Wake Perturbations in Laser- and Beam-Driven Plasma Wakefield Accelerators: A Symmetry-Based Multipole Classification
We review beam-quality physics in laser-driven (LWFA) and beam-driven (PWFA) plasma wakefield accelerators through the symmetry group of the idealised blowout wake -- axisymmetry $\mathrm{SO}(2)_ϕ$, adiabatic longitudinal translation, and propagation-direction parity. Transverse perturbations of the wake are classified by an integer azimuthal multipole order $m$ labelling the irreducible representations of $\mathrm{SO}(2)_ϕ$, with the lowest beam-quality observables coupling at a specific multipole: the bunch centroid at $m=1$, cross-plane emittance coupling at $m=2$. A symplectic analogy relates transverse matching to longitudinal beam loading. Several phenomena common to LWFA and PWFA -- hose instabilities, pulse-front-tilt jitter, spot-asymmetry emittance growth, polarisation-dependent centroid motion, resonant cross-plane mixing -- populate the two lowest non-trivial $m$-channels and admit a unified discussion. The positron-witness problem reorganises in the same language: each known mitigation abandons one specific feature of the uniform-density blowout, drawn from a finite set. The classification also raises the possibility of an $m=3$ response channel whose magnitude remains open. We note the connection to symmetry-equivariant Bayesian optimisation of plasma accelerators.