Lining Up for Laser Plasma Acceleration
On March 31, 2020, Physics Review Letters published a paper by Palastro and colleagues from the University of Rochester, where a dephasingless laser-wakefield accelerator (DLWFA) was proposed. This proposed design for plasma accelerators would use line-focused laser pulses to overcome the problem of particles outrunning the acceleration region. To highlight this work, the American Physical Society asked Jeroen van Tilborg of ATAP Division’s Berkeley Lab Laser Accelerator (BELLA) Center to write a Viewpoint perspective, reproduced here, in their online magazine Physics. The DLWFA presents a clever way of shaping the spatial and temporal laser energy delivery onto a plasma, aimed at realizing a compact (room-size) high-energy electron accelerator. This goal is very much in-line with research in BELLA Center, where, over recent years, key community milestones on laser-plasma acceleration were demonstrated. This Viewpoint article and Palastro et al., Physical Review Letters 124, 134802 (31 March 2020), will help in appreciation of the various approaches being employed to pursue the same objectives: the compact production of multi-GeV electron beams and the development of novel “table-top” radiation sources.
One way to accelerate particles is to fire an intense laser pulse into a plasma, creating a density wake whose electric field pushes charged particles like electrons to high speeds . The accelerating gradient from such a plasma wakefield is much higher than can be achieved in conventional radio-frequency-based technology. However, a central difficulty with employing laser wakefield accelerators (LWFAs) is that the electrons eventually outrun the accelerating region of the laser-driven wakefield. Because of this dephasing, a single stage of wakefield acceleration is typically limited to a few tens of centimeters in length, and it is forced to operate at a low density, which constrains the accelerating gradient.
Several ideas have been proposed to deal with this dephasing limit. A new idea from John Palastro and colleagues from the University of Rochester, New York, uses special optical components to focus the laser into points along a line, thus extending the wake region so electrons are accelerated for longer and at a higher gradient . This method faces some hurdles, such as increasing the laser power above what’s currently available, but it has the potential to accelerate electrons to TeV energies over just a few meters.
The high gradients offered by laser wakefield acceleration open the path for tabletop accelerators at universities, hospitals, and smaller R&D labs, with applications such as x-ray bio-imaging, active nuclear interrogation, and medical treatments. LWFA also enables progress towards future TeV-scale particle colliders that promise to extend our understanding of the basic structure of the Universe . In the past 20 years, the LWFA community has evolved from small-scale proof-of-principle demonstrations to milestones in the production of beams with high current, low emittance, and narrow (percent-level) energy spread. Precision control of the laser-plasma interaction with multiple lasers or density profile shaping is a central thrust in ongoing research. Efforts are also underway to increase the number of laser shots from a few per second to more than a thousand per second, which will benefit stabilizing feedback procedures and applications that require high flux.
The maximum achievable electron energy for LWFA is determined by the accelerating field strength and the length of acceleration Lacc, both of which are limited by laser and plasma physics . The field strength scales with the plasma density as Ez∼n1∕2. At currently used densities (1016–1019 cm−3), the field strength can exceed 10–100 GV/m, which is about a thousand times the acceleration in traditional radio frequency cavities used at places like CERN. Ez also depends on the laser, whose intensity is determined by the laser energy, wavelength, pulse duration, and spot size.
The effective accelerator length depends on the laser as well. Specifically, the acceleration is limited to the region over which the laser remains focused. In vacuum, this focus region is characterized by the Rayleigh length zR, which is calculated with the laser wavelength and spot size. For most setups, the Rayleigh length is in the millimeter to centimeter range, but researchers can extend the focusing region by adding light-guiding structures, as was demonstrated in a recent LWFA experiment that achieved 8-GeV acceleration energies with a 20-cm-long waveguide . However, guiding structures are not without challenges, including the need for precise alignment and transverse mode control to avoid damage from the high-power laser pulses.
Guiding structures also don’t alleviate the problem of dephasing. Dephasing occurs because the wakefield region travels behind the laser at a speed slower than the vacuum speed of light, so electrons accelerated to relativistic energies will eventually get ahead of it. As such, the wakefield speed places an upper limit on the acceleration length, which scales with density as n−3∕2. To increase this limit—and correspondingly increase the maximum electron energy—traditional setups use a low plasma density.
Recently, a group proposed a scheme to overcome this dephasing limit by obliquely intersecting two tilted-pulse-front lasers . The interference between the lasers would generate a wakefield region that travels at the speed of light in vacuum. However, this approach requires precision control of two lasers, with potential challenges from transverse wakefield asymmetries.
Palastro et al.  have a similar idea for speeding up the wakefield region that uses a single laser and special optics rather than interfering lasers. Specifically, their proposed method combines an echelon (a mirror with specially designed steps) and an axiparabola (a recently developed curved mirror) . As the team conceives it, a laser pulse would first strike the echelon, which divides the light into a number of concentric rings (Fig. 1). These rings would be separated in time, such that the outer rings arrive at the axiparabola ahead of the inner rings. The curved reflective surface of the axiparabola would focus the rings at successive points along a line. This spatiotemporal shaping of the laser pulse offers a way to generate a wakefield traveling at the vacuum speed of light, which would circumvent dephasing.
To see how this “dephasingless” LWFA compares to previous schemes, we can imagine the system is tuned so that each ring produces the same acceleration effect as a single laser pulse in the traditional nonguided LWFA. In other words, each ring generates a focal segment that is one Rayleigh-length long, and if there are N rings, the total acceleration length would be N times the Rayleigh length. In addition to increasing the acceleration length, dephasingless LWFA could operate at high plasma density, which means a higher accelerating gradient. The bottom line is that dephasingless LWFA could potentially reach higher electron energies than previous methods with the same acceleration length.
One potential hurdle to this proposal is the laser energy requirements. If each of the N rings is to produce a separate wakefield, then the single laser pulse would need roughly N times more energy than in a traditional LWFA of comparable laser intensity. For TeV-scale electron acceleration, the number of rings would need to be in the thousands, and the corresponding femtosecond-duration laser energy would be multiple kilojoules, which may pose a challenge in terms of the availability of laser systems and power consumption limitations (especially for kHz operation). There are other issues that may require further investigation, such as how well does the line-focus scale to large ring numbers, and how will the spatiotemporal laser evolution affect wakefield generation and laser-electron overlap. In addition, questions remain over the inverse accelerated charge scaling with plasma density .
Despite these uncertainties, Palastro et al. have provided a clever variation of staged acceleration, with independently timed laser “beamlets” driving acceleration without the need for intrastage optics or complex guiding structures. As such, dephasingless LWFA enables new opportunities in the choice of density and laser-plasma accelerator architecture. This could be of particular importance for leveraging future multipetawatt lasers to compactly create high-energy electron beams that can probe nonlinear quantum electrodynamics . Next on the dephasingless LWFA to-do list: performing detailed simulations of laser delivery and wakefield production at multi-GeV scale and testing the waters regarding practical limitations.
Their research was published in Physical Review Letters 124, 134802 (March 31, 2020).
E. Esarey et al., “Physics of laser-driven plasma-based electron accelerators,” Rev. Mod. Phys. 81, 1229 (2009).
J. P. Palastro et al., “Dephasingless laser wakefield acceleration,” Phys. Rev. Lett. 124, 134802 (2020).
W. P. Leemans and E. Esarey, “Laser-driven plasma-wave electron accelerators,” Phys. Today 62, No. 3, 44 (2009).
A. J. Gonsalves et al., “Petawatt laser guiding and electron beam acceleration to 8 GeV in a laser-heated capillary discharge waveguide,” , Phys. Rev. Lett. 122, 084801 (2019).
A. Debus et al., “Circumventing the dephasing and depletion limits of laser-wakefield acceleration,” Phys. Rev. X 9, 031044 (2019).
S. Smartsev et al., “Axiparabola: A long-focal-depth, high-resolution mirror for broadband high-intensity lasers,” Opt. Lett. 44, 3414 (2019).
W. Lu et al., “Generating multi-GeV electron bunches using single stage laser wakefield acceleration in a 3D nonlinear regime,” Phys. Rev. Special Topics: Accelerators and Beams 10, 061301 (2007).
A. Di Piazza et al., “Extremely high-intensity laser interactions with fundamental quantum systems,” Rev. Mod. Phys. 84, 1177 (2012).
About the Author
Jeroen van Tilborg is an experimental staff scientist at the BELLA Center at Lawrence Berkeley National Laboratory (LBNL), California. Jeroen moved to Berkeley in 2001 for a combined Ph.D. program from LBNL and the Eindhoven University of Technology, Netherlands. For his work on femtosecond bunch length measurements he received the outstanding thesis award from the APS Division of Physics of Beams. Following a postdoctoral appointment in LBNL’s chemistry division (studying molecular dynamics), he returned to the BELLA Center in 2009. In 2016 Jeroen received a five-year DOE Early Career Research Program grant, which is funding his current pursuit of laser-plasma accelerator applications towards novel radiation sources.
BELLA Center Organizing AAC2020
BELLA Center personnel are playing key roles in organizing the 2020 Advanced Accelerator Concepts Workshop. Since its inception in 1982, the biennial AAC Workshop has become the principal US and international meeting for advanced particle accelerator research and development. Some 300 scientists and research leaders in particle-beam, laser, and plasma physics are expected to attend this invitation-only event. it will be held at Asilomar Conference Grounds, Pacific Grove, California. (Update: AAC2020 has been cancelled due to COVID-19. Stay tuned.)
BELLA Center Sets New Laser-Plasma Accelerator Electron Energy Record
By accelerating electrons to an energy of 7.8 GeV in just tens of centimeters, BELLA Center researchers have nearly doubled their own previous record for laser-driven particle acceleration, set in 2014 at 4.2 GeV. To learn more about this achievement and the techniques that made it possible, visit the news release from Berkeley Lab Strategic Communications or read the technical article in the journal Physical Review Letters.
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Meet the New Leaders of BELLA Center
ATAP Interim Director Thomas Schenkel has named a leadership team for the Berkeley Lab Laser Accelerator Center (BELLA). Eric Esarey is the new Center Director, aided by Deputy Directors Cameron Geddes and Carl Schroeder.
All three are experienced BELLA Center research leaders, hold the rank of Senior Scientist, and are Fellows of the American Physical Society (Esarey since 1996, Schroeder 2012, Geddes 2016). The three were co-recipients in 2010 of the American Physical Society’s John Dawson Award for Excellence in Plasma Physics Research from the American Physical Society, and each has been twice recognized with the LBNL Outstanding Performance Award.
Eric Esarey has been performing research on intense laser-plasma interactions, advanced accelerator concepts and novel radiation sources for over 30 years. After receiving his PhD in 1986 in plasma physics at MIT, he worked for 12 years at the Naval Research Laboratory. During that time, he and his colleagues pioneered fundamental theory on nonlinear laser-plasma interactions that described the physics of laser-plasma accelerators (LPAs) and carried out groundbreaking experiments on LPAs.
Esarey joined Berkeley Lab in 1998 as a physicist within the theory group of the Center for Beam Physics, where he continued researching LPAs and related phenomena. He helped found the BELLA Center and grow it into the world leading program that it is today. Esarey had previously served as BELLA Center’s Deputy and as Senior Scientific Advisor to the ATAP Division Director. He now succeeds BELLA’s founding director, Wim Leemans, who has left LBNL for a position at DESY.
Schenkel describes Esarey as “a visionary and longtime leader in the field of LPAs with an integrating perspective on the program.”
Esarey’s recent honors include the AAC Prize from the 2018 Advanced Accelerator Concepts Workshop (an event that he will chair in 2020). Among his numerous publications are comprehensive review articles on plasma accelerators that are highly cited within the community. A technical backgrounder accompanying the announcement 2018 Nobel Prize in Physics explained the LPA concept with a diagram originally published in Physics Today by Leemans and Esarey. (See the related article, “2018 Physics Nobel Cites an ATAP Application.”)
Esarey’s deputies are described by Schenkel as “capable, energetic, and with diverse scientific backgrounds,” exemplars of a strong BELLA team “hungry for achievement.”
Carl Schroeder has been a leader of the theoretical and modeling efforts that support BELLA’s experimental work and future applications. After earning his doctorate at UC-Berkeley in 1999, followed by a postdoctoral fellowship at UCLA, he joined LBNL in 2001. His research interests range across BELLA’s intellectual portfolio, including intense laser-plasma interactions, plasma-based accelerators, advanced acceleration concepts, novel radiation sources, and free-electron lasers. “Carl is a theoretical leader not only in BELLA’s current work, but also in the long-term push toward an LPA-based lepton collider,” says Schenkel.
“Ultimately,” he adds, “mastery of laser drive plasma accelerators will enable us to explore physics beyond the Standard Model, and to make strides in understanding the nature of matter and energy, and do so with a much smaller physical and financial footprint than today’s collider technologies. Carl is driving this vision and is laying the foundation for its practical implementation.”
Cameron Geddes is the lead experimentalist in the new BELLA Center leadership team.
“Cameron has a very strong foundation in high-energy and ultrafast lasers, and since his days as a graduate student has been a driving force and leader in the projects he works on,” says Schenkel.
Geddes has led a variety of the Center’s experimental projects. This includes a new laser facility for one of the many promising near-term applications of laser-plasma accelerators: compact quasi-monoenergetic gamma-ray sources for nuclear nonproliferation and security inspection. He has broad research experience in plasma physics, which at Berkeley Lab has included experimental designs for the PW laser, demonstration of novel concepts in particle injection and beam quality, staging experiments, high energy density science, and large-scale simulations. After working at Lawrence Livermore National Laboratory and Polymath Research on inertial-fusion-related laser-plasma interactions, he earned his doctorate at UC-Berkeley and LBNL in 2005, receiving the Hertz and APS Rosenbluth dissertation prizes. He joined the LBNL staff upon graduation. “Cameron brought a strong foundation in high-energy and ultrafast lasers to us, and since his days as a graduate student has always been a leader in everything he works on,” says Schenkel.
The path forward
BELLA Center is a world leader in the study of intense laser-plasma interactions and advancing the development of LPAs. Research there has demonstrated increasing single-stage electron beam energy gains, now at several GeV, together with lower-energy experiments on staging and on achieving high beam quality.
Going to ever higher energies will require using one LPA stage’s output as the input to the next, achieving more energy than is practical for a single stage. BELLA has achieved a first demonstration of staging. A major next step is to develop and implement a multi-GeV staging experiment. This very exciting and important effort has now been started with funding from DOE High Energy Physics.
In addition to electron beam acceleration, BELLA is exploring the applications of laser-plasma accelerated electron beams, e.g., with programs to develop compact radiation sources based on LPA electron beams.
Higher average laser power will be required for a lepton collider and also for many near-term LPA applications, and the BELLA Center is performing R&D to advance the development of these lasers. Fiber-based laser systems are among the candidates for this key enabling technology.
“Eric, Cameron, and Carl are all distinguished individuals and team leaders in their disciplines, and have been working together for many years to keep BELLA at the forefront of laser-plasma acceleration,” Schenkel says, adding, “BELLA is sure to enjoy continued growth and achievement under their leadership.”
Berkeley Lab Joins LaserNetUS
To help foster the broad applicability of high-intensity lasers, Berkeley Lab is a partner in a new research network called LaserNetUS. The network will provide U.S. scientists increased access to the unique high-intensity laser facilities at BELLA Center and at eight other institutions nationwide operating high-intensity, ultrafast lasers.
LaserNetUS has had its first call for research proposals. It is anticipated that the winning proposals for this initial “Run 1” will be announced in mid-2019, with experiments to ensue through the remainder of calendar 2019.
The BELLA facilities available to outside users through LaserNetUS are described in detail here.
Expanding access to key capabilities
“High-intensity and ultrafast lasers have come to be essential tools in many of the sciences, and in engineering applications as well,” said James Symons, Berkeley Lab’s associate laboratory director for its Physical Sciences Area.
Such lasers have a broad range of uses in basic research, manufacturing, and medicine. For example, they can be used to recreate some of the most extreme conditions in the universe, such as those found in supernova explosions and near black holes. They can generate high-energy particles for high-energy physics research (being explored at the BELLA Center) or intense X-ray pulses to probe matter as it evolves on ultrafast timescales. Laser-based systems can also cut materials precisely, generate intense neutron bursts to evaluate aging aircraft components, and potentially deliver tightly focused radiation therapy to tumors, among other uses.
The petawatt-class lasers of the LaserNetUS partners generate light with at least 1 million billion watts of power. A petawatt is nearly 100 times the output of all the world’s power plants, and yet these lasers achieve this threshold in the briefest of bursts. Using a technology called “chirped pulse amplification,” which was pioneered by two of the winners of this year’s Nobel Prize in physics, these lasers fire off bursts of light shorter than a tenth of a trillionth of a second.
Maintaining U.S. leadership in a fast-moving global endeavor
The U.S. was the dominant innovator and user of high-intensity laser technology in the 1990s, but now Europe and Asia have taken the lead, according to a recent report from the National Academies of Sciences, Engineering, and Medicine titled “Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light.” Currently, 80 to 90 percent of the world’s high-intensity ultrafast laser systems are overseas, and all of the highest-power research lasers that are currently in construction or have already been built are also overseas. The report’s authors recommended establishing a national network of laser facilities to emulate successful efforts in Europe.
LaserNetUS is holding a nationwide call for proposals that will allow any researcher in the U.S. to request time on one of the high-intensity lasers at the LaserNetUS host institutions.. The proposals, due March 18, will be peer reviewed by an independent proposal review panel. This call will allow any researcher in the U.S. to apply for time on one of the high intensity lasers at the LaserNetUS host institutions. The initial “Run 1” experiments are expected to take place in the second half of calendar 2019.
2018 PHYSICS NOBEL CITES AN ATAP APPLICATION
A message from Associate Laboratory Director James Symons
Click here for a larger picture, or here for a PDF of the entire document
This year’s Nobel Prize in Physics was shared by three pioneers in the science, technology, and applications of lasers. Two of the laureates — Gérard Mourou and his then doctoral student Donna Strickland — won for a breakthrough that (among its many other benefits) made our Berkeley Lab Laser Accelerator Center possible.
Their Nobel-winning research brought “chirped pulse amplification,” a method of generating high-intensity, ultra-short pulses, to lasers. In a mere three pages, their 1985 paper “Compression of Amplified Chirped Optical Pulses” (Optics Communications 56, 3 (1 December 1985), pp. 219-221) sparked a revolution. The concept was implemented widely and almost immediately, ending a decade-long plateau in laser performance.
Today, CPA and follow-on developments are used near-universally at the peak-power frontier of very large research lasers, and also to increase the peak power of relatively small lasers for a wide variety of industrial and medical applications as well as research. (To take just one of many examples, some of you may be reading this with vision corrected by LASIK surgery, a technology made feasible for widespread use by CPA.)
We were immensely gratified to see laser-plasma acceleration, and specifically the multi-GeV electron beams obtained at the BELLA facility, mentioned as one of the examples of the benefits of CPA in the Nobel committee’s scientific background document. The BELLA Petawatt system is a 1 Hz repetition rate Ti:sapphire laser based on the CPA technique pioneered by Strickland and Mourou. In addition to the discussion, the Nobel backgrounder used a conceptual diagram of the LPA principle from the 2010 White Paper of the ICFA/ICUIL Joint Task Force on High Power Laser Technology for Accelerators —a figure that had originally appeared in an article by Wim Leemans and Eric Esarey in the March 2009 issue of Physics Today.
The white paper was produced by a joint task force, chaired by ATAP Division Director Wim Leemans, of the International Committee on Future Accelerators and International Committee on Ultra-high Intensity Lasers, and was based on a workshop series held first at GSI and then here at LBNL. The notional BELLA follow-on, which we call k-BELLA for its kilohertz repetition rate / kilowatt average power performance class, is an example of such a next-generation laser.
CPA is also one of the techniques used in an exciting collaborative project being conducted through our Berkeley Accelerator Controls and Instrumentation (BACI) Center: development of a laser system that uses “coherent combining” to achieve both high peak power and high average power from arrays of fiber-optic lasers.
Please join me in offering congratulations on the scientific stature and the widespread, ongoing societal impact of the research by Drs. Mourou and Strickland, as well as their co-laureate Dr. Arthur Ashkin. (He is a pioneer of laser trapping and the inventor of “optical tweezers” that use lasers to grasp tiny physical particles such as bacteria or viruses. His work had already figured into the 1997 Nobel Prize in Physics for our former Lab director and Secretary of Energy Steven Chu, who had worked with Ashkin at Bell Labs.) Their achievements have given us both game-changing tools and inspiration. This is a time for all of us to be proud of the important role we play as research pioneers and the resulting benefit to humankind.
Associate Lab Director
Physical Sciences Area