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<?xml version="1.0" ?> <tei> <teiHeader> <fileDesc xml:id="0"/> </teiHeader> <text xml:lang="en"> <p>B<lb/> eyond the theoretical and engineering<lb/> challenges of building particle acceler-<lb/>ators, sheer cost is a concern for physi-<lb/>cists whose work involves accelerating<lb/> and smashing subatomic particles together at<lb/> great speed. Many particle physicists think<lb/> that if the planned International Linear Col-<lb/>lider — a US$7-billion electron–positron<lb/> collider that could begin operation within a<lb/> decade — gets the go ahead, it may be the last<lb/> large accelerator to be built for many decades<lb/> as governments put a squeeze<lb/> on funding.<lb/></p> <p>The cost of accelerators is<lb/> a concern not just for those<lb/> who crave bigger and bigger<lb/> machines to probe ever higher<lb/> energy scales. Some oncolo-<lb/>gists think that proton beams<lb/> could offer superior results<lb/> to conventional X-ray treatment of some<lb/> tumours, yet they say the size and cost of the<lb/> accelerators has limited the number of studies<lb/> into their clinical effectiveness.<lb/></p> <p>" If we can reduce an accelerator's size, we can<lb/> reduce the cost of proton therapy to something<lb/> very small, " says Charlie Ma, director of radia-<lb/>tion physics at the Fox Chase Cancer Center<lb/> in Philadelphia, Pennsylvania. Buil ding a<lb/> proton-treatment centre with conventional<lb/> cyclotron or synchrotron accelerators costs<lb/> between $100 million and $200 million, which<lb/> explains why there are so few of these facilities<lb/> (see 'Targeting tumours').<lb/></p> <p>But if accelerator research continues to<lb/> progress at the rapid rate seen in recent years,<lb/> the economics could be about to change for<lb/> the better. A handful of groups are working on<lb/> a new way to accelerate particles — known as<lb/> wakefield acceleration — that<lb/> should not only help push<lb/> physicists towards the next<lb/> energy frontier, but also pro-<lb/>vide affordable, table-top accel-<lb/>erators that could revolutionize<lb/> cancer treatment.<lb/></p> <p>The technique involves<lb/> passing either a laser beam or a<lb/> beam of particles through a plasma. The beam<lb/> scatters electrons, causing an uneven distribu-<lb/>tion of charge between the scattered particles<lb/> and the plasma ions. To restore an even distri-<lb/>bution, the electrons are pulled back towards<lb/> the positive plasma ions that have congregated<lb/> towards the rear of the beam pulse. But the<lb/> electrons overshoot their original positions,<lb/> creating a wake-like disturbance called a wake-<lb/>field oscillation. Within this wake are pockets<lb/> of plasma ions, which physicists refer to as<lb/> bubbles, thanks to their spherical shape.<lb/></p> <p>The wake of a breaking wave causes turbu-<lb/>lence, and the wake generated in a plasma is no<lb/> exception. But as surfers and boat owners know,<lb/> if you hit the wave at just the right spot, you can<lb/> be accelerated by its surf. So some electrons can<lb/> surf the plasma wakefield, as can other parti-<lb/>cles, such as protons, injected into the beam,<lb/> accelerating them to very high energies.<lb/></p> <p>When particle beams are used to create the<lb/> wake, it is often simply referred to as 'plasma<lb/> wakefield acceleration' , and the disturbance<lb/> is created through electromagnetic repulsion<lb/> between the beam and plasma electrons. For<lb/> laser wakefield acceleration, the radiation<lb/> pressure from the laser beam causes the wake<lb/> formation.<lb/></p> <head>Bubble effect<lb/></head> <p>In the past three years, wakefield acceleration<lb/> has generated its own bubble of excitement.<lb/> Swapan Chattopadhyay, director of the<lb/> Cockcroft Institute, a collaborative accelera-<lb/>tor-research centre opened last year in War-<lb/>rington, UK, says that a wakefield experiment<lb/></p> <figure>Particle accelerators that use plasma technology promise to shake up the fields of high-energy<lb/> particle physics and cancer treatment. Challenges remain, but smaller, cheaper machines are<lb/> within reach. Navroz Patel reports. "</figure> <p>Experiments over<lb/> the next few years<lb/> could make or break<lb/> our field. "<lb/> — Wim Leemans<lb/></p> <p>at the Stanford Linear Accelerator Center<lb/> (SLAC) in California this year has opened up<lb/> a new chapter in accelerator physics.<lb/></p> <p>Using a 400-metre extension of the 3.2-kilo-<lb/>metre main accelerator at SLAC — the longest<lb/> linear accelerator in the world — researchers<lb/> have managed to double the energy of the<lb/> electron beam over a distance of just 85 cen-<lb/>timetres <ref type="biblio">1</ref> . Much of the beam loses energy in<lb/> setting up the plasma wakefield, but a few<lb/> (just 0.02%) of the electrons were acceler-<lb/>ated from 42 gigaelectronvolts to around 85<lb/> gigaelectronvolts. Conventional technol-<lb/>ogy would have to accelerate the<lb/> electrons for around three<lb/> kilometres to achieve<lb/> this pick-up in energy.<lb/> " This trick of sending<lb/> the SLAC's electron<lb/> beam through a<lb/> pl asma j et to<lb/> double its energy<lb/> without having to<lb/> double the size of<lb/> the facility is truly<lb/> remarkable, " says<lb/> Chattopadhyay.<lb/></p> <p>One of the SLAC<lb/> team, accelerator<lb/> physicist Chandrashek-<lb/>har Joshi based at the<lb/> University of California, Los<lb/> Angeles (UCLA), says that tak-<lb/>ing laser wakefield accelerator research<lb/> to SLAC was the logical next step for the field.<lb/> " Short-pulse lasers are powerful, but beams<lb/> typically contain energies of tens of joules, " he<lb/> explains. The energies of particle beams, on<lb/> the other hand, are of the order of kilojoules.<lb/> In other words, particle-beam technology can<lb/> reach much higher energies than contempo-<lb/>rary reliable laser technology.<lb/></p> <head>Splitting ions<lb/></head> <p>In theory, there is no limit to the energies that<lb/> plasma wakefield accelerators could reach. In<lb/> conventional accelerators, particles are accel-<lb/>erated by an electric field — the steeper the<lb/> electric gradient, the greater the acceleration.<lb/> But the field can only increase so far before<lb/> the surrounding cavity material, such as cop-<lb/>per or a superconducting material, starts to<lb/> break down as electrons are stripped from its<lb/> atoms. Because plasma, although electrically<lb/> neutral overall, is already broken down into<lb/> its atoms and electrons, it can support much<lb/> stronger electric fields.<lb/></p> <p>The SLAC experiment was a breakthrough<lb/> on several fronts. It showed that the technol-<lb/>ogy can work at larger distances — reaching<lb/> almost a metre, rather than the couple of centi-<lb/>metres previously achieved with laser technol-<lb/>ogy. It also produced enough energy to be of<lb/> interest in high-energy particle physics. But<lb/> the energy of the accelerated electrons and the<lb/> distance over which they continue to acceler-<lb/>ate are not the only important properties of an<lb/> accelerator. Other<lb/> key factors also<lb/> need to be addressed:<lb/> the number of particles<lb/> accelerated, or energy<lb/> density, should be as high as<lb/> possible, and the particles need to<lb/> have a low energy spread, which means<lb/> that they all have similar energies. With an<lb/> energy spread of 100%, the SLAC experiment<lb/> still has some way to go.<lb/></p> <p>Experiments with laser wakefield accel-<lb/>erators, although operating at lower ener-<lb/>gies and over shorter distances than plasma<lb/> accelerators, are making progress with these<lb/> key factors. In 2004, three groups used lasers<lb/> to accelerate electrons so that they had simi-<lb/>lar energies and reasonable energy densities,<lb/> exceeding 10 9 electrons per beam. These<lb/> experiments rein-<lb/>vigorated interest in<lb/> wakefield accelera-<lb/>tion, which was first<lb/> proposed <ref type="biblio">2</ref> by physi-<lb/>cists Toshiki Tajima<lb/> and John Dawson at<lb/> UCLA a quarter of a<lb/> century earlier.<lb/></p> <p>But to do particle<lb/> collision experi-<lb/>ments, such as those at SLAC, the beams need<lb/> to reach energy densities of 10 34 particles.<lb/> The tiny fraction of electrons accelerated at<lb/> SLAC is nowhere near enough for a collision<lb/> experiment.<lb/></p> <p>Late last year, researchers took wakefield<lb/> acceleration a step further. The 2004 experi-<lb/>ments had accelerated electrons over the 0.1<lb/> gigaelectronvolt range, but a collaboration<lb/> between researchers at the Lawrence Berke-<lb/>ley National Laboratory in California and a<lb/> team led by the University of Oxford's Simon<lb/> Hooker in Britain has now boosted electrons<lb/> to more than 1 gigaelectronvolt <ref type="biblio">3</ref> .<lb/></p> <head>Small steps<lb/></head> <p>This is not yet the high-energy frontier, which<lb/> sits in the region of teraelectronvolts and<lb/> beyond, but it is still a respectable gain on ear-<lb/>lier experiments. " Our next goal is to go up to<lb/> 10 gigaelectronvolts, for which we will need a<lb/> bigger laser — around one terawatt, " says Wim<lb/> Leemans, head of the group at Lawrence Ber-<lb/>keley National Laboratory.<lb/></p> <p>What's more, the researchers were able to<lb/> create narrower particle beams with tight<lb/> beam spreads — the energy spread divided by<lb/> the peak energy. Tight spreads are essential in<lb/> cancer treatment, as the energy determines<lb/> how deeply the protons will deposit their max-<lb/>imum energy<lb/> in the body.<lb/> The research-<lb/>ers achieved a<lb/> beam spread<lb/> of less than 5%,<lb/> compared with<lb/> 10% in 2004<lb/> and 100% just<lb/> a few years ear-<lb/>lier. But there's<lb/> still room for improvement. Karl Krushelnick,<lb/> a wakefield accelerator physicist at the Uni-<lb/>versity of Michigan in Ann Arbor says: " For<lb/> many processes that we would like to use these<lb/> electron beams for, this figure needs to be well<lb/> below 1%. "<lb/></p> <p>Also last year, Victor Malka and his team at<lb/> the Ecole Polytechnique in Palaiseau outside<lb/> Paris developed a technique that uses a second<lb/> counterposing laser beam to create an elec-<lb/>tron beam that can have its energy changed<lb/></p> <figure>Victor Malka uses a counterposing laser (inset) to control the injection<lb/> of electrons into plasma fields.<lb/></figure> <p>" If we can reduce an<lb/> accelerator's size, we<lb/> can reduce the cost<lb/> of proton therapy<lb/> to something very<lb/> small. " — Charlie Ma<lb/></p> <p>on the fly <ref type="biblio">4</ref> . The second laser beam is used to<lb/> control the injection of the electrons that surf<lb/> the wakefield. The resulting accelerated elec-<lb/>trons had an energy spread of less than 10%,<lb/> and by changing the way that the two lasers<lb/> overlap the researchers could tune the energy<lb/> of the beam from 15 megaelectronvolts to 250<lb/> megaelectronvolts. Importantly, the beam<lb/> was much less prone to fail than in previous<lb/> experimental set-ups.<lb/></p> <head>Particles to the people<lb/></head> <p> " We now have a good understanding and<lb/> much of the science worked out, " says Malka.<lb/> " In a sense, what we are left with is the tech-<lb/>nological work needed to improve and sta-<lb/>bilize the machines to create a commercial<lb/> product. " The commercial application that<lb/> Malka has in mind for his group's research<lb/> is cancer treament. Since 2004, he has been<lb/> collaborating with a group led by oncologist<lb/> Uwe Oelfke at the German Cancer Research<lb/> Center in Heidelberg to perform rigorous<lb/> simulations comparing proton therapy with<lb/> X-ray therapy for targeting tumours <ref type="biblio">5</ref> . The<lb/> team hopes to apply its results to patients<lb/> within the next 5 years.<lb/></p> <p>If wakefield researchers make the advances<lb/> they hope to over the coming years, then<lb/> table-top accelerators could become much<lb/> more powerful than they are now. Many<lb/> experiments that are currently the preserve<lb/> of relatively few, typically large and costly,<lb/> facilities will be carried out in the basements<lb/> of universities using compact and cheap<lb/> technology. " Experiments over the next few<lb/> years could make or break our field, " says<lb/> Leemans. " Still, I'm hopeful that we will be<lb/> able to further address issues such as beam<lb/> quality and that wakefield acceleration will<lb/> really prosper. "<lb/></p> <p>Even at the high-energy frontier, the next<lb/> generation of very large accelerators will<lb/> probably incorporate plasma. According to<lb/> Krushelnick, plasma wakefields are the only<lb/> affordable way to achieve the very large accel-<lb/>eration gradients needed to get to extremely<lb/> high energies, perhaps even the terascale.<lb/> Plasma techniques may initially be used to<lb/> boost existing accelerator technology, as with<lb/> the SLAC experiment, or in the staging of<lb/> multiple modules to build a plasma wakefield<lb/> accelerator from scratch. The SLAC team is<lb/> already trying to work out how numerous<lb/> small plasma accelerators can be combined to<lb/> create a reliable machine. And Joshi says that<lb/> he hopes that he and his team can address<lb/> all the remaining critical scientific issues and<lb/> propose an accelerator that is entirely based<lb/> on plasma within 10 years.<lb/></p> <head>■<lb/> Navroz Patel is a writer based in<lb/> New York City.<lb/></head> <p>Standard radiotherapy can restrict<lb/> tumour growth in many patients<lb/> with cancer. It works by delivering<lb/> high doses of X-rays into the body<lb/> so that enough molecules are<lb/> ionized to damage tumour cells.<lb/> But because they are difficult<lb/> to target precisely, X-rays often<lb/> damage healthy tissue around the<lb/> tumour, so doctors cannot use as<lb/> high doses as they would like.<lb/></p> <p>Proponents of proton therapy<lb/> argue that the protons in a<lb/> particle beam should be able to<lb/> target tumours more precisely<lb/> than X-rays do. This is because<lb/> protons lose most of their energy<lb/> just before coming to a standstill<lb/> when travelling through matter.<lb/> Maximum ionization will thus<lb/> occur as the protons approach<lb/> their targeted stopping point,<lb/> which depends on the energy of<lb/> the beam, leaving healthy tissue<lb/> largely untouched. Computer<lb/> simulations performed by<lb/> oncologist Charlie Ma and his<lb/> colleagues at the Fox Chase<lb/> Cancer Center in Philadelphia<lb/> support the idea that proton<lb/> beams generated by wakefield<lb/> accelerators can target tumours<lb/> much more accurately than<lb/> conventional radiotherapy<lb/> techniques (see simulations,<lb/> right).<lb/></p> <p>Others argue that the radiation<lb/> biology of proton therapy is poorly<lb/> understood and the claimed<lb/> superiority of particle beams<lb/> over conventional radiotherapy<lb/> has not been demonstrated<lb/> sufficiently in the clinic.<lb/> " Proton beams have favourable<lb/> physical characteristics, but the<lb/> question is: will that translate to<lb/> improved clinical outcomes? "<lb/> asks Steve Hahn, a radiation<lb/> oncologist at the University of<lb/> Pennsylvania's School of Medicine<lb/> in Philadelphia. " Answering<lb/> that is probably going to take<lb/> randomized phase-III trials. "<lb/></p> <p>Ma says that he has some<lb/> sympathy for this view but<lb/> argues that costs have limited<lb/> the acceptance of proton therapy,<lb/> since it was first proposed in the<lb/> 1940s. Existing proton-therapy<lb/> machines use large and expensive<lb/> conventional accelerators, and so<lb/> need a lot of space. The radiation<lb/> shielding alone can cost around<lb/> US$40 million, according to<lb/> Ma, with the total price tag for a<lb/> proton-treatment centre reaching<lb/> $100 million or more.<lb/></p> <p>With so few clinical facilities<lb/> in the world, phase-III trials of<lb/> the sort Hahn is asking for have<lb/> been few and far between. In one<lb/> of the largest clinical studies 6<lb/> reported so far, 1,255 men given<lb/> proton therapy for prostate cancer<lb/> had survival rates equal to those<lb/> for conventional radiotherapy<lb/> and surgery, but with fewer side<lb/> effects. Ma thinks that affordable<lb/> wakefield accelerators offer the<lb/> best way to address concerns over<lb/> clinical outcomes. Hahn agrees:<lb/> " Wakefield acceleration promises<lb/> to make the technology cheaper<lb/> and widely available and so<lb/> should help resolve the empirical<lb/> controversy. "<lb/></p> <p>Ma is hopeful that the laser<lb/> wakefield facility his group is<lb/> developing in the lab will soon be<lb/> converted into a clinical system.<lb/> If all goes to plan, then the Fox<lb/> Chase Cancer Center will start<lb/> treating its first patients in the<lb/> next 5–10 years, and become a<lb/> prototype clinical facility for a new<lb/> generation of compact proton-<lb/>therapy centres.<lb/></p> <p>In Germany, oncologist Uwe<lb/> Oelfke at the Cancer Research<lb/> Center in Heidelberg thinks that<lb/> he could start using wakefield<lb/> accelerators on patients with<lb/> hard-to-treat eye cancers<lb/> as soon as proton beams of<lb/> 70 megaelectronvolts are available<lb/> — some 12 megaelectronvolts<lb/> more than has been achieved so far<lb/> with laser wakefield acceleration.<lb/> " If something like this could be<lb/> built and operate reliably, it would<lb/> be a huge step, " he says.<lb/> N.P.<lb/></p> <head>Targeting tumours<lb/></head> <figure>Particles can surf along giant plasma waves.<lb/></figure> <figure>The radiation dose (coloured lines)<lb/> can be distributed more tightly<lb/> around a prostate tumour (red)<lb/> with proton therapy (bottom) than<lb/> with conventional radiotherapy.</figure> </text> </tei>