P. Muggli

Photo-ionized lithium source for plasma accelerator applications

A photo-ionized lithium source is developed for plasma acceleration applications. A homogeneous column of lithium neutral vapor with a density of 2/spl times/10/sup 15-3/ is confined by helium gas in a heat-pipe oven. A UV laser pulse ionizes the vapor. In this device, the length of the neutral vapor and plasma column is 25 cm. The plasma density was measured by laser interferometry in the visible on the lithium neutrals and by CO/sub 2/ laser interferometry on the plasma electrons. The maximum measured plasma density was 2.9/spl times/10/sup 14/ cm/sup -3/, limited by the available UV fluence (/spl ap/83 mJ/cm/sup 2/), corresponding to a 15% ionization fraction. After ionization, the plasma density decreases by a factor of two in about 12 /spl mu/s. These results show that such a plasma source is scaleable to lengths of the order of 1 m and should satisfy all the requirements for demonstrating the acceleration of electrons by 1 GeV in a 1-GeV/m amplitude plasma wake.

Development of one meter-long lithium plasma source and excimer mode reduction for plasma wakefield applications

A one meter long plasma source has been developed for studies at the Stanford Linear Accelerator Center (SLAC) for plasma wakefield excitation by a 30 GeV electron beam in an extended plasma column. The plasma is formed by ionization of a Li-vapor with an ArF excimer laser (193 nm). The Li-vapor is produced in a heat pipe oven which will be installed in the electron beam transport line. Through control of the oven temperature, neutral vapor densities reaching 2/spl times/10/sup 15/ cm/sup -3/ are produced. We report the details of the oven construction and temperature profile measurements with and without Li vapor. In the experiment, the excimer laser will be located about 15 m away from the plasma source. Beams produced by excimer lasers operating with a stable resonator cavity are unsuitable due to the large number of modes present in such beams. A significant reduction of the modal content has been obtained through the use of an unstable resonator design. Results of the implementation of this cavity configuration on the propagation characteristics of the excimer beam will be presented, including the design of the final telescope for spot size reduction.

Plasma wakefield acceleration of an intense positron beam: correlation between time-resolved and time-integrated energy diagnostics

The E162 experiment at the Stanford Linear Accelerator Center was the first experiment in which a positron beam gained energy in a plasma wakefield accelerator [B. Blue et al., Accepted for Publication in Physical Review Letters.]. A single positron bunch both excited (gave energy to) and witnessed (extracted energy from) the plasma wakefield. The energy dynamics within the single positron bunch were measured in a dispersive section of the beamline with both time-resolved (1 ps streak camera) and time-integrated (CCD camera) diagnostics. This paper will correlate the energy gain and loss measurements from both diagnostics.

Ionization injection and acceleration of electrons in a plasma wakefield accelerator at FACET

Localized injection of electrons within a relativistic plasma wake can potentially produce an ultrashort, monoenergetic electron bunch. Recent experiments at the FACET facility at SLAC explored the injection of helium electrons at the helium-lithium interface of a lithium heat pipe oven and the subsequent acceleration in the beam-produced plasma wake. Electrons accelerated to over 10 GeV in 30 cm of plasma were observed as a distinct charge bunch.

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.

Determination of Longitudinal Phase Space in SLAC Main Accelerator Beams

In the E164 Experiment at the Stanford Linear Accelerator Center (SLAC), we drive plasma wakes for electron acceleration using 28.5 GeV bunches from the main accelerator. These bunches can now be made with an RMS length of 12 microns, and accurate direct measurement of their lengths is not feasible shot by shot. Instead, we use an indirect technique, measuring the energy spectrum at the end of the linac and comparing with detailed simulations of the entire machine. We simulate with LiTrack, a 2D particle tracking code developed at SLAC. Understanding the longitudinal profile allows a better understanding of acceleration in the plasma wake, as well as investigation of related effects. We discuss the method and validation of our phase space determinations.

Plasma Dark Current in Self-Ionized Plasma Wake Field Accelerators

Evidence of particle trapping has been observed in a beam driven Plasma Wake Field Accelerator (PWFA) experiment, E164X, conducted at the Stanford Linear Accelerator Center by a collaboration which includes USC, UCLA and SLAC. Such trapping produces plasma dark current when the wakefield amplitude is above a threshold value and may place a limit on the maximum acceleration gradient in a PWFA. Trapping and dark current are enhanced when in an ionizing plasma, that is self-ionized by the beam. Here we present experimental results.

Plasma Light diagnostic for PWFA at SLAC

Summary form only given. A highly relativistic electron beam passes through an oven filled with the particular gas used in the experiment creating a plasma and a large amplitude wake field which causes the beam to lose and gain energy. The energy dumped into the plasma is dissipated through recombination and thermalization. Intensity of the plasma light is proportional to the wakefield amplitude. As the only non-beam diagnostic, study of plasma light can be used to characterize the plasma beam interaction to get the highest acceleration gradient. Moreover the unique spectrum of the gas can be used to as a reliable tool to measure the density vial the theory of Stark Broadening as an alternative to the other plasma density diagnostic tools which may not be available at the higher densities. Application of Plasma Light diagnostic to the past and future plasma experiments will be presented.

Modeling of E‐164X Experiment

In current plasma‐based accelerator experiments, very short bunches (100–150μm for E164 and 10–20 μm for E164X experiment at Stanford Linear Accelerator Center (SLAC)) are used to drive plasma wakes and achieve high accelerating gradients, on the order of 10–100GV/m. The self‐fields of such intense bunches can tunnel ionize neutral gases and create the plasma. This may completely change the physics of plasma wakes. A 3‐D object‐oriented fully parallel PIC code OSIRIS is used to simulate various gas types, beam parameters, etc. to support the design of the experiments. The simulation results for real experiment parameters are presented.

THz Cerenkov radiation from a magnetized plasma

Summary form only given. Electro-static (ES) waves with amplitudes between 1 and >100 GeV/m are routinely excited in plasma accelerators. However, the ES waves couples very weakly to vacuum modes and its energy is dissipated in the plasma. By applying a static magnetic field in the direction perpendicular to the ES wave propagation, a fraction of the ES wave is converted into electromagnetic (EM) radiation. The mode excited in the plasma is the lower branch of the magnetized plasma XO-mode. The plasma XO-mode can be viewed as the Cerenkov radiation emitted in the plasma by a particle bunch or by a laser (photon) pulse. The frequency of the EM radiation is close to the plasma frequency, and the radiation is emitted by the plasma predominantly in the forward direction (direction of propagation of the ES wave). The power of the EM radiation scales as the square of the strength of the applied magnetic field. The group velocity of the XO-mode is small (/spl Lt/c), and the radiation is expected to be emitted for the life time of the XO-mode in the decaying plasma. In the case of the UCLA-Neptune plasma beatwave accelerator (PBWA) experiment, the plasma wave is driven by a TW, two-frequency CO/sub 2/ laser pulse.