# Anomalous Beam Transport through Gabor (Plasma) Lens Prototype
## 1 Introduction
Experiments with ion beams confirmed the focusing capability of the Gabor lens and observed emittance growth [PERSON] (1947); [PERSON] (1947); [PERSON] (1947). The mechanism for electron production and the inhomogeneity in the electron density within the lens were believed to cause the observed growth in emittance [PERSON] (1947). The focusing strength of a Gabor lens is determined by the electron density. The theoretical maximum electron density is related to the electric and magnetic field strength [PERSON] (1953). Careful design of the field configuration allowed certain lenses to operate at electron densities of 61% of the theoretical maximum [PERSON] (1953). At high pressure, the electrons are lost due to the radial expansion of the plasma driven by collisions with neutral atoms. At low pressure, this radial transport can be caused by small azimuthal asymmetries in the applied electric or magnetic fields [PERSON] (1947). Further work was directed towards the design of an electrostatic lens for the space-charge-compensated transport of a high intensity heavy ion beam [PERSON] (1947). In this case, the absence of emittance growth due to the lens was reported [PERSON] (1947). Advances in simulation and finite-element analysis have been exploited to calculate the expected focusing strength of a space-charge lens and to study the resulting phase-space transformation on a beam passing through the lens. Good agreement between experimental results and beam-transport simulations [PERSON] (1947); [PERSON] (1953) suggest that plasma instabilities are a likely cause of beam aberrations. Experimental observations and numerical results [PERSON] (1947) have confirmed that the confined plasma is vulnerable to the diocotron instability [PERSON] (1947). Further studies are required to characterise the configurations under which a Gabor space-charge lens operates in a stable regime. Plasma-lens focusing for electron beams is being developed by the CERN Linear Electron Accelerator for Research (CLEAR) collaboration (1953). Evidence of aberrations in the CLEAR lens due to radial non-uniformity of the plasma temperature has been observed [PERSON] (1953); [PERSON] (1953). Previous work at the Stanford Linear Accelerator Center (SLAC) demonstrated the plasma-lens focusing of 28.5 GeV electron Gabor (1953) and positron Gabor (1953) beams. Numerical simulations were able to describe the observed non-linear focusing force in this experiment Gabor (1953). Discharge-capillary active plasma lenses were also investigated as compact devices for focusing 100 MeV-level electron beams produced by a gas-jet-based laser-plasma accelerator. Both weak and strong chromatic effects [PERSON] (1953) were observed with the potential to cause emittance degradation [PERSON] (1953). Research at SLAC seeks to demonstrate the use of such a plasma lens for staging in plasma wakefield acceleration or in radially-symmetric final focusing for linear colliders [PERSON] (1953). As compact and tunable devices, active plasma lenses [PERSON] (1953); Gabor (1954) are a promising solution for the extraction and transport of the witness bunch while removing the driver without loss of beam quality [53; 54; 55]. Finally, electron plasmas were studied in multicell traps [56; 57] to develop methods by which the number of accumulated positrons could be increased compared to the present limits coming from the requirement for long-term confinement. In this paper we report the performance of a Gabor lens prototype constructed at Imperial College London. The lens was exposed to 1.4 MeV protons at the Surrey Proton Beam Centre [58]. The effect on the beam is presented and compared to the results of a simulation of the impact of plasma instabilities on the focusing forces produced by the lens.