The energy of electron-positron colliders is limited by the acceleration gradient (energy per distance) and the length of the accelerator (=the cost). In the past the energy mainly increased from making accelerators larger. Much larger accelerators than today get prohibitively expensive, however. More compact designs could keep them affordable. While conventional cavities still make some progress in the acceleration gradient (e.g. for XFEL), they only reach about 30-35 MeV/m. Increasing this much more will need new approaches, but could lead to much higher collision energies. It would also be useful for future x-ray lasers. One proposed concept is the Compact Linear Collider (CLIC), using a high-intensity low-energy beam to create electromagnetic waves to accelerate a low-intensity high-energy beam. The goal is 100 MeV/m, three times more than conventional RF cavities. While the general concept has been demonstrated before, it is still a challenge to scale that up to a full accelerator concept. Scientists at Argonne now managed to combine two acceleration stages: Electron acceleration through two successive electron beam driven wakefield acceleration stages The stages were just 3.5 cm long and increased the energy of a beam by 2.4 MeV per stage, corresponding to 70 MeV/m. That is not enough for a proper collider, but it is an important step towards longer acceleration tracks. They hope to increase this to 300 MeV/m and more stages as demonstration object for a full accelerator. That is beyond the CLIC goals and would allow an even higher energy or a more compact design. There is also plasma wakefield acceleration as competing technology - replacing the cavities by a plasma and letting the drive beam fly through the same path as the accelerated beam. While the achieved gradients are much higher here (100 GeV/m over short distances, 50 GeV/m over about a meter) the beam quality decreases - it is unclear if this approach can be used in multiple stages and for a collider where a good focus is crucial to achieve high collision rates. Several groups are testing this. If this can be built in stages and used for an accelerator, then we might see completely new energy regions with future accelerators.