HiPER Targetry: Production and Strategy M Tolley1, F ben Saïd2, E Koresheva3, J-P Perin4 J M Perlado5, G Schaumann6, G Schurtz7, C Spindloe1 1 Central Laser Facility, STFC Rutherford Appleton Laboratory, UK 2 Commissariat a L’Energie Atomique, Valduc, DRMN,Service Microcibles, France 3 Lebedev Physical Institute, Russian Academy of Sciences, Russia 4 Commissariat a L’Energie Atomique, SBT, Grenoble, France 5 Universidad Politecnica de Madrid, Spain 6 Technische Universität Darmstadt, Germany 7 Centre Lasers Intense et Application, Université Bordeaux 1, France Main correspondent: [email protected]

ABSTRACT The HiPER project is moving into an R&D phase with an increasingly clear vision of the stages required to demonstrate inertial fusion energy (IFE) as a power source. One of the major technical challenges will be to demonstrate the production and delivery to chamber of microtargets. The project baseline targets and Targetry-relevant system requirements are reviewed. An update is given of the current status of the HiPER Targetry workpackage summarising the coordinated range of progress which has been made within the project’s Preparatory phase. A forward strategy is then presented in the context of the Targetry technology development plan. The full delivery plan is complex and only its essential structure will be presented in this paper focussing primarily on mass production issues and risk reduction. General technical issues of significance for Targetry are also discussed.

Keywords HiPER, microtarget fabrication, IFE, mass production

1 INTRODUCTION HiPER will be a European laser-driven fusion demonstration reactor facility in which Inertial Fusion Energy (IFE) can be studied 1. The main goal of HiPER is to study the technology and physics of laser-driven fusion as a basis for commercial IFE reactors. At the beginning of the Preparatory phase of the project the major technical challenges for HiPER Targetry were identified and subsequently allocated as the foci of work for individual groups. Throughout the Preparatory phase group activities were progressively integrated feeding into a coordinated, forward-looking strategy forming the Targetry sections of the HiPER Business Case. This article, published at a time which is at the end of the Preparatory phase and just as HiPER moves into its next phase, has two main and interwoven themes: to review the Targetry work accomplished so far and also to look forward and indicate how the Targetry activity can be developed to support the needs of HiPER to the point of IFE demonstration. A considerable amount of (published) work has been conducted over the past fifty years in microtargetry covering a broad sweep of microtechnologies which have been integrated. Microtargets probably represent some of the most complex and demanding micro-objects ever constructed, for example the cryogenic targets for NIF and LMJ. However, the challenges of mass production for microtargetry are only beginning to be explored, although it should be immediately noted that high

Diode-Pumped High Energy and High Power Lasers; ELI: Ultrarelativistic Laser-Matter Interactions and Petawatt Photonics; and HiPER: the European Pathway to Laser Energy, edited by J. Hein, L. O. Silva, G. Korn, L. A. Gizzi, C. Edwards, Proc. of SPIE Vol. 8080, 808023 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.891879 Proc. of SPIE Vol. 8080 808023-1 Downloaded from SPIE Digital Library on 04 May 2012 to 193.232.69.1. Terms of Use: http://spiedl.org/terms

fidelity microproduction is already an everyday capability in many commercial sectors, primarily the MEMS market. Initial experience with microtarget mass production has shown that the challenge of target placement is intimately related to microtarget production and the two activities need to be approached in an integrated way from the onset 2. (Note: Throughout this article the term “microtargetry” includes both microtarget fabrication and microtarget placement. “Placement” means placing the microtarget with sufficient linear and angular accuracy in a position for shooting. “Insertion” means holding the microtarget in position using a physical support during a shot. “Injection” means firing the microtarget into position using an injector and the microtarget does not have a physical support but is shot while in unsupported motion.)

2 HiPER TARGETRY REQUIREMENTS 2.1 Baseline Target Designs There are currently two baseline target designs for HiPER: Shock Ignition and Indirect Drive (figure 1). Throughout most of the Preparatory phase an Advanced Fast Ignition (AFI) cone + shell target design was also studied within the Targetry workpackage. Many important techniques were learned from work on AFI targets which can be generalised to other target designs and components, most notably hohlraum cans. It was understood from the project onset that the baseline designs would almost certainly be different to those of the final targets, indeed it might be impossible to actually make the baseline targets. However, it was imperative to have the baseline designs to enable Targetry work to begin. Additionally it was agreed at the beginning that throughout the entire HiPER project there would be an ongoing iterative process between the Targetry, Modelling, Facility Design and Experimental activities to mutually refine the designs at specified stages. This reflects the nature of HiPER as being an end-directed project to build an IFE power station rather than a physics driven project. Specifically, for example, target production capabilities may well require a loosening of target specifications, particularly if economic viability of a power plant is a major project driver.

Baseline Target Types SHOCK IGNITION CH shell (3μm thick): 2.088mm ID, 2.094mm OD

INDIRECT DRIVE Low-density, low-Z foam (~1 mg/cc)

Shields for P2 asymmetry

DT layer (211μm thick): 1.666mm ID, 2.088mm OD Fuel mass ~ 0.6mg Temp. 16-19.6K

High conductivity LEH window

Hohlraum radius

3.3 mm

Hohlraum length

11.9 mm

Window material

Polyimide (Kapton)

Window thickness

0.5 µm

Hohlraum wall material

Lead

Hohlraum wall thickness

25 µm

Figure 1. HiPER Baseline Target Designs.

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2.2 Repetition Rate It is stated in the shot rate document that there will be three modes of operation initially in HiPER 4a: Scenario 1: 100 shots in a burst – no yield shots (HDT or similar targets) Scenario 2: 100 shots in a burst – including 2 consecutive yield shots Scenario 3: 100 shots in a burst – including 5 equally spaced yield shots (A burst is a continual stream of targets running at 5-10 Hz.) Additionally for HiPER 4a the possibility of providing a burst of 100 yield and 1000 (mixed) shot bursts will be considered under scenarios 4 and 5. For HiPER 4b, to demonstrate power production capability, the targetry production and injection requirements are to operate at 10Hz continuously for several days with all shots potentially being full yield shots. To run an IFE reactor in continuous operation at 10 Hz will require ~900,000 targets a day, and to run at 16Hz will require ~1,500,000 targets a day. This clearly has extensive implications for mass production of components and assemblies. A key issue in HiPER will be to assess the targetry requirements for IFE high gain high repetition rate scale-up. Such targets will have the same general features as single shot targets but the emphasis will be on demonstrating high number scale-up capabilities and new processes for production will almost certainly have to be developed. Target mass-production, injection and tracking are key demonstrators for proving laser-driven fusion as a realistic option for commercial energy production. 2.3 Particular Targetry Challenges As previously mentioned early identification was made of the challenges from a microtarget fabrication perspective. There are many significant challenges that are associated with high gain IFE targets but some which received particular attention in the project follow. 1) Targets will almost certainly have a thin-walled microballoon component with an internal layer of deuterium/tritium (ice). 2) For some targets the layer may be carried on foam (particularly to remove the need for layering). 3) Indirect drive targets will require the production of hohlraums that may be made of lead. 2.4 Survivability Target survival is defined as being the retention of the capability of a target to undergo fusion at the appropriate point in its lifecycle. From its manufacture until its ignition during the fusion event within the target chamber a target must retain its fundamental properties by being able to accommodate all perturbations and environmental changes to which it is exposed. The life of a target for HiPER can be considered in eight key stages these being; production, storage, transport to injector loader, injector loading, injection, separation from sabot, steering and exposure to the chamber environmental conditions. 2.5 Responding to the Requirements In the Preparatory phase specific areas of concern within the Requirements were identified and the major aspects of the work performed, usually by individual partners, is summarised in section 3. The approach to scaling up Targetry to IFE rates (via HiPER 4a and 4b) , directed by the Requirements, is given in section 4 of this paper.

3 PREPARATORY PHASE WORK 3.1 Partners’ Work Areas During the Preparatory phase individual partners worked in particular areas of Targetry. They are listed here and the institutions are those of the authors given at the beginning of this paper. CEA (France), cryogenic single shot targets and modification of the LMJ Inserter to shoot them. LPI (Russia), cryogenic fuelled shell rapid layering. General Atomiics (US), injection, tracking and engagement. UPM (Spain), advanced target materials. TUD (Germany) medium repetition rate cryogenic targetry. STFC (UK) microtarget mass production. Good and increasingly integrated progress was made in many areas. Some specific aspects are highlighted in the following sections.

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3.1.2 CEA Single Shot Targetry and AFI Targets CEA have produced AFI targets (figure 2) by the production of thin walled micro shells and the insertion of a gold cone. The shells are produced using droplet orifice techniques to fabricate Poly(AlphaMethylStyrene) (PAMS) mandrels which are then coated with Glow Discharge Polymer. The GDP targets are laser drilled to produce openings for the cone and the fill tube and then the targets are assembled. Capillary tubes have been fabricated to fill the shells with helium and leak tests have been carried out at cryogenic temperature. Importantly the trials demonstrated that it is possible to form a glue joint that is sufficiently leak tight at 77K. (The leak rate was less than 1 x 10-8 mbar.L/s)

Figures 2a,b,c. AFI Cone with fill tube, assembly of the cone shell and the final target.

3.1.2 LPI Cryogenic Shell Rapid Layering The Lebedev Physical Institute has proposed the Free Standing Target (FST) technology for filling and layering targets for HiPER. Test models for the layering module have been designed (figure 3), and layering units for undersize shells have been built and demonstrated to produce layered shells, the layering time being of the order 10s. LPI has also proposed designs for a full scale target delivery system for the HiPER facility running in burst mode (100 shots) with sabots carrying the target through an electromagnetic injector to the target chamber.

Figure 3. FST Layering prototype for HiPER facility (designed by LPI).

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3.1.3 STFC Mass Production STFC has been developing the capability to mass produce components for IFE targets. Initial work had focussed around the batch production of AFI cone targets (figure 4a). However, this work has be transferred to demonstrate batch production of hohlraum components for the HiPER baseline design (figure 4b). Initial trails have indicated promising results in terms of surface finish and yield. Future work to develop complex geometries and other materials are in progress.

Figure 4a.b. AFI cone and hohlraum batch produced components.

4 MOVING FORWARDS AND THE BUSINESS CASE During the later stages of the Preparatory phase a coordinated review was taken of the Targetry work in the light of the Requirements (which had been defined by then) for subsequent phases of HiPER and IFE. The review then formed the basis for Targetry section of the Business Case. 4.1 The Business Case In its simplest form the Targetry section of the Business Case (BC) 1) reviews the current Technology Readiness Levels (TRLs) of all aspects of Targetry and 2) gives a detailed Technology Development Plan (TDP). This is done in a way that shows risk reduction. The main purpose is to demonstrate a credible way to develop Targetry to meet the Requirements of HiPER. Because Targetry spans many technologies the Targetry section of the BC is complex and only crucial aspects are given in this section. The particular nature of Targetry enables separate technologies to be developed almost independently early in the R&D phase and progressively integrate them throughout 4a and then fully in 4b. This allows significant de-risking during the R&D phase as well as more flexible funding opportunities. 4.1.1 Shell Production There are three potential techniques suitable for shell production for HiPER; 1) wet chemistry/ thin film coating, 2) atomic layer deposition (ALD), and 3) dielectrophoretics 3.

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The introduction of innovative production techniques, particularly those which are only recently of sufficient maturity to be applicable to shell production, may offer opportunities for cost-effective scale up. It is immediately noted that the different baseline target designs require significantly different amounts of post-production processing for shells. Most notably cone + shell targets require the placement of a re-entrant cone into the shell. Furthermore there are potentially modifications to shells that may result from the mode of filling. In all cases shell sphericity and roughness requirements are stringent. Specifications may vary between target types. 4.1.2 Non-shell component Production Current capability is for batch sizes of 50 using established ultra precision micromachining and coating techniques. However, the established process is not realistically scaleable to HiPER 4b. Several currently maturing technologies have the potential for producing the high numbers of ultraprecision microcomponents which will be required. Hot pressing and Metal Injection Molding (MIM) techniques are potentially appropriate for HiPER and IFE. 4.1.3 Micro-assembly This task is the assembly of targets consisting of more than one component (assuming that it can not be manufactured already assembled). Currently this is, at best, a semi-automated task but for volume manufacture under the levels of accuracy required increasing levels of automation will be necessary. R&D projects in robotic microassembly are underway. This includes pick and place, adhesive application (and cure) and real-time optical recognition within a production environment. The robots have a number of axes of freedom and can be combined with other assembly machines to form an integrated microassembly solution. With sub-micron accuracy and computer control software it is possible to pick and place parts ready for glue application and curing. This programme will increasingly support high rep rate target production and the initial stages of the project are showing considerable scope for addressing the many technical challenges. Alternatively, high volume production/assembly may be based on wafer-based fabrication techniques. 4.1.4 Target Fill and Layering Target manufacture, fill and layering techniques vary widely between the baseline target types. There are, however, two main modes for filling: injection filling (in which the fuel mixture is injected into the shell through a microhole bored in the wall, typically via an attached ultra small bore fill tube) and permeation filling (in which the fuel is forced to diffuse through the shell wall using elevated external pressure). Generally, however, and cutting across target types, layering, if used, will probably be performed in one of three regimes; 1) small batch processing of, say, 1 - 100 (or possibly 1000) targets at a time using a LMJ-scaled layering chamber which is of particular relevance for HiPER 4a. 2) Large batch processing of, say, 10 000 – 1 000 000 targets at a time using, for example, a fluidised bed technique for application on HiPER 4b. 3) Continuous production, running at 10 Hz possibly with parallel production units, based on, for example, microencapsulation or microfluidic technology, again for HiPER 4b. 4.1.5 Characterisation During manufacture and processing shells can currently only be characterised slowly, particularly for the outer roughness (which is data of great significance for assessing target viability). Looking ahead to IFE the characterisation needs to be done quickly, possibly on a statistical basis, and feed back to the continuously optimise the target production line. Historically shell parameters have been used which are particularly suitable for physics modelling, however, for a production environment, especially HiPER 4b, there is significant scope for choosing parameters which are rapidly applicable in a high throughput microproduction environment. Throughout production and (if it occurs) storage fuelled targets experience tritium decay/heating issues that may affect the final target ignition viability. Characterisation will need to be deployed to assess potentially deleterious changes. Characterisation equipment will be working in a challenging environment (cryogenic temperatures, elevated radiation) and also have to analyse targets in a way that does not affect the measurements. 4.1.6 Other Technologies It is clearly understood that tritium handling procedures will be a major part of the Targetry activities. However, the procedures are mature and understood well.

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4.1.7 Cross-cutting Technologies and Evolution Shell production processes, target types and fill techniques are highly inter-related issues and can not be considered in isolation from each other. Within the evolution of the full HiPER programme it is inevitable that there will be refinements and innovations in target design. Specifically, for example, foam inner layers, radially graded materials and ultra hard materials have been discussed. External (metal) coatings have also been discussed, primarily as IR reflection coatings whilst the target transits the interaction chamber.

4.2 Pragmatics of Progress The Modelling, Experimental, Targetry and Chamber Design work within HiPER are very strongly inter-related. Specifically, the progressive refinement of the (base line) target design(s) will be achieved through controlled, iterative interactions. This will be a two way process: target designs will be refined in response to results from both modelling and experiments and at the same time R&D work performed within Targetry will progressively establish the range of target designs which can be practically produced. As a specific example the target design is influenced by (a range of) factors such as the method of filling and the injection velocity. If the targets are injected at high speed, then there is less black body radiation absorption from the chamber walls; therefore there is less or no need for coating which gives suitablility for permeation filling. If the injection velocity is low (leading to a higher positional accuracy and less demand on the tracking system) then the target will be heated by the black body radiation from the chamber and a coating may be required to reflect the radiation and the coating may necessitate a fill tube for shell filling.

4.3 The HiPER Target Fabrication Facility Target production requires the following: A specifically designed building including: a. A cryogenic target production area where liquefied tritium/deuterium can be added to targets within a glovebox environment on stable foundations (local decoupled foundations) similar to semi-conductor manufacture b. Nuclear ventilation including tritium recovery processes to minimize the discharge of tritium to atmosphere c. Target characterization equipment to demonstrate compliance with finalized target specifications d. Cryogenic target storage e. Cryogenic target transfer to a magazine (or similar) suitable for interfacing with the target injector The remit of the HiPER development facility may be satisfied through manual target production which is currently demonstrable as a technology however, there are requirements to be satisfied during the lifetime of HiPER to reduce the cost per target significantly and further, to produce targets at a rate commensurate with the operation of a reactor. This can only be achieved through the automation of the target stream. This therefore imposes two further requirements on the HiPER project under the assumptions that a reactor will be operating at 10Hz and 24 hours per day during phase 4b: 1) The HiPER project shall provide the technology basis to demonstrate yield target production at a rate of ~1 million targets of an appropriate quality per day in a scalable fashion. 2) The HiPER project shall demonstrate a cost per target under the above regime of less than 1Euro per target to be achievable (this being within a factor of ten of commercial power production cost requirements). Targets within a reactor environment would also have to be filled with tritium, predominantly bred within the reactor blanket. Deuterium fuel will need to be refined from (sea) water in sufficient quantity to provide the D fuel and/or recycled from that unburned within the reactor. This imposes one further need on target production to enable the future reactor: 1) The HiPER project phase 4a shall demonstrate the viability of the process for recovery and if necessary, refining of tritium from blanket material in suitable quantities and at a suitable cost to provide a continuous fuel source for the reactor in a scalable fashion.

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5 CONCLUSIONS Significant progress was made with Targetry during the Preparatory phase of HiPER. An integrated review of the Preparatory phase work was used to respond to the Requirements for subsequent stages of HiPER and IFE. A robust Technology Development Plan (including risk reduction) has been developed for the Targetry sections of the Business Case. A credible and realistic way has been given to demonstrate target production capabilities suitable to support HiPER and IFE.

References [1] http://www.hiper-laser.org/index.asp [2] High repetition rate laser systems: targets, diagnostics and radiation protection. L A Gizzi et al, (2nd ICUIL) AIP Conf. Proc. 1209 pp 134-143 (Feb 2010). [3] http://meetings.lle.rochester.edu/TFab_2010/documents/Harding.pdf Presentation from 19th Target Fabrication Meeting, 21-26 Feb 2010, Orlando, Florida

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