NEWS & VIEWS AC C E LERATO R PHYSICS
Surf’s up at SLAC M I K E D O W N E R & R A FA L Z G A D Z A J
I
n November 2012, Guinness World Records reported that 120 surfers in Australia rode the same wave simultaneously for more than 5 seconds1. “The trick was to get them all to do the same thing at the same time,” said group leader Wes Smith. “It was an operation of military-like precision and we finally got there.” Now Litos and colleagues, in work at the SLAC National Accelerator Laboratory in Menlo Park, California, reported on page 92 of this issue2, have ‘got there’, too, by surfing half a billion 20-billion-electronvolt electrons on a steep charge-density wave about the size of a marine phytoplankton, travelling through ionized gas (plasma). The wave was driven by a companion electron bunch as it raced at nearly the speed of light through a 30-centimetrelong chamber filled with plasma (Fig. 1). Although this inaugural experiment lost about 90% of its ‘surfers’ along the way, the surviving electrons gained 1.6 billion electronvolts, or 1.6 gigaelectonvolts (GeV), in energy with unparalleled uniformity, maintaining roughly 1% energy spread throughout their wild ride, while sucking away an unprecedented fraction (up to 30%) of the wave’s energy. Such uniform, efficient acceleration required the researchers to inject the surfing electron bunch into the wave, and to adjust the bunch’s charge and shape, with a military-like precision made possible by SLAC’s recently commissioned US$15-million Facility for Accelerator Science and Experimental Tests (FACET)3. Because the plasma wave accelerated electrons 500 times faster than SLAC’s main particle accelerator, the result might herald a new generation of compact ‘plasma afterburners’ that could boost the energy of conventional particle accelerators and potentially reduce the skyrocketing cost of high-energy physics machinery4. Seven years ago, before FACET was even proposed, the same team had used single bunches of about 10 billion 42-GeV electrons and accelerated them over the full 3.2-kilo metre length of SLAC’s main machine to drive a similar plasma wave5. A handful of electrons from the tail of the drive bunch were caught in the drive bunch’s wake and were accelerated up to 84 GeV, twice the energy of the electrons in the original drive bunch, within a metre-long
1.6 GeV in 30 cm
20 GeV in 2 km
Figure 1 | Ramping up the energy. SLAC’s main accelerator, shown in aerial view, accelerates electron bunches from 0 to 20 GeV energy over 2 km, which amounts to adding 0.01 GeV to each electron every metre. The new Facility for Accelerator Science and Experimental Tests (FACET) used by Litos et al.2 then splits each 20-GeV bunch into two independently controlled tandem bunches. The leading bunch creates a new micro-accelerator inside a 30-cm chamber (top), in which it drives a charge-density wave in ionized gas, much as a boat drives a wake in water. The trailing bunch rides the lead bunch’s wake and, when optimally positioned, extracts up to 30% of its energy, boosting each electron’s energy by 1.6 GeV in only 30 cm.
4 0 | NAT U R E | VO L 5 1 5 | 6 NOV E M B E R 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
plasma chamber. However, the electrons emerging from this first-generation plasma afterburner ranged in energy from less than about 35 GeV to 84 GeV, more electrons were decelerated than accelerated, and most of the energy of the plasma wave was left untapped. FACET — which now shares SLAC with the Linac Coherent Light Source, and thus starts with 20-GeV electrons accelerated over part of SLAC’s length — was designed to correct these shortcomings. The facility exploits new particle-beam technology to split the SLAC bunches into two tandem bunches whose time separation, charge and shape are, with some limits, independently controllable. In the new experiments, the researchers used a little over half of the 20-GeV SLAC bunch to drive a plasma wave, and then timed its nearly equally charged twin to surf just a hair’s breadth behind, where its core rode the enormous electrostatic field of the drive bunch’s wake. Without the trailing surfing bunch, this field would be far from uniform, varying from 3 billion to 10 billion volts per metre (fields stronger than ordinary, nonplasma matter can withstand) just over the tiny region in which the surfing bunch was so painstakingly positioned. Had the researchers injected a lowercharged surfing bunch, it would have suffered the same fate as in the earlier experiment by broadening in energy. This would render it useless for high-energy physics applications, which require particle energy to be tuned precisely to create and identify new particles, such as the Higgs boson. However, Litos et al. took advantage of physics learned from computer simulations6 showing that a high-charge surfing bunch could ‘load’ the plasma wake, flattening its electrostatic fields locally. It is as if the 120 Australian surfers had sufficient collective weight to flatten the curved ocean wave into an inclined plane so that they could all accelerate at the same rate. This trick solved two problems simultaneously: it enabled a high-charge bunch to accelerate nearly monoenergetically while maximizing energy extraction from the plasma wake. Can plasma surfing meet future needs of high-energy physics research, which include electron bunches with sufficiently high energy, charge, repetition rate and focusability that they
SLAC NATL ACCELERATOR LAB
A ‘plasma afterburner’ just 30 centimetres long accelerates electrons hundreds of times faster than giant conventional accelerators. The result may ultimately open up a low-cost technology for particle colliders. See Letter p.92
NEWS & VIEWS RESEARCH Mike Downer and Rafal Zgadzaj are in the Physics Department, University of Texas at Austin, Texas 78712-1081, USA. e-mail: [email protected]
5. Blumenfeld, I. et al. Nature 445, 741–744 (2007). 6. Tzoufras, M. et al. Phys. Rev. Lett. 101, 145002 (2008). 7. Wang, X. et al. Nature Commun. 4, 1988; http:// dx.doi.org/10.1038/ncomms2988 (2013). 8. Caldwell, A., Lotov, K., Pukhov, A. & Simon, F. Nature Phys. 5, 363–367 (2009). 9. Schroeder, C. B., Esarey, E. & Leemans, W. P. Phys. Rev. ST Accel. Beams 15, 051301 (2012). 10. Lebedev, V. & Nagaitsev, S. Phys. Rev. ST Accel. Beams 16, 108001 (2013). 11. Schroeder, C. B., Esarey, E. & Leemans, W. P. Phys. Rev. ST Accel. Beams 16, 108002 (2013).
1. www.worldrecordacademy.com/sports/most_ surfers_riding_the_same_wave_120_surfers_set_ world_record_113137.html 2. Litos, M. et al. Nature 515, 92–95 (2014). 3. Hogan, M. J. et al. New J. Phys. 12, 055030 (2010). 4. Lee, S. et al. Phys. Rev. ST Accel. Beams 5, 011001 (2002).
D EVELO PM E N TA L B I OLOGY
Cells unite by trapping a signal Gradients of fibroblast growth factors often induce cells to adopt different fates. A study in zebrafish embryos reveals another, unexpected role when the factors are trapped in small spaces by a special arrangement of cells. See Letter p.120 JAMES SHARPE
B
uilding complex, multicellular organs during embryonic development is not just about making different cell types, it is about getting the right cells in the right place. For a cell to have some sense of where it is, it must integrate diffusible signals released by its neighbours. On page 120 of this issue, Durdu et al.1 provide evidence for a surprising new way by which diffusible signals such as fibroblast growth factors (FGFs) are controlled — by trapping them in small, closed extracellular spaces called microlumina, from which they have access to only a discrete collection of cells. Exactly how signalling molecules provide b
a FGF signalling levels
can create detectable amounts of new particles that may be lurking in the cosmic underworld? The jury is still out. The present 1.6-GeV energy gain (starting from 20 GeV) is no greater than that achieved by plasma accelerators driven by light pulses from lasers (starting from zero)7, which are much smaller and less-expensive instruments than SLAC. Nevertheless, electrondriven plasma accelerators scale more readily to gains of tens of gigaelectronvolts than do their laser-driven counterparts, as demonstrated in previous work5. Improved bunch-shaping technology will better match surfing bunch to plasma wave, increasing electron-survival rate and thus the number of accelerated electrons. Yet the Higgs boson has a mass equivalent to 126 GeV, and physical theories such as supersymmetry predict additional particles that have even greater mass than the Higgs and may be the source of the elusive ‘dark matter’ that seems to comprise about 25% of the Universe. Creating and identifying these new denizens of the Universe could set the next energy frontier at many thousands of gigaelectronvolts. Reaching these energies will probably require synchronized, multistaged plasma accelerators — a daunting, and largely unexplored, technical challenge in view of the micrometre dimensions of plasma waves. An interesting alternative proposal is to drive plasma waves with very energetic proton bunches, which because of their greater mass can push plasma waves for hundreds of metres, potentially accelerating electrons to the energy frontier in a single stage8. In either case, plasma acceleration of positrons (anti electrons) lags far behind electron acceleration because plasma waves shaped like those in the current experiment defocus surfing positron bunches, degrading their usefulness. Positron acceleration is important because high-energy collisions of electrons and positrons, a natural matter–antimatter pair, create a richer collection of products with higher efficiency than, say, electron–electron collisions, and thus offer one of the most promising routes to particle discovery. FACET, with access to SLAC’s companion positron beam, is uniquely positioned to explore new ways to shape plasma waves in order to advance plasma-based positron acceleration. Finally, even if the energies and charges required for an electron–positron collider are achieved, debate rages over whether focused, plasma-surfed particle beams can yield particle-discovery events at rates competitive with those achieved with conventional accelerator technology9–11, which underlies proposed tens-of-kilometres-long machines such as the International Linear Collider and the Compact Linear Collider. These uncertainties notwithstanding, Litos et al. have overcome one of the most difficult challenges so far in the long quest for small, affordable accelerators, and have given the plasma-surfing community every reason to surge ahead. ■
enough spatial information to build complex organisms is still obscure, but studies of the principles of signalling generally split into two types. Those of the upstream part of signalling ask how the movement of diffusible molecules — sometimes called morphogens — is controlled to form appropriate spatial gradients2,3, for example by ‘sticky’ molecules in the extracellular matrix4. Studies of the downstream part ask how these spatial distributions are used by receiving cells to control cellular ‘decisions’. A well-characterized example of the upstream part is the FGF family of secreted proteins5. These often form coherent spatial gradients within which different levels of signalling divide the responding cell
Rosette
Microlumen Direction of migration
Threshold
c Rosette
FGFCells induced expressing cells by FGF
Figure 1 | Roles of fibroblast growth factors (FGFs). a, A common role for FGFs is to induce different cell fates through spatial variations in FGF levels determined by the distance from FGF-expressing cells. In this schematic, cells that experience FGF levels above a threshold value are induced, whereas the others are unaffected. b, During the development of zebrafish embryos, a group of cells called the primordium, shown here from above, migrates from near the head-end to the tail. As it migrates, cells cluster into rosette structures that drop off at regular intervals and then develop into mechanosensory organs. Durdu et al.1 report that confinement of FGFs to the microlumen at the centre of a rosette coordinates the cells within that cluster, so that the rosette drops off in a well-organized manner. c, The microlumina are enclosed by a patchwork of membrane sections contributed by all the surrounding cells, as shown in this side view of a rosette. The right-hand graphic shows how one cell contributes to the microlumen, which is not shown to scale. 6 NOV E M B E R 2 0 1 4 | VO L 5 1 5 | NAT U R E | 4 1
© 2014 Macmillan Publishers Limited. All rights reserved
Surf’s up at SLAC M I K E D O W N E R & R A FA L Z G A D Z A J
I
n November 2012, Guinness World Records reported that 120 surfers in Australia rode the same wave simultaneously for more than 5 seconds1. “The trick was to get them all to do the same thing at the same time,” said group leader Wes Smith. “It was an operation of military-like precision and we finally got there.” Now Litos and colleagues, in work at the SLAC National Accelerator Laboratory in Menlo Park, California, reported on page 92 of this issue2, have ‘got there’, too, by surfing half a billion 20-billion-electronvolt electrons on a steep charge-density wave about the size of a marine phytoplankton, travelling through ionized gas (plasma). The wave was driven by a companion electron bunch as it raced at nearly the speed of light through a 30-centimetrelong chamber filled with plasma (Fig. 1). Although this inaugural experiment lost about 90% of its ‘surfers’ along the way, the surviving electrons gained 1.6 billion electronvolts, or 1.6 gigaelectonvolts (GeV), in energy with unparalleled uniformity, maintaining roughly 1% energy spread throughout their wild ride, while sucking away an unprecedented fraction (up to 30%) of the wave’s energy. Such uniform, efficient acceleration required the researchers to inject the surfing electron bunch into the wave, and to adjust the bunch’s charge and shape, with a military-like precision made possible by SLAC’s recently commissioned US$15-million Facility for Accelerator Science and Experimental Tests (FACET)3. Because the plasma wave accelerated electrons 500 times faster than SLAC’s main particle accelerator, the result might herald a new generation of compact ‘plasma afterburners’ that could boost the energy of conventional particle accelerators and potentially reduce the skyrocketing cost of high-energy physics machinery4. Seven years ago, before FACET was even proposed, the same team had used single bunches of about 10 billion 42-GeV electrons and accelerated them over the full 3.2-kilo metre length of SLAC’s main machine to drive a similar plasma wave5. A handful of electrons from the tail of the drive bunch were caught in the drive bunch’s wake and were accelerated up to 84 GeV, twice the energy of the electrons in the original drive bunch, within a metre-long
1.6 GeV in 30 cm
20 GeV in 2 km
Figure 1 | Ramping up the energy. SLAC’s main accelerator, shown in aerial view, accelerates electron bunches from 0 to 20 GeV energy over 2 km, which amounts to adding 0.01 GeV to each electron every metre. The new Facility for Accelerator Science and Experimental Tests (FACET) used by Litos et al.2 then splits each 20-GeV bunch into two independently controlled tandem bunches. The leading bunch creates a new micro-accelerator inside a 30-cm chamber (top), in which it drives a charge-density wave in ionized gas, much as a boat drives a wake in water. The trailing bunch rides the lead bunch’s wake and, when optimally positioned, extracts up to 30% of its energy, boosting each electron’s energy by 1.6 GeV in only 30 cm.
4 0 | NAT U R E | VO L 5 1 5 | 6 NOV E M B E R 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
plasma chamber. However, the electrons emerging from this first-generation plasma afterburner ranged in energy from less than about 35 GeV to 84 GeV, more electrons were decelerated than accelerated, and most of the energy of the plasma wave was left untapped. FACET — which now shares SLAC with the Linac Coherent Light Source, and thus starts with 20-GeV electrons accelerated over part of SLAC’s length — was designed to correct these shortcomings. The facility exploits new particle-beam technology to split the SLAC bunches into two tandem bunches whose time separation, charge and shape are, with some limits, independently controllable. In the new experiments, the researchers used a little over half of the 20-GeV SLAC bunch to drive a plasma wave, and then timed its nearly equally charged twin to surf just a hair’s breadth behind, where its core rode the enormous electrostatic field of the drive bunch’s wake. Without the trailing surfing bunch, this field would be far from uniform, varying from 3 billion to 10 billion volts per metre (fields stronger than ordinary, nonplasma matter can withstand) just over the tiny region in which the surfing bunch was so painstakingly positioned. Had the researchers injected a lowercharged surfing bunch, it would have suffered the same fate as in the earlier experiment by broadening in energy. This would render it useless for high-energy physics applications, which require particle energy to be tuned precisely to create and identify new particles, such as the Higgs boson. However, Litos et al. took advantage of physics learned from computer simulations6 showing that a high-charge surfing bunch could ‘load’ the plasma wake, flattening its electrostatic fields locally. It is as if the 120 Australian surfers had sufficient collective weight to flatten the curved ocean wave into an inclined plane so that they could all accelerate at the same rate. This trick solved two problems simultaneously: it enabled a high-charge bunch to accelerate nearly monoenergetically while maximizing energy extraction from the plasma wake. Can plasma surfing meet future needs of high-energy physics research, which include electron bunches with sufficiently high energy, charge, repetition rate and focusability that they
SLAC NATL ACCELERATOR LAB
A ‘plasma afterburner’ just 30 centimetres long accelerates electrons hundreds of times faster than giant conventional accelerators. The result may ultimately open up a low-cost technology for particle colliders. See Letter p.92
NEWS & VIEWS RESEARCH Mike Downer and Rafal Zgadzaj are in the Physics Department, University of Texas at Austin, Texas 78712-1081, USA. e-mail: [email protected]
5. Blumenfeld, I. et al. Nature 445, 741–744 (2007). 6. Tzoufras, M. et al. Phys. Rev. Lett. 101, 145002 (2008). 7. Wang, X. et al. Nature Commun. 4, 1988; http:// dx.doi.org/10.1038/ncomms2988 (2013). 8. Caldwell, A., Lotov, K., Pukhov, A. & Simon, F. Nature Phys. 5, 363–367 (2009). 9. Schroeder, C. B., Esarey, E. & Leemans, W. P. Phys. Rev. ST Accel. Beams 15, 051301 (2012). 10. Lebedev, V. & Nagaitsev, S. Phys. Rev. ST Accel. Beams 16, 108001 (2013). 11. Schroeder, C. B., Esarey, E. & Leemans, W. P. Phys. Rev. ST Accel. Beams 16, 108002 (2013).
1. www.worldrecordacademy.com/sports/most_ surfers_riding_the_same_wave_120_surfers_set_ world_record_113137.html 2. Litos, M. et al. Nature 515, 92–95 (2014). 3. Hogan, M. J. et al. New J. Phys. 12, 055030 (2010). 4. Lee, S. et al. Phys. Rev. ST Accel. Beams 5, 011001 (2002).
D EVELO PM E N TA L B I OLOGY
Cells unite by trapping a signal Gradients of fibroblast growth factors often induce cells to adopt different fates. A study in zebrafish embryos reveals another, unexpected role when the factors are trapped in small spaces by a special arrangement of cells. See Letter p.120 JAMES SHARPE
B
uilding complex, multicellular organs during embryonic development is not just about making different cell types, it is about getting the right cells in the right place. For a cell to have some sense of where it is, it must integrate diffusible signals released by its neighbours. On page 120 of this issue, Durdu et al.1 provide evidence for a surprising new way by which diffusible signals such as fibroblast growth factors (FGFs) are controlled — by trapping them in small, closed extracellular spaces called microlumina, from which they have access to only a discrete collection of cells. Exactly how signalling molecules provide b
a FGF signalling levels
can create detectable amounts of new particles that may be lurking in the cosmic underworld? The jury is still out. The present 1.6-GeV energy gain (starting from 20 GeV) is no greater than that achieved by plasma accelerators driven by light pulses from lasers (starting from zero)7, which are much smaller and less-expensive instruments than SLAC. Nevertheless, electrondriven plasma accelerators scale more readily to gains of tens of gigaelectronvolts than do their laser-driven counterparts, as demonstrated in previous work5. Improved bunch-shaping technology will better match surfing bunch to plasma wave, increasing electron-survival rate and thus the number of accelerated electrons. Yet the Higgs boson has a mass equivalent to 126 GeV, and physical theories such as supersymmetry predict additional particles that have even greater mass than the Higgs and may be the source of the elusive ‘dark matter’ that seems to comprise about 25% of the Universe. Creating and identifying these new denizens of the Universe could set the next energy frontier at many thousands of gigaelectronvolts. Reaching these energies will probably require synchronized, multistaged plasma accelerators — a daunting, and largely unexplored, technical challenge in view of the micrometre dimensions of plasma waves. An interesting alternative proposal is to drive plasma waves with very energetic proton bunches, which because of their greater mass can push plasma waves for hundreds of metres, potentially accelerating electrons to the energy frontier in a single stage8. In either case, plasma acceleration of positrons (anti electrons) lags far behind electron acceleration because plasma waves shaped like those in the current experiment defocus surfing positron bunches, degrading their usefulness. Positron acceleration is important because high-energy collisions of electrons and positrons, a natural matter–antimatter pair, create a richer collection of products with higher efficiency than, say, electron–electron collisions, and thus offer one of the most promising routes to particle discovery. FACET, with access to SLAC’s companion positron beam, is uniquely positioned to explore new ways to shape plasma waves in order to advance plasma-based positron acceleration. Finally, even if the energies and charges required for an electron–positron collider are achieved, debate rages over whether focused, plasma-surfed particle beams can yield particle-discovery events at rates competitive with those achieved with conventional accelerator technology9–11, which underlies proposed tens-of-kilometres-long machines such as the International Linear Collider and the Compact Linear Collider. These uncertainties notwithstanding, Litos et al. have overcome one of the most difficult challenges so far in the long quest for small, affordable accelerators, and have given the plasma-surfing community every reason to surge ahead. ■
enough spatial information to build complex organisms is still obscure, but studies of the principles of signalling generally split into two types. Those of the upstream part of signalling ask how the movement of diffusible molecules — sometimes called morphogens — is controlled to form appropriate spatial gradients2,3, for example by ‘sticky’ molecules in the extracellular matrix4. Studies of the downstream part ask how these spatial distributions are used by receiving cells to control cellular ‘decisions’. A well-characterized example of the upstream part is the FGF family of secreted proteins5. These often form coherent spatial gradients within which different levels of signalling divide the responding cell
Rosette
Microlumen Direction of migration
Threshold
c Rosette
FGFCells induced expressing cells by FGF
Figure 1 | Roles of fibroblast growth factors (FGFs). a, A common role for FGFs is to induce different cell fates through spatial variations in FGF levels determined by the distance from FGF-expressing cells. In this schematic, cells that experience FGF levels above a threshold value are induced, whereas the others are unaffected. b, During the development of zebrafish embryos, a group of cells called the primordium, shown here from above, migrates from near the head-end to the tail. As it migrates, cells cluster into rosette structures that drop off at regular intervals and then develop into mechanosensory organs. Durdu et al.1 report that confinement of FGFs to the microlumen at the centre of a rosette coordinates the cells within that cluster, so that the rosette drops off in a well-organized manner. c, The microlumina are enclosed by a patchwork of membrane sections contributed by all the surrounding cells, as shown in this side view of a rosette. The right-hand graphic shows how one cell contributes to the microlumen, which is not shown to scale. 6 NOV E M B E R 2 0 1 4 | VO L 5 1 5 | NAT U R E | 4 1
© 2014 Macmillan Publishers Limited. All rights reserved
