D.K. Johnson

Beam Matching to a Plasma Wake Field Accelerator using a Ramped Density Profile at the Plasma Boundary

An important aspect of plasma wake field accelerators (PWFA) is stable propagation of the drive beam. In the under dense plasma regime, the drive beam creates an ion channel which acts on the beam as a strong thick focusing lens. The ion channel causes the beam to undergo multiple betatron oscillations along the length of the plasma. There are several advantages if the beam size can be matched to a constant radius. First, simulations have shown that instabilities such as hosing are reduced when the beam is matched [1]. Second, synchrotron radiation losses are minimized when the beam is matched. Third, an initially matched beam will propagate with no significant change in beam size in spite of large energy loss or gain. Coupling to the plasma with a matched radius can be difficult in some cases. This paper shows how an appropriate density ramp at the plasma entrance can be useful for achieving a matched beam. Additionally, the density ramp is helpful in bringing a misaligned trailing beam onto the drive beam axis. A plasma source with boundary profiles useful for matching has been created for the E-164X PWFA experiments at SLAC.

Energy Measurements of Trapped Electrons from a Plasma Wakefield Accelerator

Recent electron beam driven plasma wakefield accelerator experiments carried out at SLAC indicate trapping of plasma electrons. More charge came out of than went into the plasma. Most of this extra charge had energies at or below the 10 MeV level. In addition, there were trapped electron streaks that extended from a few GeV to tens of GeV, and there were mono-energetic trapped electron bunches with tens of GeV in energy.

Acceleration and Focusing of Electrons and Positrons Using a 30 GeV Drive Beam

A series of plasma wakefield acceleration (PWFA) experiments are being conducted with a 30 GeV drive beam from the Stanford Linear Accelerator Center (SLAC). These experiments continue to address the application of meter-scale plasmas to focus and accelerate electrons and positrons in the context of future applications to high-energy accelerators.

Positron Source from Betatron X-Rays Emitted in a Plasma Wiggler

In the E-167 plasma wakefield accelerator (PWFA) experiments in the Final Focus Test Beam (FFTB) at the Stanford Linear Accelerator Center (SLAC), an ultra-short, 28.5 GeV electron beam field ionizes a neutral column of Lithium vapor. In the underdense regime, all plasma electrons are expelled creating an ion column. The beam electrons undergo multiple betatron oscillations leading to a large flux of broadband synchrotron radiation. With a plasma density of 3 × 1017cm-3, the effective focusing gradient is near 9 MT/m with critical photon energies exceeding 50 MeV for on-axis radiation. A positron source is the initial application being explored for these X-rays, as photo-production of positrons eliminates many of the thermal stress and shock wave issues associated with traditional Bremsstrahlung sources. Photo-production of positrons has been well-studied; however, the brightness of plasma X-ray sources provides certain advantages. In this paper, we present results of the simulated radiation spectra for the E-167 experiments, and compute the expected positron yield.

Electron Bunch Length Measurements in the E‐167 Plasma Wakefield Experiment

Bunch length is of prime importance to beam driven plasma wakefield acceleration experiments due to its inverse relationship to the amplitude of the accelerating wake. We present here a summary of work done by the E167 collaboration measuring the SLAC ultra-short bunches via autocorrelation of coherent transition radiation. We have studied material transmission properties and improved our autocorrelation traces using materials with better spectral characteristics.

Bunch Length Measurements Using Coherent Radiation

The accelerating field that can be obtained in a beam-driven plasma wakefield accelerator depends on the current of the electron beam that excites the wake. In the E-167 experiment, a peak current above 10 kA will be delivered at a particle energy of 28 GeV. The bunch has a length of a few ten micrometers and several methods are used to measure its longitudinal profile. Among these, autocorrelation of coherent transition radiation (CTR) is employed. The beam passes a thin metallic foil, where it emits transition radiation. For wavelengths greater than the bunch length, this transition radiation is emitted coherently. This amplifies the long-wavelength part of the spectrum. A scanning Michelson interferometer is used to autocorrelate the CTR. However, this method requires the contribution of many bunches to build an autocorrelation trace. The measurement is influenced by the transmission characteristics of the vacuum window and beam splitter. We present here an analysis of materials, as well as possible layouts for a single shot CTR autocorrelator.

Modeling of beam-ionized sources for plasma accelerators

When considering intense particle or laser beams propagating in dense plasma or gas, ionization plays an important role. Impact ionization and tunnel ionization may create new plasma electrons, altering the physics of wakefield accelerators, creating and modifying instabilities, etc. Here we describe the addition of an ionization package into the 3-D object-oriented fully parallel PIC code OSIRIS [R.G Hemker, F.S. Tsung, V.K. Decyk, W.B. Mori, S. Lee, and T. Katsouleas, Development of a parallel code for modeling plasma based accelerators, IEEE Particle Accelerator Conference 5, 3672-3674 (1999).]. Using intense beams to tunnel-ionize neutral gas may become a new source of plasma. For the beams whose electrical fields are right above threshold, the optimal gas density for maximize electrical field is about 7 n/sub 0/(n/sub 0/ is the optimal density according to linear theory /spl omega//sub p//spl sigma//sub z//c=2/sup 1/2/ [E164 proposal, unpublished.]). We apply the simulation tool to the parameters of the current E164 [R. Keinigs and M.E. Jones, Phys. Fluids 30, 252 (1987)] Plasma Wakefield Accelerator experiment at the Stanford Linear Accelerator Center (SLAC). We find that tunnel ionization affects the wakefield and energy gain of E-164 experiment.

Towards a compact 0.1-10 MeV broadband betatron photon source

When a highly relativistic electron is injected off-axis into an ion channel, the restoring force of the radial field of the ions will cause the electron to accelerate towards the axis, overshoot, and begin to undergo oscillations about the ioncolumn axis at a characteristic frequency; the betatron frequency. This so-called betatron motion will cause the electron to radiate hard x-rays in the forward direction. In two recent experiments at the Stanford Linear Accelerator Center (SLAC), betatron x-rays in the 1-20kV range and in the 1-50MV range were produced with an electron beam with an energy of 28.5 GeV for ion densities of about 1 x 1014 cm-3 and 1 x 1017cm-3, respectively. To make such an x-ray source more compact, the 3km long SLAC linac would be replaced by a source of electrons from a Laser Wakefield accelerator (LWFA). To increase the efficiency of converting laser into photons at high photon energies, we propose adding a second stage where the LWFA electrons radiate via a second ion channel, independent of the accelerating process. This two stage concept allows one to control the critical frequency of the emitted radiation as well as the efficiency of the process.

Energy doubling of 42GeV electrons in a particle-driven, plasma-based accelerator

Summary form only given. Plasma-based accelerators have made remarkable progress in the last few years. In particular, the energy doubling of 42GeV incoming electrons has been demonstrated over a plasma length of only 85cm in a particle beam-driven plasma-based accelerator or plasma wakefield accelerator (PWFA). This distance is about 2000 times shorter than the conventional linear accelerator that produced these particles. In the plasma the accelerating gradient is ap 50GeV/m, suggesting that energies of the order of ITeV could be reached in only ap 20 m of plasma. In the experiment the energy gain scales linearly with the plasma length. The maximum energy gain is obtained at a plasma density such that the plasma wavelength is approximately equal to the electron bunch. This scaling is verified for long (ap 700 mum) and short (ap 20 mum) bunches with corresponding optimum plasma densities of ap1.8 x 1014cm-3 and ap2.7 x 1017cm-3. In the short bunch experiments, the plasma is produced through field ionization of a lithium vapor confined to the hot region heat- pipe oven by a helium buffer gas at room temperature. Above a wake field amplitude of ap 30 GV/m, trapping of plasma electrons is observed. The trapping was identified as resulting from the field-ionization ionization of neutral helium in the lithium to helium transition regions of the heat-pipe oven. These free electrons born at rest inside the wake and have a much lower trapping threshold that the lithium electrons born ahead of the wake. Spectral interference measurements show that the trapped electrons form one or more short bunches (a few fs). Simulations suggest that they have a peak current larger and an emittance lower that those of the drive bunch, possibly making them interesting for radiation source applications. Experimental results will be presented and compared to scaling laws. The prospects for the application of plasma-based accelerators to a future linear particle collider will be discussed.

Demonstration of a Novel Positron Source Based on a Plasma Wiggler

A new method for generating positrons has been proposed that uses betatron X‐rays emitted by an electron beam in a high‐K plasma wiggler. The plasma wiggler is an ion column produced by the head of the beam when the peak beam density exceeds the plasma density. The radial electric field of the beam blows out the plasma electrons transversely, creating an ion column. The focusing electric field of the ion column causes the beam electrons to execute betatron oscillations about the ion column axis. If the beam energy and the plasma density are high enough, these oscillations lead to synchrotron radiation in the 1–50 MeV range. A significant amount of electron energy can be lost to these radiated X‐ray photons. These photons strike a thin (.5Xo), high‐Z target and create e+/e− pairs. The experiment was performed at the Stanford Linear Accelerator Center (SLAC) where a 28.5 GeV electron beam with σr ≈ 10μm and σz ≈ 25μm was propagated through a neutral Lithium vapor (Li). The radial electric field of the dense...