar to that
from a high yield fusion shot on one of the larger systems,
albeit more directional in nature. In addition, there will be an
electromagnetic pulse (EMP) effect due to the short pulse
interaction with the target, with predicted transient fields of
7 kV/ m.9 Protection against the gamma radiation must address effects due to both the instantaneous dose rate and the
total lifetime radiation dose. Protection against the EMP
should be provided by the construction of suitably screened
enclosures for sensitive electronic equipment, whether this is
in the form of screened and filtered electronic racks within
the Target Area, or through the use of an “airbox” (in which
the electronics operate in a sealed compartment at atmospheric pressure despite being held within the vacuum chamber) on TIM-based diagnostics, effectively acting as a
screened compartment. It should be noted that while ancillary systems such as the target positioner can be switched off
at shot time in order to minimize problems due to these
effects, this is obviously not true of target diagnostic systems
which have to be active throughout the laser-target interaction.
Many of the core Orion diagnostics will be familiar from
decades of long pulse experiments: soft x-ray spectrometer
systems (“Dante”)10 for Hohlraum characterisation, shock
breakout11 and interferometric techniques, streaked x-ray
spectroscopy, and gated x-ray imaging. Wherever possible
these will be adapted for use with experiments with a fundamentally different time scale, for example, through the acquisition of subpicosecond resolution streak cameras, both
optical and x ray. In other cases complementary techniques
need to be developed. Many of the techniques adopted by the
evolving short pulse field will also be available, however.
These include the use of radiochromic film and track detection in CR39 film for the detection of ionizing radiation, and
the challenge for Orion is to integrate these with the long
pulse techniques. One attractive possibility is the use of proton beams for probing plasma experiments, either for “radiography” or for mapping electric and magnetic fields.12
Other diagnostics which have seen more widespread use
in the short pulse regime, will be available on Orion, are
analyzers such as the Thomson parabola and the electron
spectrometer, which are used to gain deeper understanding of
the fundamental interaction processes, for example, in enhancing the electron yield for the generation of high photon
energy backlighting sources.
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4224
Oades et al.
Rev. Sci. Instrum., Vol. 75, No. 10, October 2004
IV. TARGET DATA ACQUISITION AND TIMING
SYSTEMS
Wherever feasible, target diagnostics will be interfaced
to the target data acquisition system through a set of racks
that network the data to the operations rooms. Timing trigger
signals for the target diagnostics will be controlled through
the timing system through its own specialized software. The
outputs will be either electrical or optical, with the amplitude
adjustable over a range of fixed voltages or optical power
levels. Optical and electrical fiducials derived from very low
jitter, short pulse, delay generators will also be delivered to
diagnostic equipment in the Target Area.
V. SUMMARY
The conceptual design of a target diagnostics suite for
the planned Orion laser has been presented, together with
discussion of some of the challenges still to be overcome.
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