Omega Laser Facility Users Group 2010 Workshop

28–30 April 2010

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Findings and Recommendations of Executive Committee

1. Introduction | 2. OMEGA (60-Beams) | 3. OMEGA EP | 4. Other Facility Improvements
OLUG Executive Committee
Richard Petrasso, Committee Chair, Massachusetts Institute of Technology
Hector Baldis, University of California-Davis
James Cobble, Los Alamos National Laboratory
Paul Drake, University of Michigan
James Knauer, LLE, University of Rochester (designated)
Roberto Mancini, University of Nevada, Reno
Peter Norreys, Rutherford Appleton Laboratory
1. Introduction

The OMEGA Laser Users Group warmly thanks LLE's management for taking actions on the recommendations from our last report and for expeditiously addressing the issues identified therein. It is a strong testament to the professionalism of the facility staff that the tasks were executed with good speed and due diligence. The activities undertaken are a great credit to the Laboratory and to the University of Rochester. They are certainly highly appreciated by the academic and research user community.

The OMEGA and OMEGA EP lasers are world-class facilities that provide academic access to cutting-edge energies and intensities on target. As with any high-performance device, however, critical enhancements are needed to continue to see progress in forefront science. This document describes the academic and research user community's observations, distilled from its April 2010 workshop, of those elements and components that can maintain LLE's leading position at the forefront of high-energy-density physics (HEDP). The OLUG looks forward to hearing progress on these recommendations at the semiannual satellite meeting in November (at the APS Conference) and at its next annual workshop (27–29 April 2011).

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2. OMEGA (60-Beams)
  1. Three independent legs will add substantial improvement and flexibility to future experiments. Greatly expanded experimental design options would develop if it were possible to use all three of the existing drivers, with each driving one of the OMEGA legs. Ideally, each driver would be able to drive any leg to accommodate the constraints imposed by diagnostic configurations. Improved experiments and new classes of experiments would then become possible. The OLUG considers this a high priority.

    Request: Present a plan to the Executive Committee on the implementation of three independent legs. In addition, include this and additional options for discussion at next year's workshop.

    LLE Response: Preliminary concepts for this project have been developed and will be advanced further in FY11. Pending the availability of LLE resources, the implementation will commence in FY11. At a minimum, LLE will complete a plan for the implementation.

  2. Request: Compile a list of qualified diagnostics that can be fielded for joint operations.

    LLE Response: The requested information will be made more readily accessible, and the OLUG Executive Committee will be notified as soon as the data are published on the Website.

  3. It is common to drive experiments with ten OMEGA beams and very common to conduct two separate experiments in one day. The workhorse distributed phase plates (DPP's) are the SG8's. Many users have found that the limited number of these specific DPP's has compromised the quality or productivity of their experiments, especially when they had shots on the second half-day of a shared day. In the area of chamber optics, OLUG would prioritize this item second behind the provision of four 750-µm phase plates for OMEGA EP.

    Request: Acquire enough SG8's to make 20 available to the user community (if this will not jeopardize the higher-priority 750-µm phase plate purchase needed for OMEGA EP).

    LLE Response: The acquisition of additional SG8 DPP's is included in LLE's FY11 budget. The implementation date is contingent on vendor availability. There are currently 12 SG8 DPP's available, and these can continue to be shifted to accommodate campaign transitions as required in the interim period.

  4. Request: Acquire a high-resolution neutron spectrometer to measure neutrons with energies in the 2- to 3-MeV range.

    LLE Response: LLE is working with users to develop the requirements for this diagnostic and will then determine if it is feasible. If so, LLE will initiate a project to implement it.

  5. Upgrade the backlighter driver to operate at the nominal specified performance. The existing backlighter driver is unable to support the full energy that OMEGA's 60 beams are capable of producing. Because most backlighting and x-ray–probing experiments are photon limited, this is a significant shortfall for some experiments.

    Recommendation: Improve the backlighter driver in order to support full-energy operation of the beams that it drives.

    LLE Response: The backlighter source will be reworked in FY11 to increase the available operating envelope to match the other laser drivers.

  6. OMEGA EP petawatt (PW) beam delivering two foci onto target. There is a need for simultaneous pump/probe experiments on OMEGA using the OMEGA EP PW beam. The availability of this feature would greatly increase the capability of the facility and would lead to the undertaking of many new experiments. Examples include hard x-ray radiography of integrated fast-ignition experiments. The OLUG recognizes, however, that space limitations impose a restriction on the provision of a single off-axis focusing parabolic mirror. The OLUG considered the possibility of splitting the PW beam into two beamlets at the final turning mirror to generate two focal spots.

    Request: The OLUG requests that LLE investigate the possibility of splitting the petawatt beam and report back to the Executive Committee.

    LLE Response: In FY11 the beam-combiner optic in the grating compressor chamber will be reinstalled. With this optic, the two short-pulse beams can be independently directed to two foci with a small separation. This capability will be developed and demonstrated on the OMEGA EP target chamber initially in FY11 and may be available on OMEGA in late FY11. The requirements for focal-spot separation, quality, and timing need to be refined to determine if this flexibility will meet user requirements. LLE will provide status updates on this effort to the OLUG Executive Committee. Should a "split mirror" be required, the design, fabrication, and integration effort will likely take of the order of 18 to 24 months. LLE recommends that users who need this feature refine their requirements so that this assessment can be made as soon as practical.

  7. Integrated experiments require smaller focal spots for the petawatt beam. This is the same problem as Issue (g) in the section devoted to OMEGA EP.

    Endorsement: OLUG strongly endorses the development and implementation of the new phase-front corrector as a matter of high priority.

    LLE Response: The FY11 phase-front–correction effort will initially concentrate on a proof-of-principle experiment. If this demonstration is successful, a second-generation device will be fabricated at large aperture to refine the technique. Concurrent with this work is LLE's effort to operate the existing adaptive-optic, closed-loop system as close as practical to shot time to improve repeatability. Finally, high-spatial-resolution phase-front control will be investigated using the active device presented to the OLUG community on 29 April 2010. Note that the implementation of dynamic control may not be complete until FY12.

  8. OMEGA 60-beam timing measurement accuracy. At present, for delays or advances (offsets) of beams beyond about 10 ns from time zero, the accuracy with which one knows the beam timing drops to ±0.5 ns. This often becomes the largest uncertainty in experiments with offsets in the 10-ns to 30-ns range.

    Request: Develop the means to determine the beam's temporal offsets actually achieved on each shot, with an accuracy on the order of ±0.1 ns. Such a capability would significantly improve the quality of experimental results from certain experiments.

    LLE Response: The P510 streak camera system has an option currently available for a 40-ns sweep speed that is calibrated to give timing to <0.1 ns over the requested 10- to 30-ns window. LLE recommends consulting with the Laser Facility Manager during the proposal phase (~2 months prior to shot day) to optimize configuration of the laser streak system to yield satisfactory pulse-shape measurement and precision timing simultaneously.

  9. Photographic documentation of some diagnostic and related systems. To understand complex mechanical systems, it is often helpful to have photographic (and/or CAD) images to complement sets of mechanical drawings. This is definitely true of the ten-inch manipulators (TIM's). We suggest that such images be made available online for the TIM's. Users and the facility should both consider whether there are other systems that would also merit photographic and/or CAD documentation.

    LLE Response: LLE concurs with the request to make a photograph-enhanced diagnostic glossary and will develop a standardized "cut sheet" for each diagnostic. The OLUG Executive Committee will be notified as soon as the data are published on the Website.

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  1. Bring OMEGA EP performance up to its full specification. We fully appreciate that the OMEGA EP IR and UV beam energy must be limited to ensure minimum damage to expensive and difficult-to-obtain optical components during operations with the existing gratings and UV optics. On the other hand, it is OLUG's duty to note that the user community needs access to the full performance of the laser facility to ultimately allow one to explore the exciting new regimes in high-energy-density physics. The constraints, while essential, limit the experiments that have been conducted as well as the results that have been obtained so far. Although it should be pointed out that outstanding results have been obtained with the present operating parameters of OMEGA EP, it is still the desire of the user community to have the facility operating at its specified performance at the earliest practical opportunity.

    Endorsement: OLUG endorses LLE's plan to develop and acquire high-laser-damage-threshold IR and UV optics that will make it possible for OMEGA EP to routinely operate at its design performance. OLUG requests that facility management keep the OLUG Executive Committee apprised of the research and development plans that will make it possible for OMEGA EP to reach design performance.

    LLE Response: LLE is aggressively working to expand the operational envelope limits of OMEGA EP. The IR and UV energies on target are currently limited by laser-induced damage. The constraints are due to the laser-damage–threshold limits of gratings (short-pulse IR) and transmission optics (long-pulse UV). LLE will continue to share progress and plans on this item with the OLUG Executive Committee.

  2. Bring design options forward to next OLUG workshop for 2ω and 3ω conversion of the PW beam. There are some preliminary and tentative indications, from experiments performed in France, that the fast electron energy transfer and heating of background plasma is enhanced with frequency-converted light. This past summer at the TITAN facility, more evidence was gathered that added to the knowledge base. Certainly, much greater control over the energy content of the amplified spontaneous emission pedestal associated with petawatt laser pulses is ensured with conversion to 2ω and 3ω. This has been established as an important factor in cone-guided experiments. It is prudent to have available engineering plans for frequency conversion of the PW beam if required for future experiments by the user community.

    Recommendation: The OLUG requests that facility management provide options for discussion at the next OLUG Workshop for frequency conversion of the PW beam.

    LLE Response: LLE will commence a scope study to identify viable concepts for implementation of short-pulse frequency conversion. There is a high likelihood that conversion crystals, mounts, diagnostics, beam transport, and focusing systems will constitute a significant investment of LLE resources and require an NNSA-approved construction project. A case study representing the need for this item would be beneficial in garnering NNSA support for a capital project.

  3. Two Cu Kα imaging crystal spectrometers are needed for both OMEGA 60 and OMEGA EP. Understanding the physics of fast electron energy transport from petawatt-power laser–plasma interactions with solid targets requires the deployment of sophisticated x-ray diagnostics. Cu Kα imaging spectrometers have been developed in the United States over the past decade and have proven to be powerful tools in diagnosing the heating of dense matter using intense laser pulses. These instruments are required on both chambers so that independent experiments can be carried out simultaneously. In each target chamber, they need to be deployed in orthogonal directions so that spatial nonuniformities can be identified and characterized.

    Recommendation: The OLUG requests that LLE management validate and deploy these imaging spectrometers as a matter of high priority in the coming year. The OLUG recognizes that the deployment of orthogonal instruments may pose difficulties, but the requirement for these instruments for each chamber is vital to the user program. We request that OMEGA provide a deployment plan to the Executive Committee for their consideration and report to the workshop next year on arising issues.

    LLE Response: A prototype crystal imager is being developed for the Multi-Terawatt laser with plans to install a complete system on OMEGA EP in FY11. In the interim a "fast-track" crystal imager was installed on OMEGA EP in FY10. In FY11, LLE will initiate projects to add a second crystal imager on OMEGA EP and two on OMEGA. Depending on resources available and other priorities, these projects may be carried out in FY11.

  4. Two electron spectrometers are needed for both target chambers. Electron spectrometers are important tools for characterizing the behavior of intense laser–plasma interactions. They have applications in many areas, including wakefield acceleration, betatron x-ray source characterization, channel formation, and positron generation, among many others. The development of a university-based program requires the deployment of spectrometers with electromagnets that provide a wide window up to GeV of particle energies. OLUG understands that Lawrence Livermore National Laboratory (LLNL) colleagues have validated and deployed low-energy electron spectrometers in their experiments; these instruments should be made available to the wider community.

    Recommendation: The OLUG requests that LLE management initiate a design and validation program for an electromagnet-based electron spectrometer for wakefield acceleration studies up to GeV particle energies. They request that electron spectrometers based on the LLNL design be made available to the wider academic community.

    LLE Response: In FY11, LLE will initiate projects to ensure that there are two electron spectrometers on OMEGA EP and two on OMEGA. Depending on resources available and other priorities, these projects may be carried out in FY11.

  5. LLE's contrast-ratio improvement program. The OLUG was impressed with the presentation of Christophe Dorrer relating to the intensity contrast issues of the petawatt beam. The audience appreciated the identification of all the different components in the laser chain that contribute to the pedestal over its full range—some nanoseconds ahead of the pulse, some on its leading edge. The enhancement program was warmly received by the audience.

    Endorsement: The OLUG strongly endorses LLE's contrast-ratio improvement program. They fully endorse the time frame outlined and want to be updated on progress at the next OLUG Workshop.

    LLE Response: The OLUG Executive Committee will be kept abreast of all developments in the short-pulse laser's contrast enhancement.

  6. Status of the 4ω probe. The OLUG was impressed with the presentation of Wolfgang Theobald on the fourth-harmonic probe status. The probe commissioning is proceeding with good speed and the audience was delighted with progress so far. Nevertheless, concern was raised that the seeds are not from the same source and much closer attention to minimizing and measuring timing jitter is needed. In addition, the pulse energy is marginal and needs to be increased by an order of magnitude.

    Recommendation: The OLUG requests that the timing-jitter issue be addressed and an additional amplifier stage be added so that the probe energy can be increased when required. These changes should be reported to the Executive Committee.

    LLE Response: The concerns of the OLUG have been communicated to the development team and will be closely monitored through the initial implementation of the 4ω probe laser at its baseline performance level. Any requirements analysis for specific experiments that users can provide to the team would be helpful in motivating changes to the baseline requirements for this system. LLE will work to include design features for operation of the probe with maximum precision timing and as much energy as practical.

  7. OMEGA EP focal-spot size of the PW beam. The community welcomed Brian Kruschwitz's characterization of the OMEGA EP focal-spot quality. They were satisfied that the encircled energy measurements he reported were consistent with other plasma diagnostics. Concern was raised, however, that the focus of the OMEGA EP was insufficient for many future experiments and that effort is needed to reduce the 80% encircled energy radius by a least a factor of 2 (from 40 µm to 20 µm). They were delighted to hear about the new phase-front–corrector technology that will make this development possible.

    Endorsement: OLUG strongly endorses the development and implementation of the new phase-front corrector as a matter of high priority.

    LLE Response: See response to OMEGA Item (g)

  8. Polarization smoothing on all four UV beams. The community was very impressed by the quality of the presentation from Samuel Morse. They received with great pleasure the news of the concerted effort to address issues raised at the last workshop. They were particularly struck by the quality of his argument that the polarization-smoothing technology had reached sufficient maturity to implement on OMEGA EP and that the advantages over smoothing by spectral dispersion were apparent.

    Endorsement: OLUG unanimously accepted LLE's recommendation for polarization smoothing on all beams. They look forward to rapid implementation of the proposal and to receiving an updated progress report at next year's meeting.

    LLE Response: User need for this feature remains unclear. If there is any specific analysis of an experiment that would benefit from this capability, please communicate to LLE since it will aid in justification of the cost. LLE is pursuing this item with no guarantee that the acquisition will be selected for funding in FY11. The time frame for implementation of polarization smoothing is 18 to 24 months after optics are ordered. Every effort will be made to expedite this acquisition as well as the mounts that will support the optics, once the funding commitment is secured.

  9. The planar cryogenic target handler should be implemented on OMEGA EP for users. A number of academics in the community raised the issue of fielding planar cryogenic-deuterium targets for the OMEGA EP chamber. They expressed concern that this technology was not available for cutting-edge transport and hydrodynamic experiments.

    Recommendation: OLUG strongly endorsed the implementation of planar cryogenic target–handling technology as a high priority. The community wants to see this target–handling technology made available at the earliest opportunity.

    LLE Response: LLE will consider the OLUG endorsement of this project in prioritizing laboratory resources in FY11. It is likely to take 12 to 18 months from project inception to initial capability deployment. LLE will keep the OLUG Executive Committee informed of the project status.

  10. A full set (4) of 750-µm phase plates is needed. The community expressed great pleasure that a number of phase plates had been acquired in response to their requests at last year's workshop. After considerable debate over different phase-plate sizes, it was felt that a full set (4) of 750-µm phase plates is needed. These would be useful in the context of the long-duration hydrodynamics and laboratory astrophysics experiments that OMEGA EP uniquely makes possible by stacking beams in time. By running the specified maximum of 6 kJ in a 10-ns pulse, a 750-µm phase plate produces an average intensity of 1.4 x 1014 W/cm2. Using an 1100-nm phase plate reduces the intensity to below 7 x 1013 W/cm2. This is an important difference. One definitely wants to be above 1014 W/cm2 for most experiments. In solid plastics, shocks will be driven not much more than 1 mm in 40 ns; therefore, 2-D expansion is a smaller concern than having adequate intensity.

    Endorsement: A full set (4) of 750-µm phase plates should be purchased and made available to the community.

    LLE Response: LLE appreciates the aggregation of OLUG requirements and distillation to a well-defined recommendation. The acquisition of additional 750-µm distributed phase plates to obtain a full set of four operational optics is in the FY11 acquisition plan.

  11. Requirement for a limited-reservoir gas-jet target. Some academics expressed the view that gas-jet targets should be implemented on OMEGA EP. They accepted the facility management's argument that an unlimited reservoir gas line might put the OMEGA EP compressor grating actuators at risk. They recognized, however, that many experiments would benefit from a gas-jet arrangement. It was felt that the burden of commissioning this technology should not fall on a single institution but should be shared as a common resource. They agreed that a limited reservoir would add capability to a wide range of experiments.

    Recommendation: The facility should design, validate, and implement a limited-reservoir gas-jet target and make it available to the user community.

    LLE Response: Development of equipment to provide a gas jet in the Omega Facility will require significant resources. Users interested in this capability should contact John Soures, who will facilitate formation of a subcommittee to investigate the development of this capability. LLE resource allocation to this project is subject to balancing laboratory priorities. Project selection depends on developing a set of requirements that can be met within the system's safety constraints.

  12. Requirement for added flexibility for long-pulse operation. The community was delighted by the performance of the long-pulse beams in their current configuration. The arrangement had produced some outstanding results that were received by the audience with acclaim. After much debate, the community felt that if different ports, originally earmarked for long-pulse operation, were made available, it would add substantial flexibility to experiment design and effectiveness. They endorsed the option of irradiation from opposite sides as one example.

    Recommendation: The OLUG recommends that all ports originally earmarked for long-pulse operation be brought into facility capability. The OLUG welcomes a report at next year's meeting on progress with options.

    LLE Response: The 48°-cone-angle ports were proposed as an option in the OMEGA EP project, but it was not supported by the users or NNSA at that time. It would be very expensive to fit out with UV-beam–transport paths, which would take significant facility time and would require capital project support from NNSA. A potential smaller-scale, higher-payoff alternative, suggested at the OLUG meeting in April 2010, was to bring one or more beams to the back side of the target chamber. A feasibility study will be conducted to assess potential UV-beam routing to the opposite side of the target chamber from the existing 23° UV-beam ports.

  13. Equivalent-plane monitors for all UV beams. The community felt that while the focal-spot monitor was adequate for some experiments, all experiments were complicated by a lack of knowledge of the quality of all beams away from their foci.

    Recommendation: Equivalent-plane monitors should be implemented for all beams. The OLUG welcomes a report on this issue at next year's meeting and trusts that this can be implemented with little disruption to operations.

    LLE Response: The request for UV far-field measurements on all beams will be carefully considered. It would be helpful if the experimentalists requesting this feature would provide a range of spot sizes of interest for measurement. The UV diagnostic package has provision for locating a far-field monitor, and the LLE plan is to implement a phase-plate holder in the diagnostic path for at least one beam. LLE will keep OLUG apprised of the progress in meeting this recommendation.

  14. A new gamma-ray spectrometer diagnostic is required. The community felt that a new gamma-ray spectrometer needs to be designed, validated, and commissioned for the facility. Users requested that the spectrometer have a spectral range up to 20 MeV.

    Recommendation: The OLUG requests that management consult the Executive Committee on the design, validation, and implementation of this new instrument for the community.

    LLE Response: OLUG scientists interested in this spectrometer should contact John Soures, who will facilitate formation of a subcommittee on high-energy photon spectroscopy. Once a consensus of requirements for sensitivity, spectral range, resolution, and other features is generated, the subcommittee and OLUG Executive Committee can provide LLE with more definition of what is desired. LLE resource allocation to this project is subject to balancing laboratory priorities.

  15. See also Items (f) and (g) of OMEGA (60 beams).

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4. Other Facility Improvements
  1. Dedicated user support for experiment design and theoretical modeling. To maximize the productivity of both OMEGA (60 beams) and OMEGA EP, it is important to have access to theoretical and computational support for both experiment design and data interpretation for university-based academics. Individual academics, who are motivated by curiosity-driven research in high-energy-density science, often find that they do not have access to the necessary sophisticated computational models at all stages in the training cycle. The very nature of university life, where there is a high turnover of doctoral and postdoctoral research fellows, means that modeling skills nutured by the teams and applied to specific problems can suddenly evaporate, leaving them without vital tools at critical stages in the research effort.

    Ideally, the theoretical support team must be located within the larger-scale facilities. It must be of critical size to provide the facility users with a wide range of modeling capability (e.g., one- and two-dimensional magnetohydrodynamic simulations, implicit and explicit particle-in-cell and hybrid models for high-intensity laser–matter interactions, quantum molecular dynamics for warm dense matter studies, etc.) and have dedicated access to large-scale, high-performance computing resources. Here are some desirable characteristics of the support team:

    • Team members should be involved at all stages in the experiment cycle, obviously requiring a collaborative approach by all parties.
    • The team must be university focused and highly responsive to user demands. The team should assist with interpretation of smaller-scale experiments undertaken on university-scale facilities. This is particularly important to the reduction in cost and size of intense laser systems and the proliferation within academic institutions.
    • The team must be of sufficient size so that members have enough time to develop their own research interests, in addition to their support duties. This should allow team members time to devote to the development of new codes, algorithms, and possibly visualization routines.
    • The team must be involved in training students and postdoctoral fellows in high-energy-density science, e.g., by co-supervision of Ph.D. students, etc.

    Clearly, a balance must be struck between supporting experiments themselves and maintaining a critical size in the modeling support team.

    Recommendation: The OLUG requests that LLE management establish a university-focused support team, ideally comprising four staff members, with academics from other programs at LLE and/or other institutions. The team will make a huge difference in the quality and depth of publications arising from the facility.

    LLE Response: After the 2010 OLUG report is completed, LLE will request additional funding from NNSA and the Office of Science to support this request. LLE may request a letter from the Executive Committee endorsing this request.

  2. Increased support from DOE to the National Laser Users' Facility (NLUF) Program. The OLUG recognizes and applauds the increased funding to the NLUF Program in FY11/12 from the DOE ($1.3M to $1.6M). This has allowed facility management to increase access under the NLUF Program for the university community. The OLUG is also highly appreciative of the efforts of DOE officials to achieve this increase. The OLUG firmly believes that the establishment of new academic positions for young researchers in high-energy-density physics, working in partnership with the established academics and consortia (e.g., the Fusion Science Center), is a high priority for the growth of the field.

    Recommendation: The OLUG strongly urges DOE to maintain the percentage increase in funding to the NLUF.

    Recommendation: The OLUG urges DOE to develop and fund a program to accelerate the career progression for the brightest young dynamic researchers working in experimental high-energy-density physics by sharing the cost of their appointment with universities while they are at the assistant-professor level.

    LLE Response: LLE recognizes the value of these requests, but clearly it is an issue for OLUG to communicate directly with DOE.

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