Kenneth A. Marsh

Energy doubling of 42 GeV electrons in a metre-scale plasma wakefield accelerator

The energy frontier of particle physics is several trillion electron volts, but colliders capable of reaching this regime (such as the Large Hadron Collider and the International Linear Collider) are costly and time-consuming to build; it is therefore important to explore new methods of accelerating particles to high energies. Plasma-based accelerators are particularly attractive because they are capable of producing accelerating fields that are orders of magnitude larger than those used in conventional colliders. In these accelerators, a drive beam (either laser or particle) produces a plasma wave (wakefield) that accelerates charged particles. The ultimate utility of plasma accelerators will depend on sustaining ultrahigh accelerating fields over a substantial length to achieve a significant energy gain. Here we show that an energy gain of more than 42 GeV is achieved in a plasma wakefield accelerator of 85 cm length, driven by a 42 GeV electron beam at the Stanford Linear Accelerator Center (SLAC). The results are in excellent agreement with the predictions of three-dimensional particle-in-cell simulations. Most of the beam electrons lose energy to the plasma wave, but some electrons in the back of the same beam pulse are accelerated with a field of ∼52 GV m-1. This effectively doubles their energy, producing the energy gain of the 3-km-long SLAC accelerator in less than a metre for a small fraction of the electrons in the injected bunch. This is an important step towards demonstrating the viability of plasma accelerators for high-energy physics applications.

Demonstration of the frequency upshifting of microwave radiation by rapid plasma creation

A technique for frequency-upshifting electromagnetic radiation is demonstrated. By ionizing azulene vapor contained in a resonant cavity using a laser pulse, the frequency of the incident RF wave at 33.3 GHz is upshifted by 5% with greater than 10% efficiency. Maximum frequency upshift of 2.3 times the source frequency is observed. There are two mechanisms thought to be operative in producing the observed frequency upshift: the time-dependent dielectric constant due to increasing plasma density, and rapid Q-switching of the cavity. This technique has the potential of being able to generate tunable and chirped radiation over a very broad ( Delta f/f>or approximately=1) frequency range. >

Experiments on laser driven beatwave acceleration in a ponderomotively formed plasma channel

A 10 ps long beam of 12 MeV electrons is externally injected into a ∼3-cm long plasma beatwave excited in a laser ionized hydrogen gas. The electrons have been accelerated to 50 MeV with a gradient of ∼1.3 GeV/m. It is shown that when the effective plasma wave amplitude-length product is limited by ionization-induced defocusing (IID), acceleration of electrons is significantly enhanced by using a laser pulse with a duration longer than the time required for ions to move across the laser spot size. Both experiments and two-dimensional simulations reveal that, in this case, self-guiding of the laser pulse in a ponderomotively formed plasma channel occurs. This compensates for IID and drives the beatwave over the longer length compared to when such a channel is not present.

Investigation of a channeling high-intensity laser beam in underdense plasmas

The interaction of an intense short pulse laser (>5/spl times/10/sup 18/ Wcm/sup -2/) with underdense plasma was extensively studied. The beam is found to be highly susceptible to the forward Raman scattering instability. At sufficiently high growth rates, this can lead to wavebreaking with the resultant production of a high flux of accelerated electrons (>10/sup 11/ for E>2 MeV). Some electrons are found to be accelerated well above the dephasing energy, up to 94 MeV. Self-scattered images intimate the presence of high-intensity channels that extend more than 3.5 mm or 12 Rayleigh lengths. These filaments do not follow the axis of laser propagation, but are seen to be emitted within an f4 cone centered around this axis. Spectra of the self-scattered light show that the main contribution of the scattering is not from light captured within these filaments. But there is evidence for self-phase modulation from effects such as ionization and relativistic self-focusing. However, no clear correlation is observed between channel length and the number or energies of accelerated electrons. Evidence for high intensities within the channels is given by small-angle Thomson scattering of the plasma wave generated therein, with this method, the intensity is found to be of the order of 10/sup 18/ Wcm/sup -2/ greater than 12 Rayleigh lengths from focus.

Demonstration of a positron beam-driven hollow channel plasma wakefield accelerator

Plasma wakefield accelerators have been used to accelerate electron and positron particle beams with gradients that are orders of magnitude larger than those achieved in conventional accelerators. In addition to being accelerated by the plasma wakefield, the beam particles also experience strong transverse forces that may disrupt the beam quality. Hollow plasma channels have been proposed as a technique for generating accelerating fields without transverse forces. Here we demonstrate a method for creating an extended hollow plasma channel and measure the wakefields created by an ultrarelativistic positron beam as it propagates through the channel. The plasma channel is created by directing a high-intensity laser pulse with a spatially modulated profile into lithium vapour, which results in an annular region of ionization. A peak decelerating field of 230 MeV m−1 is inferred from changes in the beam energy spectrum, in good agreement with theory and particle-in-cell simulations.

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.

Collinear Thomson scattering diagnostic system for the detection of relativistic waves in low-density plasmas

A Thomson scattering technique is used to study relativistic plasma waves at plasma densities as low as 2×1015 cm−3. A spatial-spectral filter is utilized to simultaneously attenuate the probe light ∼109 times and collect the weak scattered light which is shifted 3–24 A from the probe wavelength. The plasma waves, excited by a two-wavelength CO2 laser pulse with intensities up to 1015 W/cm2, are probed by a 532 nm laser pulse in a collinear geometry. Both the red- and blueshifted sidebands of Thomson scattered light are simultaneously resolved in time and frequency.

A Meter‐Scale Plasma Wakefield Accelerator

Plasma wakefield accelerators (PWFA) have recently shown substantial progress, attaining accelerating fields of more than 30 GV/m. The goal of the present experiment is to show that such accelerating fields can be sustained over the scale of a meter, resulting in a total energy gain comparable to the entire SLAC linear accelerator. We also seek to determine which factors limit the length of the interaction and determine the maximum achievable energy.

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.

Exact forward scattering of a CO2 laser beam from a relativistic plasma wave by time resolved frequency mixing in AgGaS2

In the UCLA plasma beat wave accelerator, a high intensity two frequency CO2 laser (λ1=10.6 μm, λ2=10.3 μm) is used to drive a large amplitude relativistic plasma wave. The plasma wave acts as a moving phase grating and scatters the incident pump waves into Stokes and anti-Stokes sidebands (ω1−ωp, ω2+ωp). The observation of these sidebands in the forward direction confirms the presence of the relativistic plasmon, and also gives an estimate of the amplitude–length product (n1/n0×L) of the wave. Since the Stokes and anti-Stokes signals are picosecond pulses at 10.9 and 10.0 μm, respectively, this light cannot be time resolved directly on a conventional detector or streak camera. The forward scattered light can be analyzed, however, by mixing the 10 μm light with visible light from a laser diode (670 nm) in a nonlinear crystal (AgGaS2) to produce frequency shifted light at 630 nm. The intensity of the 630 nm light is proportional to the product of the intensities of the two incident laser pulses, and can be...