Berkeley Lab

ATAP News, December 2016

DIRECTOR’S CORNER

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Years of Laying the Groundwork Led to Today’s Exciting Opportunities


In the top highlight of a year with many candidates for that honor, the Advanced Light Source Upgrade, which is among the Laboratory’s highest prorities and will be among the biggest endeavors in its history, received Critical Decision Zero, the “statement of mission need” from the Department of Energy that marks its beginning as an official project.

We have also been making ongoing critical contributions to major projects elsewhere in the DOE complex, including the Linac Coherent Light Source-II project at SLAC and the High-Luminosity Upgrade of the Large Hadron Collider. These efforts bring together longtime core competencies of ATAP and are reminders of our origin in Lawrence’s collaborative, multidisciplinary team-science approach.

This multi-institutional teamwork may be seen in smaller-scale efforts as well: recent examples described in this issue highlight achievements of our newly constituted Accelerator Modeling Program, which has made a technical breakthrough in a vexing problem together with colleagues at the University of Maryland as well as the University of Hamburg and DESY; and an accelerator stewardship-funded project that teams us with the University of Michigan and Lawrence Livermore National Laboratory in developing ways to combine high peak power with high average power in ultrafast lasers.

I invite you to read on and learn more about these efforts, and to take a look back at our 2016 news as we turn our eyes toward new opportunities for progress in 2017.

REVIEWS, TECHNICAL PROGRESS HIGHLIGHT LAB’S LCLS-II EFFORTS

— SXR, HGVPU, LLRF, Injector Source, Modeling All Make Strides

The SLAC-based multi-laboratory project to build LCLS-II, a next-generation free-electron laser light source, recently had two major reviews. The SLAC Director’s review of LCLS-II was held August 30–September 1, and a DOE review followed on October 12–14.

The DOE review was held at Fermilab, so attendees also had an opportunity to see the first of two prototype cryomodules under test. Production and testing of this prototype cryomodule (on which our part of the low-level RF controls effort has been taking data) is a significant development in the project.

LBNL’s contributions to the project received very positive comments from the review committees, and we have made great progress in production of major systems for LCLS-II.

Pre-Production SXR Undulator Tuned, Measurements Underway

Final design and production changes have been implemented on the LBNL-designed pre-production soft-X-ray (SXR) undulator, and magnetic measurements and adjustments to provide the required exquisite field quality have been made in the LBNL magnet measurement facility. Tuning of the device is complete, and performance is excellent, exceeding the stringent specifications for this X-ray FEL.

LCLS-II_SXR_preproduction_600x448y Pre-production SXR undulator segment. Its main features include magnetic modules and a strongback structure to position them — and move them upon command — with reproducible micron accuracy in the face of tens of thousands of kg of magnetic force. Each undulator array will consist of many 3.4 m long individual segments like this one: 21 in the case of the SXR undulator.

Some production undulator segments will be assembled by mechanical vendors, others by LBNL. The main LBNL assembly shops in Building 77 are preparing for the production run, which will begin in early 2017.

High-precision magnetic field measurements are made to determine the small field errors that remain even after the very precise mechanical assembly of the undulator components. The LBNL design allows for correction of these errors using various techniques so that the errors stay within tight tolerances even as the undulator gap is exercised over its full range.

sxr_reproducibility_700x385y As shown at left, the SXR undulator module tuned and tested in our magnetic measurements facility has displayed outstanding field reproducibility across several cycles of its full range of adjustment. The excellent performance achieved with the pre-production unit, incorporating all components and assembly processes that will be used in production, gives great confidence that the devices to be installed at SLAC will perform flawlessly. Indeed, the magnetic field achieved by the design exceeds the requirements; it should be possible to extend the tuning range of the devices and thus produce a larger range photon energies than required by the project’s Key Performance Parameters.

Production is proceeding with vendors, and plans call for delivery of the first assembled units to SLAC in early 2017. Integration of the magnetic and mechanical sub-systems into final assemblies ready for tuning will be done both by a vendor and at LBNL, supporting a fast and flexible production schedule.

HGVPUs Advance Into Pre-Production

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HGVPU pre-production magnet module

A pre-production HGVPU (Horizontal Gap Vertically Polarized Undulator) module for the hard-X-ray beamline is in the final stages of assembly at LBNL’s Building 77 assembly shop. The completed module will be moved to the magnet measurement facility in the same building, where it will be measured and tuned in a similar manner to the pre-production SXR undulator.

Modifications to the magnet measurement facility required to measure the vertically polarized undulator, as opposed to the more usual horizontally polarized devices, will be made in December in preparation for receiving the pre-production device.

In parallel with the pre-production unit development, procurements are being placed with vendors for the 31 production units that will be used for the hard-X-ray beamline of LCLS-II.

Injector Source Moves Cleanly Into Fabrication

The VHF gun component fabrication is well advanced, with several sub-assemblies at or near completion. The gun, an evolution of LBNL’s Advanced Photoinjector Experiment (APEX) gun design, is a complex of sub-assemblies, so it requires many steps of precision machining, brazing and welding, quality-assurance tests, and integration. The LBNL engineering team and Building 77 machine shop staff are working closely with each other and with vendors to ensure quality and successful completion of the gun.

gun nosepiece gun cavity wall Some of the components of the VHF gun being machined and assembled at LBNL; brazing and e-beam welding are performed by local vendors. Far left: Nose assembly with welded water fitting connections. Left: VHF gun cavity wall machining with vacuum grill, undergoing coordinate measuring machine (CMM) measurements. Below: Gun cathode wall assembly in the braze oven.

cathode wall assembly

Cleaning and assembly of the injector source gun and beam line vacuum surfaces is critical to keep particulate matter from contaminating the superconducting RF cavities connected to the downstream end of the injector source. Equipment for cleaning these surfaces using dry ice (frozen CO2) has been delivered, and modifications to the LBNL clean room in the assembly shop area are under way to provide an isolated area within the clean room with the even lower particle count required by this delicate operation.

Modeling To Have Direct IMPACT on X-Ray Output

Following the recommendation of the Facilities Advisory Committee, which met in July, the LBNL LCLS-II accelerator physics team has been funded for additional studies using the LBNL-developed IMPACT codes to optimize the FEL output. In the past, optimizations have been based on electron beam parameters. We are now developing approaches to adjust the accelerator settings to optimize X-ray pulses directly, based on Ming Xie parameters (FEL scaling laws formulated by the late Ming Xie, a scientist with ATAP’s predecessor, the Accelerator and Fusion Research Division).

Low-Level RF Systems Progress

A prototype low-level RF (LLRF) control system, developed under LBNL technical leadership by a team that includes Fermilab, Jefferson Lab, and SLAC, is installed at the Fermilab cryomodule test facility, and is taking data on the first LCLS-II prototype cryomodule. This provides critical information on cavity field stability and is an important part of the prototype cryomodule testing plan. LBNL staff have been involved in hardware installation and operation at Fermilab, and also remote data acquisition and analysis.

These systems will also be tested on the second prototype cryomodule built at Jefferson lab. Testing is planned on the cold cryomodules at Fermilab and at Jefferson Lab through the winter.


STACKING THE LASER EXPERTISE OF MULTIPLE LABS FOR FUTURE ACCELERATORS

Laser physicists are always hungry for speed, power, control, and efficiency (and if you ask them which is the most important for any given application, the answer might well be, “Yes!”) A multi-institutional project headed by ATAP is developing a new way to give it to them by combining the beams of many fiber-optic lasers.

The researchers are developing a “coherent combining” approach that will stack ultrafast laser pulses to achieve both high peak power and (by obtaining the high peak power pulses at a fast repetition rate) high average power.

“These are enabling technologies for laser parameters that the laser wakefield accelerators of the future will need,” says ATAP Director Wim Leemans. Laser wakefield acceleration is a promising young technology in which ATAP’s Berkeley Lab Laser Accelerator Center (BELLA) is one of the leaders.

One of the most important facets of this project, and the one where LBNL has made its greatest contributions, is developing ways to combine the pulsed beams of many fiber-optic lasers. “We’re working on scaling the combination of ultrafast pulses to the numbers of lasers now only possible with continuous signals,” says Russell Wilcox, ATAP’s senior laser engineer on the project.

pulse stacking layout
Block diagram of laser architecture shows the interrelationships of the concepts we are working to combine in order to achieve stepping-stone and, ultimately, HEP LPA driver levels of performance.

Berkeley Lab expertise in both lasers and optical control systems may be the key, together with the leadership in large-core, ultrashort-pulse fiber lasers at the University of Michigan and in high-power fiber lasers, as well as modeling, at Lawrence Livermore National Laboratory. The idea came from Dr. Almantas Galvanauskas of the University of Michigan while he was on sabbatical here, and is now moving forward in collaborative efforts at all three institutions under the overall leadership of ATAP Director Wim Leemans.

The advancements that the researchers are exploring include

  • Coherent Pulse Stacking. Extractable pulse energy is presently limited by detrimental nonlinear effects, as well as by the potential for optical damage. This effort will ensure efficient extraction of energy from fiber lasers while minimizing nonlinear effects.
  • Coherent Beam Combining. The energy that can be stored in a fiber is limited by the aperture size. This work will add the output of many individual fiber lasers in order to scale pulsed power from 50 mJ/fiber to 3 J/system.
  • Coherent Spectral Combining. Pulse duration is limited to hundreds of femtoseconds by a phenomenon called “gain narrowing.” By coherently combining several spectrally adjacent pulses, we hope to improve the pulse width of current ultrafast fiber lasers from a best case of about 300 fs to less than 80 fs.
  • Pulse Quality Improvement. This area of research will help us understand what is needed to make fiber lasers give the pulse quality of titanium:sapphire lasers such as BELLA in order to meet the needs of specific potential stewardship customers.

“The team has demonstrated both spatial and temporal pulse combining with optical interferometers,” says Wilcox. An important next step in their efforts is a proof-of-principle demonstration in which spatial and temporal pulse combining, hitherto achieved separately, work together. Adding the spectral dimension is among the near-term goals of the three-year project.

Benefits en route to the ultimate goal

This technology offers a long-term R&D path towards an electron linear collider that could meet the science needs of high-energy physics with much lower cost and smaller footprint than today’s collider technologies. In the relatively near term, LPAs as “tabletop” synchrotron-light sources would have societal benefit in fields ranging from materials-science research to nuclear security.

The dream of an LPA relevant to high-energy physics calls for lasers far beyond anything we can build today; the requirements are high energy (~40 joules) in short pulses (30–100 femtoseconds) at a high repetition rate (>10 kilohertz), with overall “wall plug” efficiency of 30% or greater. A natural stepping-stone, beyond the present state of the art but within reach, would be a 3 J, 1 kHz laser. This project is developing enabling technologies for a next step in that class.

The effort is aligned with the Advanced Accelerator Development Strategy Report, a DOE Office of High Energy Physics document that lays out a roadmap for reducing the cost of a next-generation electron collider, including ten-year goals for three potentially game-changing technologies, including the laser wakefield acceleration—the technology of BELLA Center. (For more about the roadmap, see “Laser R&D Focuses on Next-Gen Particle Collider” by Glenn Roberts, Jr. of Berkeley Lab Public Affairs.) Leemans describes the roadmap report as “giving us an anchor in the whole accelerator program” outlined for the DOE national laboratory complex.



AVOIDING THE WAVE THAT WASN’T THERE

— New Computational Approach Circumvents The Numerical Cherenkov Instability

It lurks at the dark corners of equations and strikes without warning, introducing unphysical false results into a class of simulations that are used in important areas of physics. It’s the numerical Cherenkov instability. Now a multi-institutional team that includes researchers from ATAP’s Accelerator Modeling Program appears to have found a way to avoid it for most practical cases.

The simulation of relativistic flowing plasmas plays an important role in a number of fields, including astrophysics and laser-plasma acceleration. Particle-in-cell (PIC) codes provide a computationally reasonable means of performing these simulations. Unfortunately, this has remained a major challenge due to a problem called the numerical Cherenkov instability (NCI). The NCI can crop up at certain places and ruin the results by introducing large plasma waves that do not actually exist, hence the name “numerical” instability. The problem builds and builds in a runaway manner.

Several means have been proposed to reduce the NCI, but some may not leave the underlying physics intact, and can moreover impose impractical constraints on the simulation, such as requiring a very small timestep.

A group of scientists that included two ATAP researchers recently proposed a new method to eliminate the NCI. In this scheme, Maxwell’s equations (which describe the electromagnetic fields that act upon the plasma) are solved analytically using a Galilean comoving coordinate system. Instead of having the plasma move along a fixed coordinate system, the coordinate system moves together with the plasma.

galileanscheme_hires_900x484y While the standard PIC method integrates Maxwell’s equations on a fixed grid, the new method integrates the equations in a Galilean frame moving with the plasma.

“The new “Galilean Frame” method is the first to succeed fully in eliminating the numerical Cherenkov instability for a drifting relativistic plasma,” says Dr. Brendan Godfrey of the University of Maryland. A leading figure in plasma modeling and frequent ATAP collaborator, Godfrey exposed the effect in a seminal paper in 1974. Since then, many researchers, himself included, have attempted to eliminate it, but with only partial success. This new approach is remarkable, he adds, because NCI “has been the bane of relativistic streaming plasma simulations for over forty years.”

Unlike previous methods, the method does not require a small timestep, and can be used with Cartesian or quasi-cylindrical coordinates. Detailed empirical and theoretical stability analysis has been conducted for a uniform flowing plasma, demonstrating the capabilities of this technique and showing that it indeed mitigates the NCI. Computer simulations of laser-plasma accelerators in an optimal Lorentz boosted frame demonstrated the effectiveness of the method for real applications.

galilean-psatd_hires_900x517y Two simulations of a plasma wake, performed using a Lorentz boosted frame PIC code, show the improvement possible with the Galilean coordinate method (top) versus the standard method with a fixed grid (bottom), distorted by the numerical Cherenkov instability.

“The new method represents a key advance for simulations of plasma accelerators,” says Wim Leemans, director of the ATAP Division and of its Berkeley Lab Laser Accelerator (BELLA) Center, which performs laser-plasma accelerator R&D.

Analytically integrable spectral Maxwell solvers — a class of simulation-code modules that have been the subject of revived interest in recent years by ATAP’s researchers — have been a key to implementing the new approach. They enable use of the full power of the Lorentz boosted frame technique pioneered by Jean-Luc Vay, head of ATAP’s Accelerator Modeling Program.

“It is a great example of how a team with a diversity of approaches came together to solve a difficult problem,” says Vay. “Young researchers with new insights teamed up with researchers with decades of experience and deep knowledge of the problem. It led to an unexpected solution: conceptually simple, yet mathematically sophisticated.”

The researchers from or affiliated with ATAP included postdoctoral fellow Remi Lehe; Manuel Kirchen, a doctoral student from the University of Hamburg and the German laboratory DESY; Godfrey, a very active ATAP collaborator in recent years; and Vay. Their Hamburg colleagues included Dr. Andreas R. Maier, group leader of LUX, DESY’s laser-plasma accelerator program; doctoral student Irene Dormair, and master’s student Sören Jalas.

To learn more…

The new approach to eliminating the numerical Cherenkov instability is described in a pair of recent papers:

M. Kirchen (LBNL and University of Hamburg); R. Lehe; B.B. Godfrey (University of Maryland and LBNL); I. Dornmair, S. Jalas, K. Peters (University of Hamburg); J.-L. Vay; A.R. Maier (University of Hamburg), “Stable discrete representation of relativistically drifting plasmas,” Physics of Plasmas 23, 100704 (2016); http://dx.doi.org/10.1063/1.4964770

R. Lehe; M. Kirchen (LBNL and University of Hamburg); B.B. Godfrey (University of Maryland and LBNL); A.R. Maier (University of Hamburg); and J.-L. Vay, “Elimination of numerical Cherenkov instability in flowing-plasma particle-in-cell simulations by using Galilean coordinates”, Physical Review E 94, 053305 (2016), https://doi.org/10.1103/PhysRevE.94.053305

The effect was first described in Brendan B. Godfrey, “Numerical Cherenkov instabilities in electromagnetic particle codes,” Journal of Computational Physics 15, 4 (August 1974), pp. 504-521; http://dx.doi.org/10.1016/0021-9991(74)90076-X

The outreach article below is a minor variation on an already approved story published in Today at Berkeley Lab


ATAP RESEARCHERS REACH OUT AT APS-DPP MEETING

At the annual meeting of the American Physical Society’s Division of Plasma Physics, ATAP scientists could be found not only in the technical sessions but also in the science-outreach expo booth.

The meeting was held this year in San Jose, California, inspiring ATAP Director Wim Leemans to endorse the expo-booth effort. “With a number of our people giving papers at a relatively local conference, we had a highly leveraged and cost-effective opportunity to connect with future scientists and the community,” said Leemans. ATAP is home to most of the Laboratory’s work in areas relevant to the meeting, and some 15 presentations involved ATAP authors.

geddes_apsdppbooth_749x401y Cameron Geddes of ATAP’s BELLA Center uses spectrographic gratings to reveal the inner secrets of incandescent, compact fluorescent, and LED light bulbs. This is a gateway to discussion of the nature of light and then such facilities as the Advanced Light Source and free-electron lasers. The ever-popular demonstration is a staple of our educational outreach activities.

At the expo, which is a traditional feature of the meeting, universities, laboratories, and other organizations host booths to educate and engage the public about plasma physics and other sciences. The expo is open to the general public in the evening and has hands-on science experiments for middle- and high-school groups during the day. (Of course, attendees taking a break from the technical sessions enjoyed some “playtime” with the hands-on exhibits as well!)

ATAP Education and Outreach Coordinator Ina Reichel and BELLA Center researcher Cameron Geddes, chair of the local organizing committee for the APS-DPP meeting, were the principal booth volunteers for the November 3-4, 2016 expo. Conference attendees Qing Ji, Greg Penn, Peter Seidl, and Jean-Luc Vay took shifts in the booth or helped with setup and logistics. The Lab’s Workforce Development and Education Office provided some of the exhibit materials.

ATAP’s demonstrations explained the nature of light and used ordinary lamps to help understand extraordinary ones: particle accelerators that serve as sources of x-ray light with exquisite properties, enabling a broad array of science experiments. The booth also featured posters that illustrate how we use laser-induced plasmas to accelerate particles in unprecedentedly short distances, and on the new applications this may enable.

Related Resources
The hands-on activities provided by ATAP include a variety of materials that can be checked out by ATAP staff for demonstrations at the Lab, at local schools or at conferences.

The following items are easily portable and do not require personal protective equipment (note: the permanent magnet weighs more than 10 pounds; the magnetic field is such that people with a pacemaker or similar device should keep a 5 inch distance).

  • A cathode ray tube with a “magnetic wand” to show how magnets can deflect particle beams (works in daylight).
  • A permanent magnet quadrupole with an inner aperture of about 3 inches with iron filings in a plexiglass box to show the field lines.
  • A 3D-printed model of one arc of the ALS main storage ring.

ATAP also has some other demonstration experiments that involve liquid nitrogen and therefore require some training and PPE. The best known one is a maglev train that our superconducting magnet group put together years ago. These can only be used onsite unless the school/conference can provide liquid nitrogen; the maglev train also requires two people to transport the “tracks.”

For more information or requests to use these materials, please contact Ina Reichel (x4341, IReichel@lbl.gov). If a number of people are interested in learning more about these materials and activities, including the related science that we usually discuss, we can arrange group instruction.

Fund for Postdocs Carries on Mike Zisman’s Legacy

Excerpted from an article by LBNL science writer Keri Troutman in the 2 November 2016 issue of Today at Berkeley Lab.

MSZismanA Senior Scientist with ATAP’s Center for Beam Physics and a Fellow of the American Physical Society, Michael Zisman was passionate about accelerator science. And he generously shared his passion through his energetic and heartfelt commitment to mentoring young scientists.

Even throughout a difficult illness, Zisman considered his time at work to be precious, and it gave him strength to keep fighting, says his wife, Andrea Chan. “Mike liked to say, “how lucky we are to get paid for what we love to do,” when he talked about working in science,” says Chan. After he passed away last August, his family set up a fund in his name to further his passion of not only caring deeply about the science, but also about the people who make those scientific achievements possible.

Chan worked with Ivy Clift, President of Berkeley Lab Foundation, to develop the fund. It was established with an initial $15,000 installment, which will be spent each year to fund ATAP postdoc activities. Zisman’s family hopes to renew and grow the fund each year.

“Throughout his career, when Mike dove into something he really dove into something,” adds ATAP Director Wim Leemans, who knew Zisman for 25 years. “Everything was really done to perfection and he thought deeply about all the angles, not just the scientific but the managerial and political.”

Leemans says the fund is a fantastic opportunity for his division. “I’d like to see the money go towards the things that Mike was passionate about — sending young people to conferences and giving them training, as it would be a beautiful testimonial to the memory of his contributions.”

“Throughout his career, Mike was always investing in the people who do the science, promoting care for one another,” says Chan. “We want that passion for young people in science to continue; I think it’s something that would make him smile.”


WORKPLACE LIFE

Level II Electric-Vehicle Charging Comes to the M2 Lot (Building 71)

evchargingonly_200x291yAs part of the Laboratory’s campaign to reduce the environmental footprint of our commute, electrical-vehicle charging stations are being made available in more and more places across the site. One of the newest is in the M2 parking lot between Buildings 71 and 71B, where it conveniently serves about half of ATAP’s employees. The station has a Level II (high current 220 V) charger and an EVSE or charging cable.

Owners of electric vehicles may reserve this space for two-hour blocks of time. Sometimes it is empty but has been reserved for later in the day, so please refrain from parking regular vehicles there — or using it as an all-day parking space even for an EV.

To learn about getting an EV-charging permit, scheduling blocks of time at this or other reserved spaces, arranging payment, etc., please visit the Lab’s EV-charging website.

For the latest information on parking, traffic, and alternatives to the single-passenger car, or to offer suggestions, visit commute.lbl.gov/ .

Survey Seeks to Shape Mentoring Progam: Has Your Voice Been Heard?

mentoring_200x152yTo improve career development and collaboration within the Lab, the Physical Sciences Workplace Life Committee (PSWLC) is exploring the viability of a mentorship program. The PSWLC has developed a brief (2 minute) survey for Physical Sciences Area employees in order to gauge interest in, and help shape, this potential program.

Mentoring programs have been linked to benefits for both the participants and the organizations that foster them. Amongst other benefits, mentors report improved interpersonal relationship skills and greater satisfaction, while mentees report improved self-confidence and openness to feedback. The program would be outside supervisory lines and could encompass both professional and personal advice. Scientists and engineers and support personnel are all welcome to participate.

If you’d like to either tap the Laboratory’s vast experience base or use your hard-earned wisdom to help others advance their careers become more effective contributors to our efforts (or both!), please take the survey and share your perspective. Click here to get started.


SAFETY: THE BOTTOM LINE

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Poster by Lucky Cortez, ATAP Operations Team

Let’s Think On Our Feet This Winter


With its rainy weather, long nights, leaves on the sidewalks and stairways, and holiday decorations almost within easy reach, this is a time to bear down on slip, trip, and fall prevention both at the Lab and at home. Let’s walk mindfully (which includes paying attention to the physical world around us rather than the virtual world in handheld devices), use handrails, clean up slippery spills before others encounter them, and use stepstools and ladders properly.

More tips to keep easily prevented accidents from spoiling the festivities are available from the National Safety Council and the US Fire Administration.

Here’s wishing you and yours a happy holiday season and a safe return in the new year!