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CERN prepares to test revolutionary mini-accelerator

Machines that ‘surf’ particles on electric fields could reach high energies at a lower price.

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The home of the Large Hadron Collider (LHC), the world’s largest particle accelerator, is getting a new machine — and this time, the whole point is to keep it small.

On 18 September, the council that governs CERN, Europe’s premier particle-physics laboratory, near Geneva, Switzerland, approved a boost in funding for a planned experiment called the Advanced Wakefield Experiment, or AWAKE. Due to switch on next year, AWAKE will accelerate particles by ‘surfing’ them on waves of electric charge created in a plasma, or ionized gas. It is a method that could allow future accelerators to probe matter and the forces of nature at ever-higher energies, without the usual accompanying increase in the instruments’ size and therefore cost.

Although plans are afoot to build bigger machines once the LHC reaches the end of its life in the 2030s, many fear that accelerator size is nearing its limit and that such proposals may simply prove too expensive to implement.

“When you look at cost estimates for these machines and the scale of machines, you understand that maybe a new breakthrough regime is needed,” says Nick Walker, an accelerator physicist at DESY, Germany’s high-energy-physics laboratory in Hamburg.

Conventional colliders, such as the 27-kilometre-long LHC, use electric fields to move charged particles through a tunnel; the fields switch from positive to negative at a frequency that means the particles are constantly nudged forward, gaining energy with each push. But such colliders use metal-walled cavities that spark if the electric field is too strong. As a result, the only way to further increase the particles’ speed, and therefore energy, is to lengthen the tunnel.

Plasma wakefield accelerators, which were first proposed in the 1970s, are designed to break this cycle, says physicist Allen Caldwell at the Max Planck Institute for Physics in Munich, Germany, who will lead the AWAKE experiment. They send a pulse of charged particles or laser light through a plasma, which sets electrons and positively charged ions oscillating in its wake. The resulting regions of alternating negative and positive charge form waves that accelerate further charged particles. Injected at just the right time, these particles effectively surf the waves (see ‘Wakefield acceleration’). Crucially, as the electric fields are much stronger than those in a conventional collider, the acceleration can be as much as 1,000 times greater over the same distance.

Such accelerators exist in prototype at several facilities around the world, but AWAKE will be the first time that CERN has experimented with the technology. “CERN is the world’s high-energy physics lab right now, and the fact that it has decided this is an important field to get involved in is a bit of validation for this community,” says Mark Hogan, an accelerator physicist at the SLAC National Accelerator Laboratory in Menlo Park, California.

Different groups have different ways of setting the plasma oscillating: Hogan’s team at SLAC uses pulses of electrons, for example. AWAKE will be the first to use pulses of protons, which have some big advantages.

Because protons have greater mass than electrons, each proton pulse penetrates further into the plasma, setting up a longer series of charged regions, which in turn provides greater acceleration per pulse. A proton machine is also compatible with the LHC, which accelerates and collides protons.

“The fact that CERN has decided this is an important field to get involved in is a bit of validation for this community.”

For now, AWAKE will use the proton bunches that feed the LHC to test whether protons can generate the electric fields necessary to accelerate particles in plasma.

The latest investment from CERN — worth 2.6 million Swiss francs (US$2.7 million), from the total of 21.4 million Swiss francs so far committed to the experiment — is intended to allow AWAKE to test the concept before the end of 2018, when CERN is scheduled to shut down its accelerators for an upgrade. Success will depend on whether these proton bunches, which are long relative to what is needed to create plasma waves, can be efficiently chopped up into short pulses.

Eventually, it might be possible to inject the much-higher-energy protons that have been accelerated by the LHC into a plasma wakefield machine for further acceleration. Hogan estimates that a machine just a few kilometres long could produce electrons with 6 times the energy of those that would be produced by the next planned conventional accelerator, the 31-kilometre-long Inter-national Linear Collider.

Despite such promise, plasma accelerators are decades from practical use because, to do better than existing accelerators, they must also match them in efficiency — supplying focused, accelerated particles at high rates as well as high energies, says Walker. Still, he adds, “right now, this is the only thing I see that might work”.

The technology might also be useful elsewhere. Wakefield-accelerated electrons could drive X-ray free-electron lasers, which probe matter using powerful bursts of light that are short enough to capture the motions of molecules. These are currently kilometres long — but using wakefield technology might allow them to fit into labs or hospital basements. “I think this is more realistic as a potential application,” says Walker, “and I would say a mandatory first step before the plunge into trying to achieve high-energy-physics experiments.”

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  1. Avatar for Rodney Bartlett
    Rodney Bartlett
    Referring to "CERN to test revolutionary mini-accelerator" by Elizabeth Gibney: Nature, 8 October 2015, Volume 526, p. 173 - It's doubtful this version of Wakefield acceleration will work as well as CERN hopes. The article states as the proton pulse speeds through the plasma, electrons are attracted towards the centre but overshoot because the pulse has moved on by the time the electrons arrive at the centre. It seems that positive charge would accompany the proton pulse, and that it's unlikely there would be enough residual positive charge in the region vacated to enable cycling from positive to negative charge to set up a wave of electron density that can be used to accelerate injected electrons. Continuously accelerating injected electrons could be achieved by attaching them behind the proton pulse. The injected electrons would be attracted forwards to the positive pulse and also repelled forwards by the plasma electrons that are at the centre (before they overshoot it). An alternative is implied by pages 66-67 of "A Brief History of Time" by Stephen Hawking (Bantam Press, 1988) which says - "What the spin of a particle really tells us is what the particle looks like from different directions. A particle of spin 1 ... looks different from different directions. Only if one turns it round a complete revolution (360 degrees) does the particle look the same." (Spin 1 is illustrated by the playing card Ace of Spades.) To me, turning a revolution (or part of one) suggests quantum spin might be analogous to warping. This stuck in my mind, and I combined it with a paper by Einstein which I discovered some 15 or 20 years later - A. Einstein, “Speilen Gravitationfelder in Aufbau der Elementarteilchen eine Wesentliche Rolle?” (Do gravitational fields play an essential role in the structure of elementary particles?), Sitzungsberichte der Preussischen Akademie der Wissenschaften, (Math. Phys.), 349-356 (1919) Berlin. In this way, I equated quantum spin with gravitational warps (more about this soon). Professor Hawking writes on p. 67, "there are particles which do not look the same if one turns them through just one revolution: you have to turn them through two complete revolutions! Such particles are said to have spin 1/2 (and they make up the matter in the universe)". I combined this turning through two complete revolutions with the fact that you have to go around a Möbius Strip twice to arrive at your starting point (more about the Möbius soon, too). The weak energy of gravitational waves combines with the 10^36-times-stronger energy of electromagnetic waves to make matter and mass. String theory says everything's composed of tiny, one-dimensional strings that vibrate. Replacing the word "strings" with "bits" (BInary digiTS - 1's and 0's); translation of fluctuating, 1-D bits into matter could be via electromagnetic and gravitational waves being disturbances in fields. These disturbances known as virtual particles are equivalent to energy pulses that produce the binary digits of 1 and 0 encoding pi, e, √2 etc. (“Our Mathematical Universe” by cosmologist Max Tegmark – Random House/Knopf, January 2014 believes the universe has a mathematical foundation). Matter particles [and even bosons like the Higgs, W and Z particles] are given mass by the energy of electromagnetic and gravitational waves interacting in "wave packets” (interaction within this term from quantum mechanics results in wave-particle duality). There are 2 forms of spin - classical (e.g. a rotating top) and quantum. The latter can't be explained classically but may possibly be explained by particles and space mutually affecting each other. According to General Relativity, matter causes a gravity field by its mass creating depressions in space that can be pictured as a flexible rubber sheet. Space could affect particles through its curvature (gravity) infiltrating particles, thus giving them quantum spin. If space-time is curved as a result of being modeled on the twisted Mobius strip, particles of matter and antimatter would also be twisted up to 180 degrees. This gives them a non-rotating “quantum spin” which does not have unlimited values (as visualizing the continuous curvature of a Mobius strip might imply) but is restricted to certain values by the more fundamental operation – that of the 1’s and 0’s (I'm suggesting that this more fundamental operation can replace descriptions using quarks). Remember, 1's and 0’s need not only represent “on” and “off” – they can also represent “increase” and “decrease”; resulting in spins of 0, 1/2, 1, 3/2, 2, etc.) There would be the ordinary matter we see and touch, which could be labeled positive. At the extremity of 180 degrees; there would also exist an inverted, negative form of that matter. This would be as invisible to us as the curving of space, and only detectable through its gravitational effects. It would be referred to as Dark Matter existing in what can only be called a 5th-dimensional hyperspace. If combined with the intense strength of electromagnetism (some 10^36 times more powerful), concentration of repelling gravitational waves might account for universal expansion. Imagine waves constituting a particle and its quantum spin being curved and twisted to a degree that exceeds the warping responsible for the particle's quantum spin, but is less than that responsible for dark matter. Then neither a particle of matter nor a particle of dark matter would be in existence. Since gravitational waves can create electromagnetism*, we'd have a particle whose electric charge could be the opposite of the original particle ie we could have a negative electron's antiparticle, the positive positron. And (the alternative to attaching electrons behind proton pulses) appropriate interaction between gravity and the electromagnetics would change the positive charge of the protons to negative charge like an electron while the plasma's electrons were at the centre of the plasma. When they've overshot and moved towards the outside, the protons are reverted to positivity so they can attract other plasma electrons to the centre as they pulse forward. Then the protons again become negative antiprotons to be repelled by the plasma electrons at the centre, and positive protons to attract more electrons from the plasma's outside. The cycling from positive to negative charge, and back again, keeps on repeating ... Given sufficient interaction between the gravitational and electromagnetic components, positive and negative and neutral particles with a variety of masses could exist within the range of gravitational warping associated with any particular quantum spin. This permits existence of a "particle zoo" which is unified into one thing: gravitational-electromagnetic interaction. * When Einstein penned E=mc^2, he used c (c^2) to convert between energy units and mass units. The conversion number is 90,000,000,000 (light's velocity of 300,000 km/s x 300,000 km/s) which approx. equals 10^11. Gravity waves with a strength of 1 are, via quantum gravitational lensing, concentrated 10^24 times after they’re focused to form matter (to 10^25, weak nuclear force’s strength - giving the illusion that a weak nuclear force* that is not the product of gravitation exists). Waves are magnified by the matter's density to achieve electromagnetism’s strength (10^36 times gravity's strength) i.e. 10^25 is multiplied by Einstein's conversion factor [10^11] and gives 10^36 (this gives the illusion of the existence of electric and magnetic fields that are not a product of gravitation). (The gluons that bind mesons would likewise be either products of gravitation or, like quarks, replaceable by the more fundamental 1's and 0's.) After absorption by atoms, the depleted remnant of the gravity waves is re-radiated from stars, interstellar gas and dust, etc. It’s radiated as gravitational waves (a Gravity Wave Background, challenging the idea that Cosmic Inflation was necessary to generate gravitational waves) which have lost most of their energy or strength during formation of forces (returning to a strength of 1). Since gravity can produce electromagnetism, it’s also radiated as electromagnetic waves – including an infrared background whose heat output exceeds that of the stars alone, in addition to a microwave background. The latter challenges the idea that existence of the cosmic microwave background proves the universe began with the traditional Big Bang (for a nontraditional Big Bang, see "Binary Digits and Topology Create Hybrid Big-Bang/Steady-State Universe Unified as One Qubit" by R. Bartlett, July 2015, binary-digits-and-topology-create-hybrid-big-bang-steady-sta-t591.html) * Remember, this is only one example: the so-called weak force’s “strength isn’t constant” and varies with distances [“The Strengths of the Known Forces” by theoretical physicist Matt Strassler [May 31, 2013] - ].
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