Electron acceleration at AWAKE
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The LHC deliver kJ-level proton bunches, more energy than any other lepton or laser pulse driver ever produced or envisioned on earth. As a result, because these proton bunches carry so much energy, they promise to drive a plasma accelerator capable of generating 1 TeV electron bunches in a single stage, in a plasma with a few hundreds of meters long [caldwell'07]. This is the main motivation for the development of the AWAKE experiment at CERN. AWAKE is plasma acceleration research program that uses proton bunches from the Super-Proton-Synchrotron (SPS) ring to excite plasma waves enabling high gradient electron acceleration.
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The proton bunches at CERN are long, with a length of about 10 cm. This is much longer than the sub-mm plasma wavelengths in AWAKE. As a result, plasma waves need to be excited gradually, as the proton bunch propagates in the plasma, through the so called seeded self-modulation instability (S-SMI) [kumar'10]. In the S-SMI, protons residing in the defocusing field regions are expelled from the plasma radially. As a result, this instability modulates the initially 10 cm long proton bunch into a train of proton beamlets, separated by the plasma wavelength and with a length close to the plasma wavelength [turner'19,adli'19]. In this configuration, the protons can excite plasma waves resonantly, just as if we strike a pendulum at its natural frequency of oscillation.
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The movie below illustrates the dynamics of a long particle bunch in a plasma in the conditions of AWAKE. The first part of the movie shows the development of the S-SMI. The second part of the movie shows a scenario where a competing instability that leads to beam-break-up occurs. We have found the conditions that lead to the suppression of beam-break-up after the saturation of S-SMI [vieira'14]
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The large amplitude plasma waves that result from the S-SMI, can then accelerate electrons. development of the S-SMI This was recently shown in experiments at CERN, which the proved many of the necessary steps for a successful plasma accelerator at CERN. To know more please find the paper here: E. Adli et al. Nature 121, 054801 (2018).
Bibliography
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[caldwell'07] A. Caldwell et al, Nature Physics 5, 363 (2009)
[kumar'10] N. Kumar et al, Phys. Rev. Lett. 104, 255003 (2010)
[turner'19] M. Turner et al, Phys. Rev. Lett. 122, 054801 (2019)
[adli'19] E. Adli et al, Phys. Rev. Lett. 122, 054802 (2019)
[vieira'14] J. Vieira et al, Phys. Rev. Lett. 112, 205001 (2014)
[adli'18] E. Adli et al, Nature 561, 363–367 (2018)