The plasma lens was proposed(P. Chen, Part. Acc. 20), 171 (1987). as a final focusing mechanism to achieve high luminosity for future high energy linear colliders. Previous experiments(See, for example, R.~Govil~et al.), Phys. Rev. Lett, 86, No. 16, 3202 (1999), and references therein. to test this concept were carried out at low energy densities. In this talk, results from the SLAC E-150 experiment(P.~Chen~et al.), Proposal for a Plasma Lens Experiment at the Final Focus Test Beam, SLAC Expt. Prop. E-150, April 1997. on plasma lens focusing of a high energy density beam with parameters relevant to linear colliders are presented and compared with theoretical expectations. The experiment was carried out at the SLAC Final Focus Test Beam, with nominal parameters of 30 GeV beam energy, 1.5\times 10^10 electrons per bunch, bunch length \sigma_z = 0.7~mm and beam cross-section \sigma_x^* \times \sigma_y^* = 7~\mum \times 3~\mum. The plasma lens was produced by a fast pulsing gas-jet providing a neutral Nitrogen gas column with density up to 5\times 10^18 / cm^3. The gas was then ionized by the leading portion of the incident high energy density electron beam, while the rest of the electrons in the same bunch were focused by the strong plasma pinching force and a reduction in the beam size of up to 40% was measured. The beam waist was also measured and compared with detailed numerical calculations with a particles-in-cell code. The reduction in focal length indicated a focusing strength approximately 100 times that of the FFTB final focus magnets. The synchrotron radiation with critical energy in the 1-10 MeV range due to the strong bending of beam particles inside the plasma lens was observed for the first time.
[C6.002] Results from SLAC Experiment on Plasma Wake field Acceleration over One Meter
Christopher E. Clayton (University of California at Los Angeles)
In the E-157 experiment, a 30 GeV electron beam of 2e10
electrons in a 0.65mm long bunch is propagated through a
1.4m long lithium plasma (created by UV ionization) of
density \sim 2e14/cm^3. The beam density is greater than the
plasma density and the head of the bunch expels the plasma
electrons leaving behind an ion channel with transverse
focusing fields of up to several thousand Tesla/m. There are
three types of ongoing studies: (1) The zeroth order effect
is the ``thick plasma lens" where the induced
focusing-channel causes the beam to undergo so-called
betatron oscillations where the envelope of the beam
oscillates radially. (2) Transverse head-tail effects are of
higher order: non-axially-symmetric longitudinal charge
distributions cause different slices of the beam to
oscillate radially with different periods, or may even be
unstable, distorting the beam further. (3) For electron
bunch lengths on the order of half the plasma wavelength,
the ion column immediately behind the head of the electron
bunch will cause the main bunch to lose energy. The plasma
electrons expelled from the beam will rush back to the axis
and produce a strong accelerating force (order GeV/m) for
the particles in the tail of the same bunch. The betatron
oscillations are studied by scanning the plasma density and
observing time-integrated images of optical transition
radiation and Cherenkov radiation from foils downstream of
the plasma. Energy changes in the beam are observed from
time-resolved images of the Cherenkov radiator located in a
dispersive section of the downstream beamline. Head-tail
effects are seen in these images and can be isolated from
energy gain by acquiring time-resolved images from the
Cherenkov radiator in the non-dispersive direction.
Beam-position monitors and beamline feedback signals also
reveal information regarding beam centroid motion induced by
the plasma. Progress on the experiment will be reported.
[C6.003] Laser pulse evolution and electron acceleration in plasmas
Eric Esarey (LBNL)
Laser-driven plasma-based accelerators(For a review
see, E. Esarey et al., IEEE Trans. Plasma Sci. 24, 252
(1996).) require the propagation of intense laser pulses
over long distances in plasmas, the generation of large
amplitude wakefields, and the injection and acceleration of
electrons. This talk will discuss the nonlinear propagation
of short laser pulses in plasmas, with or without channels.
Non-paraxial effects will be analyzed and simulated,
including finite pulse duration, finite group velocity, and
dispersion(E. Esarey et al., Phys. Rev. Lett.,
submitted.). These effects on the evolution of the forward
Raman and self-modulation instabilities, that lead the
generation of wakefields, will be examined. Also discussed
are methods for self-trapping and injecting electrons into
the wakefield. Application to ongoing experiments at
LBNL(W.P. Leemans et al., Phys. Plasma 5, 1615
(1998); in preparation.) will be discussed.
[C6.004] Development and application of laser-plasma driven electron and ion accelerators
Donald Umstadter (Center for Ultrafast Optical Science, University of Michigan, Ann Arbor)
Intense laser pulses interacting with plasmas are giving
rise to novel table-top accelerators of electrons and ions.
With laser intensities of 10^19 W/cm^2 and wavelengths
of 1-\mum, we observe the acceleration of MeV-energy
electrons and protons into well collimated beams. In the
case of electrons, we observe from an underdense plasma a
narrow beam with less than a three-degree divergence angle.
Detailed studies of the dynamics of the interaction are
discussed. In the case of protons, we observe from an
overdense plasma a beam with less than a forty-degree
divergence angle. Also discussed is the application of ion
beams to the preparation of medical isotopes.
[C6.005] Three-dimensional simulations of beam driven plasma accelerators
W.B. Mori (Departments of Physics and Astronomy and of Electrical Engineering, University of California at Los Angeles)
There have been many recent developments in plasma-based
accelerator research. One of the more exciting and important
is the ongoing E-157 experiment at SLAC in which the goal is
high-gradient acceleration over meter distances. The nomimal
SLAC beam has ~ 2 x 10^10 electrons at 30 GeV with a
bunch length of .63mm. In the experiment this beam is sent
through 1.4 meters of ~ 1.5 x 10^14 cm-3 density plasma.
In so doing, under ideal conditions it generates a ~ 400
MeV/m peak accelerating gradient which can accelerate the
tail of the drive beam. Motivated to understand the key
physics and diagnostic issues of this and future plasma
wakefield accelerator (PWFA) experiments, we have been
performing numerous 3D, fully explicit, particle-in-cell
(PIC) simulations. We use the newly developed PIC code
OSIRIS which is a fully parallelized, fully relativistic,
and Fortran 90 based object-oriented PIC code. In this talk
we will describe recent results from OSIRIS on the effects
of spot-size asymmetries and tails on the acceleration
gradient, on the dymanics of the spot-size due to the
transverse focusing forces of the wake, and on the sloshing
and hosing of the tail due to initial head-tail offsets of
the beam. In particular we will describe how these effects
complicate the experimental diagnostics and how they impact
future designs of follow-on PWFA experiments.
[C6.006] PROPAGATION INSTABILITIES AND MAGNETIC FIELDS IN HIGH INTENSITY SHORT PULSE LASER INTERACTIONS WITH UNDERDENSE PLASMAS
Zulfikar Najmudin (Physics Dept., Imperial College, University of London)
The interaction of an intense (> 5e18 Wcm-2) short pulse laser with underdense matter has been studied using the Vulcan CPA laser at the Rutherford Appleton Laboratory. Such interactions have been of great interest recently due to the production of high energy (> 94 MeV) electrons as a result of high intensity laser self-modulation. We will report recent nuclear activation measurements of the relativistic electrons generated from these experiments. With such intense lasers the propagation of the laser is also greatly modified by the plasma, through multiple non-linear effects such as the relativistic quiver velocity, ponderomotive blow-out, large-amplitude plasma wave generation and ultrafast optical field ionization. This results in pronounced effects such as whole beam self-focusing, filamentation, and long-wavelength laser hosing which have been observed in our experiments. We have also measured magnetic fields greater than 1 MGauss in the underdense plasma due to the inverse Faraday effect which will be discussed as well.