Director’s Corner
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ATAP’s diverse research portfolio and many years of experience at the leading edge of science and technology are bearing fruit in many ways. Better particle-therapy delivery gantries, improved beam intensity for high-energy-density physics, a way to focus electron beams from our laser-plasma accelerators without using magnets, and a new contribution to what will be a flagship facility in x-ray science are among the highlights you can read about in this month’s newsletter.
I’d also like all our people to take some time to focus on fire and chemical safety: vital for the workplace and also life-saving for the home, especially with the holidays coming on. |
Canted Cosine Theta Contributes to Cancer Therapy
Novel magnet could make particle-beam treatment gantries far lighter
ATAP and the Engineering Division are applying their superconducting-magnet expertise, along with knowledge from ATAP’s program in accelerator-physics support for the Advanced Light Source, to reduce the weight of particle-therapy beam delivery systems by nearly a factor of 10. Supported by a grant from the DOE Accelerator Stewardship Program, they are working together with colleagues from Switzerland’s Paul Scherrer Institute and industrial partner Varian Technologies.
Ion-beam therapy, a way of treating cancer (sidebar), requires that the tumor be irradiated from a wide variety of directions to keep radiation doses to healthy tissue as small as possible while maximizing the dose the tumor receives. It is not feasible to keep the particle beam fixed in position and rotate the patient because internal organs would move around, so it is the particle beam that must be moved — preferably a full 360 degrees around the patient.
Particle beams of the requisite energy — especially beams of heavy ions such as carbon, which are the next frontier of this form of treatment — must be steered by strong magnetic fields. As a result, the traditional isocentric “gantry,” which uses a system of normal-conducting magnets to guide the beam to the patient, is impressive in its weight and cost. (One state-of-the-art heavy-ion therapy facility has a 630-ton gantry three storeys high.) A lighter, less expensive beam-delivery system has been a longstanding desire, especially for hospital-based treatment centers.
They knew that, in order to significantly reduce the gantry size, the strength of the bend magnets would have to be increased beyond 1.8 tesla, which typically is the limit for normal-conducting bend magnets. Superconducting magnets can reach far higher fields, and they do not require an iron yoke, further reducing the weight of the gantry. The main drawback of superconducting magnets is that changing or “ramping” the magnetic field is slower.
To avoid this problem, designers have sought to enlarge momentum acceptance, meaning that the magnet can accommodate a wider range of beam energies without having to change the magnetic field. (Using different beam energies adds the necessary third dimension to the goal of irradiating the tumor while sparing adjacent healthy tissues. Usually a technique called pencil beam scanning is employed, where a narrow beam is rapidly scanned both transversely and in energy.)
The LBNL team devised an achromatic magnet that has much larger acceptance and also zero dispersion at the treatment area. The magnet furthest downstream in the system has a 15-cm bore radius.
A momentum aperture of order 25% can be achieved, enough for multiple-liter tumor volumes to be treated without ramping the magnetic field and with minimal distortion of the beam shape. It is estimated that the overall weight will be 10% that of current gantries.
| This design for a fixed-field alternating-gradient (focusing and defocusing, as shown) uses a winding called left-right canted cosine theta (LR-CCT). The magnet is the subject of a provisional application for patent. It combines dipole (bending) and quadrupole (focusing) fields in its four layers of conductor, and aberrations can be locally corrected with additional superimposed fields. Various achromatic layouts have been postulated using, typically, three such magnets. The magnet furthest downstream has a 15-cm bore radius; low fringe fields also contribute to its large momentum acceptance. |
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To Learn More:
Weishi Wan, Lucas Brouwer, Shlomo Caspi, Soren Prestemon (LBNL); Alexander Gerbershagen and Jacobus Maarten Schippers (PSI); and David Robin (LBNL), “Alternating-gradient canted cosine theta superconducting magnets for future compact proton gantries,” Physical Review Special Topics: Accelerators and Beams 18, 103501 (23 October 2015).
U.S. Department of Energy and U.S. Department of Health and Human Services, National Institutes of Health, National Cancer Institute, “Workshop on Ion Beam Therapy: Summary Report” (January 2013).
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CAPILLARIES, NOT QUADRUPOLES, FOCUS LPA e– BEAMS
Active plasma lensing for relativistic laser-plasma-accelerated electron beams
Electron beams from laser-plasma accelerators (LPAs) are being considered for a variety of applications, including driving a free-electron laser, producing gamma rays, and multi-stage acceleration. These applications take advantage of some of the unique properties of LPAs, such as the compact footprint, few-femtosecond beam durations, intrinsic femtosecond synchronization to other radiation sources, and electron/photon energy tunability.
Compact transport of the e-beams from LPA source to the application is challenging for these beams, which have few-milliradian divergence and several-percent energy spread. One would have to combine at least three quadrupole lenses to provide symmetric transport at the target energy. Such a triplet configuration severely reduces the effective focal strength, thus compromising the system compactness, and is not easily tunable over an appreciable energy range.
As an alternative to using quadrupoles, at the BELLA Center we recently revived an existing concept, namely, the use of the focusing magnetic fields carried by strong current pulses through plasma-discharged capillaries. The scheme is known as active plasma lensing. Not only does a single lens provide radially symmetric focusing, the focusing power is over an order of magnitude stronger (>3000 T/m) than quadrupoles, and is easily tunable by varying the discharge current. With our in-house ability to produce cm-length capillaries at sub-mm diameter, we have access to a supply of truly unique lenses.
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The active-plasma-lensing concept and a key result that we achieved.
We installed an active plasma lens on one of our stable gas-jet-based LPA lines, and focused the electron beam to a magnetic spectrometer to measure the energy-dispersed e-beam size. The figure below shows single-shot images for a variety of currents, scanned by changing the arrival time of the electron beam with respect to the discharge pulse. The data agree well with simulations.
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Images I and II show the e-beam in absence of a current, XVIII-XIX highlight focusing at currents around 20-60 amperes, and XV shows how the e-beam can over-focus to exit the lens diverging, while VI-XIV show the beams converging after a double beam size oscillation inside the lens. In the latter case, the chromatic dependence is strongly enhanced. |
We published our results in Physical Review Letters and presented them at conferences, finding that the need for, and advantages of, such a strong tunable symmetric lens were acknowledged by the accelerator community.
To Learn More: J. van Tilborg, S. Steinke, C.G.R. Geddes, N.H. Matlis, B.H. Shaw, A.J. Gonsalves, J.V. Huijts, K. Nakamura, J. Daniels, C.B. Schroeder, C. Benedetti, E. Esarey, S.S. Bulanov, N.A. Bobrova, P.V. Sasorov, and W.P. Leemans, “Active Plasma Lensing for Relativistic Laser-Plasma-Accelerated Electron Beams,” Physical Review Letters 115, 184802 (28 October 2015), doi:10.1103/PhysRevLett.115.184802.
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NEW ION SOURCE ADDS INTENSITY TO NDCX-II
Improves capabilities for users exploring warm dense matter
The Neutralized Drift Compression Experiment-II facility has begun using a new multi-cusp, multi-aperture ion source to enable higher-intensity short ion pulses for materials science and warm dense matter experiments. To date, the team has focused 15 nanocoulombs of charge within a radius of 1 mm. This puts the facility’s capabilities into an interesting range for studying the dynamics of radiation-induced defects on a variety of materials, as well as exploration of materials modifications with pulsed ion beams near the onset of the warm dense matter regime.
Understanding “warm dense matter” — a certain regime of high temperature and pressure — is important in many areas of science. In astrophysics, for example, warm dense matter is found in planetary cores. This knowledge also provides foundational data to the quest for inertial fusion energy. It could also lead to the discovery of novel materials and compounds.
This state of matter cannot be directly accessed in the natural world, so laboratory plasmas (such as those which can be created by accelerators) are important tools for measuring its characteristics. NDCX-II, an ion accelerator, is supported by the DOE Office of Fusion Energy Science to provide unique capabilities for user science in these fields.
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With its new helium ion source, NDCX-II can provide 1-MeV helium ion beams with an energy deposition greater than 0.7 joules/cm2. The full temporal distribution of the pulses is ~25 ns, with a ~1 ns rise time, though still with a broader tail than desired; a near-term objective will be to achieve a 1-2 ns pulse duration via acceleration and bunching waveform adjustments. Higher intensities are also a goal, enabling heating of thin samples to 10,000 degrees, or 1 electron-volt, since much greater charge is available from the new helium source. |
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NDCX-II is the latest application to take advantage of ATAP expertise in ion sources that use a multicusp magnetic field for plasma confinement. With this new helium source, NDCX-II has delivered 15-nC bunches within a 2-mm spot size. The beam energy is 1 MeV and the energy deposition is greater than 0.7 joules/cm2.
To Learn More: Peter A. Seidl, Arun Persaud et al., “Short intense ion pulses for materials and warm dense matter research,” Nucl. Instrum. Meth. A 800 (2015), pp. 98-103, doi: 10.1016/j.nima.2015.08.013.
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R&D PROGRESS AND AN ADDED ROLE FOR LBNL IN LCLS-II
Final Design Report under review; LBNL adds vertically polarized undulator task
The LCLS-II project has issued its Final Design Report (including major ATAP contributions) for review. The resulting report will be a key piece of input to the DOE project review planned for December. ATAP and Engineering staff have continued to make progress on their contributions to LCLS-II, including the undulator prototype HXU-32, the Advanced Photoinjector Experiment, and beam dynamics studies, and have accepted a major new responsibility: vertically polarized undulators.
LCLS-II Final Design Report In Review
The Linac Coherent Light Source-II (LCLS-II) Final Design Report has been revised and issued to the Facilities Advisory Committee for their review. LBNL is a key partner in LCLS-II, multi-institutional project to be built at SLAC National Accelerator Laboratory. ATAP and Engineering Division staff made significant contributions to this document, primarily in the chapters on the injector source, FEL undulators, accelerator physics, and cryogenic systems.
The Final Design Report is required for the DOE project review planned for December, which will assess the project baseline design and readiness for construction to start. Comments from the Facility Advisory Committee will be addressed, and the final document issued, later in the year.
This is the latest in several recent reviews involving LBNL participants:
- September 28-30 – Cryo Systems Final Design Review
- October 13-15 – LCLS-II Facilities Advisory Committee Meeting
- October 19 – LCLS-II Collaboration Meeting
- October 20-22 – Director’s CD-2/3 Review
The LBNL LCLS-II team is now preparing for the injector source gun and load-lock final design review, SXR and HXR undulators final design review, and cost drill-downs as part of the DOE critical decision process, all during November.
LBNL Testing of HXU-32 Nearly Complete
Tests of the undulator prototype, known as “HXU-32”, by the LCLS-II undulators team and staff from the Berkeley Center for Magnet Technology, are almost complete and have already demonstrated that the magnetic design meets project requirements and specifications. Environmental tests that thermally cycle the whole device by ±15° C are now in the final stages.
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The LCLS-II undulator team with a prototype of one section of an FEL undulator for LCLS-II (there will be 21 such sections in all for a soft x-ray beamline, 32 for hard x-rays).
Once complete, the undulator will be shipped to SLAC for long-term testing. Meanwhile, mechanical tests of the frame and drive systems of a pre-production unit have been successfully performed at the vendor facility by a team of LBNL engineers. Magnetic module assemblies are also under fabrication at a vendor, and we will take delivery of components for the pre-production unit in December. |
An Added LCLS-II Role: Vertically Polarized Undulators
The LCLS-II project has asked LBNL to take responsibility for the delivery of vertically polarized undulators for the hard X-ray line (HXR). We are now in the process of developing our successful magnetic modules design and adopting an Argonne National Laboratory-developed mechanical support structure to meet this need.
Computer-aided design model of the LCLS-II vertically polarized undulator concept under development by Engineering Division staff.
This new approach for the HXR has advantages for X-ray experiments, and is planned to be included in the project baseline. Selection of LBNL to deliver such an advanced device for a state-of-the-art X-ray science facility reflects confidence in our technical and management capabilities.
APEX Embarking on Phase II
APEX, the Advanced Photoinjector Experiment, is dedicated to the development and test of a new concept high-repetition rate high-brightness electron injector optimized to operate at the performance required by a high-repetition-rate x-ray FEL. The baseline for the injector design for LCLS-II is the VHF gun of APEX, which is based on a new concept developed at LBNL. As one of its major contributions to that facility, LBNL is responsible for the design and construction of the LCLS-II injector source and will be involved in its commissioning.
APEX Phase II hardware has been completed and commissioned, and initial results have demonstrated a beam energy of ~15 MeV, sufficient to allow meaningful measurements of relativistic electron bunch properties. The LCLS-II project is funding accelerator physics studies by a team from ATAP and SLAC to verify the performance of the APEX systems upon which the LCLS-II injector source design is based. This important activity is a high priority for ATAP as well as the LCLS-II project, and is meant to retire risk from the project by proving the capability to produce high-repetition-rate, high-brightness electron beam.
Details of the injector source low-energy beamline design are converging, in collaboration with SLAC and Fermilab, to an integrated layout with component spacing that meets physics requirements.
Beam Dynamics Beyond the Injector
Electron-beam modeling of next-generation FELs poses unique challenges and is important for achieving the highest-quality photon beams, which of course are the entire point of the project. ATAP staff are involved in modeling the beam dynamics throughout the LCLS-II machine, including optimizing the injector layout. ATAP has a suite of unique tools that together can provide high-resolution multiphysics modeling from end to end, including FEL output. Critical distances for the required beam dynamics performance — the cathode, focusing solenoids, and first superconducting accelerating cavity—are being defined in light of the space needed for the physical hardware.
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HILUMI-LHC PROJECT TRANSITIONS FROM DESIGN STUDY TO MACHINE CONSTRUCTION PHASE
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At the end of October, the HiLumi LHC Project, whose aim is an order-of-magnitude beam luminosity upgrade of CERN’s Large Hadron Collider, officially moved from the design study to the machine construction phase. Steve Gourlay, leader of the Berkeley Center for Magnet Technology (BCMT) and ATAP’s Superconducting Magnet Program, was among the 230 scientists and engineers from around the world who marked this occasion at the annual joint meeting of HiLumi LHC and LARP, the LHC Accelerator Research Project. |
LARP and HiLumi History and Context
Major accelerators tend to be repeatedly upgraded over the years, maximizing the scientific return on the investment. Before the LHC was even completed in its present version (famed as the discovery site of the Higgs boson), scientists and engineers were planning upgrades. LBNL has had a primary role since the inception of the four-laboratory US Large Hadron Collider Accelerator Research Program (US-LARP) collaboration.
The latest effort is aimed at an order-of-magnitude increase in beam luminosity. ATAP and the BCMT’s primary role will involve the first major use of the brittle high-field niobium-tin (Nb3Sn) superconductor in an operating accelerator, for high-performance interaction-region quadrupoles. The IR quadrupoles are among the keys to higher luminosity, as their function is final focusing of the beams destined for collision. These quadrupoles will double the luminosity all by themselves, a major contribution to the factor-of-10 luminosity upgrade envisioned by the HiLumi project.
The technology developed by LARP over the last decade will allow these magnets to achieve higher fields in significantly larger aperture than the current ones, and to provide greater temperature margin than would be possible with the ductile NbTi superconductor of the present interaction-region quadrupoles.
ATAP’s ability to develop such high-field magnets dates back decades. Even in the days of the Superconducting Super Collider, researchers here and program managers at the DOE Office of High Energy Physics had the foresight to realize that NbTi would reach its limits, and further progress would require learning to build magnets with high-field superconductors such as Nb3Sn. The heat treatment that makes these new materials superconductive also makes them brittle, so different ways of building magnets had to be devised.
LBNL’s superconducting magnet program pioneered both conductor and magnet construction techniques, and built a series of R&D magnets such as D20 (1997, 13.5 tesla), RD-3 (2001, 14.7 T), and HD-1 (2004, 16 T) that showed the capabilities of the new materials and set a series of field-strength records for accelerator-style dipoles.
Building the Future
Now, as the magnet R&D phase of the HiLumi-LHC transitions to machine construction, the focus of the LARP magnet program pivots as well, turning toward reducing the construction project risk by developing the design and demonstrating the performance of the new IR quadrupoles, as well as ensuring compatibility of the final focus system with the other improvements that add up to the order-of-magnitude improvement to integrated luminosity.
Besides superconducting magnets, other LBNL and Engineering Division contributions to LARP have included accelerator physics studies, as well as design, fabrication and commissioning of critical accelerator instrumentation. Crab cavities (radiofrequency devices that slip the otherwise separately orbiting beams onto a head-on collision course) and high-bandwidth feedback systems for optimization of the upgraded interaction regions, are among the efforts that will involve us. Accelerator physics studies and modeling, especially with regard to beam-beam effects, will also be prominent.
The end result will be a major upgrade of the LHC in the 2020 timeframe.
Looking even further into the future, there might be an energy upgrade of the LHC, which would also place great demands on the world’s collective expertise regarding superconducting materials and magnets. The exact nature and specifications of such a machine remain the subject of lively debate among scientists, but one thing seems very likely: LBNL will play an important role.
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WORKPLACE LIFE
Advanced Light Source Brightness Upgrade Team Honored
The ALS Brightness Upgrade Team was honored with this year’s LBNL Director’s Award for Exceptional Achievement. Accepting on behalf of the team will be Arnaud Madur of the Engineering Division (project manager) and ATAP physicist Christoph Steier (project lead).
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Steier (far left) and Madur (far right) discuss installation as technicians work on the ring.
Although the project included a number of installations and upgrades, the major challenge was replacing 50 corrector magnets with an equal number of combined-function magnets (sextupoles serving as dipoles, skew quadrupoles and sextupoles), which amounted to major surgery on the storage ring. The four-year project culminated in about one and a half months of installation and commissioning in 2013. As shown below, 48 new combined function sextupoles (four per arc section, indicated by the green circles) were installed as part of the Brightness Upgrade. It was all accomplished under budget and months ahead of schedule, with no “teething period,” a testament to the planning and management of the project and the thorough understanding of the ring achieved through years of study. |
The Brightness Upgrade, a $5.8M project supported by American Recovery and Reinvestment Act stimulus funds) was the largest upgrade of the ALS storage ring in its 20+ year history, eclipsing Superbends (superconducting bend magnets) in 2001 and Top-off injection in 2009. The brightness upgrade was completed under budget and months ahead of schedule, with no “teething period.”
The upgrade solidified the ALS’s place as one of the world’s brightest sources of soft x-rays. It reduced the horizontal emittance of the electron beam from 6.3 nm to 2.0 nm, delivering to the users a 3x photon-beam brightness improvement in bend-magnet beamlines, and 2x in the insertion-device beamlines (with existing undulators; more will be achieved with future ones).
This success also demonstrated success in a major upgrade with minimal disruption to users, which bodes well for the proposed “ALS-U,” a future upgrade that would turn the ALS into a diffraction-limited light source.
Madur and Steier will accept the honor in the Labwide awards ceremony on December 2nd at 2:30 in the Building 50 Auditorium.
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Rémi Lehe, ATAP Postdoc, Wins Metropolis Award
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Rémi Lehe, a postdoctoral fellow in ATAP’s modeling efforts, has won the 2016 Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics. His thesis was cited for “development, implementation, and application of new algorithms toward the improvement of laser-wakefield accelerators.”
The award is given annually by the American Physical Society in recognition of doctoral-thesis work that shows “outstanding quality and achievement” in computational physics. He will receive it, and present a lecture on the work, at the upcoming APS Division of Computational Physics annual meeting. |
Lehe received his PhD in 2014 from École Polytechnique under Dr. Victor Malka. His thesis work concerned particle-in-cell (PIC) simulations of laser-plasma accelerators. In ATAP, his present research focuses on high-accuracy spectral PIC algorithms, and on various means to speed up large-scale simulations of laser-plasma accelerators.
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Women @ the Lab Event Slated For November 18
Update Dec. 8: a video of the event is available.
Join us at the Cafeteria, 3:30-5 p.m. Wednesday, November 18, for the second Women @ The Lab event. It celebrates the achievements of the 14 women profiled this year who work in or support science, technology, engineering, and mathematics at Berkeley Lab.
The Women Scientists and Engineers Council and the Lab’s Diversity & Inclusion Office organizes this occasion. The Women @ The Lab website has more information about the honorees; there you can discover what inspired them to work in these fields, what excites them about their roles at LBNL, how to get people from underrepresented demographics engaged in the STEM fields, and more.
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SAFETY: THE BOTTOM LINE
Fire prevention, evacuation readiness are easy ways to stay safe
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If you were at LBNL on January 13, 2015, you probably heard about this fire in a (non ATAP) building that took some labs out of operation for months. It started with a common radio/cassette player and could have ended up much worse than this.
In this case there was not enough left of the radio to tell exactly what went wrong, but we can check electrical appliances for notorious trouble spots like frayed or broken cords, overloaded power strips, and loose connections. |
Here are some simple things you can do to help prevent that from happening in your work area. Fire safety is especially important now that cold weather is upon us… and you can apply these tips at home too, especially during the holidays. (According to the National Fire Protection Association, three times as many fires occur on Thanksgiving as on a typical day; unattended cooking is the main reason, and the hot-oil-immersion turkey fryer presents special perils.)
Start with Housekeeping
Keep boxes and papers away from sources of ignition, such as space heaters and electrical outlets. Sort through boxes and papers regularly, and recycle the ones you don’t need. Consider scanning documents into electronic form to reduce paper.
Check Electrical Equipment…
Use only electrical equipment marked as approved by UL, an equivalent nationally recognized testing laboratory, or the LBNL electrical equipment safety program. Take damaged or defective electrical equipment out of service immediately and either arrange for proper repair or dispose of it as electronic waste. Your Electrical Safety Advocate or Safety Coordinator would be glad to help.
…Especially Space Heaters
If your work area is too cold, first check with your Building Manager to see if the main heating in your work area can be improved. If you must use a space heater, order one approved by LBNL through eBuy.
Smoking (If You Must)
Consider participating in a smoking cessation program — your health as well as safety will benefit.
If you must smoke, use designated smoking areas only. LBNL has experienced grass fires and wooden deck fires from smokers who wandered off to enjoy the views. Dispose of butts using the safety containers provided in the designated smoking areas.
Store Chemicals Safely
Store flammable chemicals in proper containers (with secondary containment trays) inside flammables cabinets. Don’t forget to label the containers.
Be Ready, Just In Case
Fire prevention doesn’t always succeed; preparedness can save lives. At work and at home, know how to evacuate and keep the way clear.
- Keep fire doors closed. This will slow down the spread of heat and smoke to improve your chances of evacuating safely.
- Keep exit corridors, stairwells, and doorways free of obstructions and flammable materials.
- Know where the closest exit is… and other exits, and paths to them, as well.
Learn to use a fire extinguisher through LBNL’s online and hands-on classes.
At home, if you haven’t done it yet (traditionally, when you turn back the clock at the end of Daylight Savings Time), put fresh batteries in your smoke detectors. (Unless of course yours are hardwired to 120V or are of the kind that have ten-year batteries… in which case, use the occasion to press their test buttons.)
