R. Tikhoplav

Angular momentum measurement of the FNPL electron beam

In the flat beam experiment at Fermilab/NICADD Photoinjector Laboratory(FNPL), it is essential to have a nonvanishing longitudinal magnetic field on the photocathode. The canonical angular momentum of the electron beam generated by this magnetic field is an important parameter in understanding the round to flat beam transformation. In this paper, we report our measurements of the canonical angular momentum, which is directly related to the skew diagonal elements of the beam matrix before beam is made flat. The measurements of the other elements of the beam matrix are also reported.

Observations of low-aberration plasma lens focusing of relativistic electron beams at the underdense threshold

Focusing of a 15 MeV electron bunch by a plasma lens operated at the threshold of the underdense regime has been demonstrated. The strong, 1.7 cm focal length, plasma lens focused both transverse directions simultaneously and reduced the minimum area of the beam spot by a factor of 23. It is shown through analytic analysis and simulation that the observed spherical aberration of this underdense lens, when expressed as the fractional departure of the focusing strength from its linear expectation, is ΔK/K=0.08±0.04. This is significantly lower than the minimum theoretical value for the spherical aberration of an overdense plasma lens. Parameter scans showing the dependence of focusing performance on beam charge, as well as time resolved measurements of the focused electron bunch, are reported.

Observations of underdense plasma lens focusing of relativistic electron beams

Focusing of a 15 MeV, 19 nC electron bunch by an underdense plasma lens operated just beyond the threshold of the underdense condition has been demonstrated in experiments at the Fermilab NICADD Photoinjector Laboratory (FNPL). The strong 1.9 cm focal-length plasma-lens focused both transverse directions simultaneously and reduced the minimum area of the beam spot by a factor of 23. Analysis of the beam-envelope evolution observed near the beam waist shows that the spherical aberrations of this underdense lens are lower than those of an overdense plasma lens, as predicted by theory. Correlations between the beam charge and the properties of the beam focus corroborate this conclusion.

RESULTS FROM THE UCLA/FNPL UNDERDENSE PLASMA LENS EXPERIMENT

A gaussian underdense plasma lens with peak density 5 x 10{sup 12} cm{sup -3} and a full width half maximum (FWHM) length of 2.2 cm has been used to focus a relativistic electron beam. This plasma lens is equivalent in strength to a quadrupole magnet with a 150 T/m field gradient. The lens focused a 15 MeV, 16 nC electron beam with initial dimensions {sigma}{sub x,y} {approx} 650 {micro}m and {sigma}{sub z} {approx} 6.5 mm onto an optical transition radiation (OTR) screen {approx}2 cm downstream of the lens. The average transverse area of the plasma focused electron beam was typically demagnified by a factor of 23. The evolution of the beam envelope in the area near the beam waist was measured for both round beams and asymmetric beams with x:y aspect ratios as large as 1:5. The light from the OTR screen in the round beam case was also imaged into a streak camera in order to directly measure the correlation between z and {sigma}{sub r} within the beam.

UCLA/FNPL Underdense Plasma Lens Experiment: Results and Analysis

Focusing of a 15 MeV, 16 nC electron bunch by a gaussian underdense plasma lens operated just beyond the threshold of the underdense condition has been demonstrated. The strong 1.9 cm focal length plasma lens focused both transverse directions simultaneously and reduced the minimum area of the beam spot by a factor of 23. Analysis of the beam envelope evolution observed near the beam waist shows that the spherical aberrations of this underdense lens are lower than those of an overdense plasma lens, as predicted by theory. Time resolved measurements of the focused electron bunch are also reported and compared to simulations.

The UCLA/FNPL Underdense Plasma Lens Experiment

Summary form only given. An underdense plasma lens experiment is underway as a collaboration between UCLA and the Fermilab NICADD Photoinjector Laboratory (FNPL). The experiment will focus on measuring the variation of the plasma focusing along the longitudinal beam axis and comparing these results with theory and simulation. The experiment utilizes a thin Gaussian underdense plasma lens with peak density 5times1012 cm-3 and a FWHM length of 2.2 cm. This plasma lens will have a focusing strength equivalent to a quadrupole magnet with about 180 T/m field gradient. A 15 MeV, 8-12 nC electron beam with nominal dimensions sigmar = 600 mum and sigmaz= 2.1 mm will be focused by this plasma lens onto an OTR screen approximately 2 cm downstream of the lens. The light from the OTR screen will be imaged into a streak camera in order to directly measure the correlation between z and sigmar within the beam. Status and progress on the experiment are reported

Status of the UCLA/NICADD Plasma Density Transition Trapping Experiment

Plasma density transition trapping is a recently proposed self‐injection scheme for plasma wake‐field accelerators. This technique uses a sharp downward plasma density transition to trap and accelerate background plasma electrons in a plasma wake‐field. This paper recounts the first attempt to demonstrate density transition trapping experimentally. The goal of the experiment is to capture a ∼ 100 pC, 1.5 MeV beam with 4% rms energy spread out of a 2.5×1013 cm−3 peak density plasma using a 6nC, 14 MeV drive beam. The first experimental run occurred at the Fermilab NICADD Photoinjector Laboratory (FNPL) between January and May 2004. While several key objectives were achieved, we were unable to achieve the drive beam parameters necessary for the experiment due to technical problems. We are in the process of resolving these problems in preparation for a second experimental run.