Plasma wakefield accelerators: beam quality and staging

4 problems

• Fundamental trade-off between efficiency and beam-breakup instability

  Partially resolved

  To be efficient, the accelerated bunch must extract much of the wake's energy, but that also makes the bunch wobble sideways and break up. How efficient a stable plasma accelerator can be is not known.

• Ion motion in plasma wakes driven by collider-density beams

  Open

  Collider bunches are so dense that they pull the plasma's ions inward while passing, which spoils the smooth focusing the wake normally gives. How much this damages beam quality at collider parameters is not settled.

• Collider-quality positron acceleration in plasma

  Open

  The usual plasma wake focuses electrons but pushes positrons apart, so positrons cannot be accelerated in it with good quality. A collider needs both, and no scheme yet works for positrons in experiment.

• Multistage plasma acceleration with preserved beam quality

  Open

  Each plasma stage can only add a limited energy, so a collider needs many stages in a row. Passing a beam from one stage to the next without losing particles or blurring it has not been demonstrated.