CERN Accelerating science

AWAKE: More plasma = more acceleration

The helicon plasma cell was commissioned in Building 169, where plasma generation tests are ongoing (Image: CERN)

In May 2018, the AWAKE experiment carried out the first ever acceleration of electrons using a wakefield created by protons flowing through plasma. AWAKE demonstrated that it is not only possible but also efficient to use plasma wakefields generated by proton beams to accelerate charged particles, thereby fulfilling the objective of the AWAKE Run 1 phase. The experiment was carried out over a distance of 10 metres, with a rubidium plasma cell.

The next phase, AWAKE Run 2, will start after the LS2 and involves maintaining the quality of an electron beam when it is accelerated and demonstrating the feasibility of the technology over several hundred metres. “The AWAKE plasma is currently produced by sending a laser pulse which transforms rubidium gas into plasma by ionisation. This works well as the AWAKE cell is 10 metres long, but this ionisation method is not appropriate for a larger scale”, explains Alban Sublet, an applied physicist in the Vacuum, Surfaces and Coatings group within the Technology Department.

This is where the helicon plasma cell comes in. A helicon wave is a low frequency electromagnetic wave capable of generating very high-density plasmas, like those needed for AWAKE. “We are currently working with a 1-metre prototype helicon plasma cell developed by the Institute for Plasma Physics in Greifswald (Germany). In this set-up, helicon waves are generated by radiofrequency antennas, which surround a quartz tube filled with argon at low pressure”, explains Alban Sublet. In theory, this set-up should enable very long cells to be created as the tube can be extended and antennas added to spread the plasma over long distances.

Ensuring that the generated plasma remains homogeneous throughout the cell remains a challenge. How can we be sure that the density required for AWAKE is uniformly reached throughout the cell? “For the time being we only have a diagnostic tool that enables us to measure the density profile of the plasma locally”, points out Alban Sublet. “So far, we have deduced the density of the rubidium plasma cell indirectly by measuring the density of the rubidium gas before it is ionised”, adds Edda Gschwendtner, technical coordinator and leader of the AWAKE project at CERN.

To resolve this problem, the team responsible for the tests is currently working in collaboration with the University of Wisconsin (UW), Madison (United States) and the Swiss Plasma Center (SPC) at the EPFL in Lausanne. Two plasma diagnostic techniques are currently being studied and will be tested at CERN in 2020. “Developing a diagnostic tool capable of measuring the uniformity of the plasma with the precision required by AWAKE throughout the length of the cell is a significant challenge”, explains Alban Sublet. “Once we have developed a reliable diagnostic method, we will be able to optimise the helicon plasma cell and then design a longer helicon cell for use in AWAKE in a few years.” Synergies exist with the field of coatings and surface treatments, where this type of plasma might be used in the future, as is the case for certain industrial applications.

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Small proton bundles are needed to separate the charges in the plasma, thus creating surfable waves for the electrons. 

Plasma wakefield acceleration technology has the potential to revolutionize linear lepton colliders.  Plasma can sustain accelerating fields that are many orders of magnitude higher than the fields of conventional radio-frequency cavities. Thus, plasma-based acceleration could substantially decrease the length (and possibly cost) of a future linear accelerator.

Plasma wakefields can be excited by a highly relativistic particle bunch, the so-called drive bunch. As the bunch enters the plasma, plasma electrons respond to its electric field and start oscillating at the plasma frequency; their oscillations sustain the wakefield. The wakefield’s longitudinal and transverse components accelerate and focus a witness bunch. Similar to a surfer on a water-wave, the witness bunch gains energy from the wakefield.

The drive bunch excites large-amplitude plasma wakefields, when it is about one plasma wavelength long (typically < 3 mm) and its density similar to the plasma electron density (> 1014/cm3). Such short dense particle bunches are available at facilities such as SLAC, but their energy content is small (<100 J) and when exciting wakefields, their energy depletes over a short distance.

Proton bunches at CERN carry much larger amounts of energy, in excess of 10 kJ. This is enough to drive several GV/m field amplitudes over hundreds of meters. Unfortunately, their bunch length is on the order of 6-12 cm, at least 20 times too long.  Additionally, their particle density is approximately 10-100 times too low.

The AWAKE experiment at CERN recently demonstrated for the first time that - using the seeded self-modulation process - one can use those long proton bunches to excite high amplitude wakefields. As the long bunch enters the plasma, it drives low amplitude seed wakefields (~10 MV/m). The transverse wakefields act back on the bunch and periodically focus and defocus it. Defocused protons leave the bunch. After all defocused protons have left, focused regions form a train of ‘micro-bunches’. Each micro-bunch drives its own wakefield and these fields add resonantly, resulting in a large amplitude wave.

Figure 1. As the long bunch enters the plasma, it drives low amplitude seed wakefields (~10 MV/m). (Image: AWAKE)

The plasma itself modulates the proton bunch. Thus, micro-bunches are spaced at the plasma wavelength. This is a fundamental physics property of the process. AWAKE measured the proton micro-bunch structure using a streak camera with pico-second time resolution and showed directly that the self-modulation occurs. Measurements of the micro-bunch spacing (or frequency) are in good agreement with the theoretically predicted value for various plasma densities.

Figure 2. Streak camera measurement of the self-modulated proton bunch. (Image: AWAKE)

Further, AWAKE measured the transverse deflection of the protons that were defocused during the self-modulation process on a screen after the plasma exit. It was shown that such a large transverse displacement can only be achieved if the transverse wakefield amplitudes exceed the amplitude driven by the unmodulated proton bunch. This proves that – as expected - the wakefields have grown along the bunch. Time-resolved measurements of the defocused protons confirm that the displacement of the defocused protons increases along the bunch.

This is clear evidence of proton bunch self-modulation and excitation of high amplitude wakefields in plasma. AWAKE also accelerated externally injected ~18 MeV witness electrons to 2 GeV in these wakefields as published in a Nature article last year. The longitudinal wakefield amplitude reached values comparable to the transverse one. All physics concepts required for proton-driven wakefield acceleration have now been validated, paving the way for more in-detailed studies.

The path to a plasma-based collider is still long as there are many challenges that need to be overcome. But the outstanding success of this proof-of-principle experiment now opens the door for further breakthroughs: demonstration of good witness bunch quality after acceleration, scalability of the plasma and acceleration process and very high witness electron bunch energy. These will the primary goals of AWAKE Run 2 which will start in 2021.

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