P. Raimondi

Test of the electron hose instability in the E157 experiment

The E157 experiment is designed to demonstrate high gradient plasma wake field acceleration over a significant length. It has been suggested that the electron hose instability of the drive beam will degrade the performance of this experiment because the hosing tail electrons will not fully sample the highest acceleration field. In this paper a parasitic experiment designed to test the extent of the hosing instability is described. In particular, we discuss how the initial beam conditions are determined so that the extent to which any transverse perturbations grow due to hosing can be determined.

Synchrotron oscillation damping by beam-beam collisions in DAFNE

In DA{Phi}NE, the Frascati e+/e- collider, the crab waist collision scheme has been successfully implemented in 2008 and 2009. During the collision operations for Siddharta experiment, an unusual synchrotron damping effect has been observed. Indeed, with the longitudinal feedback switched off, the positron beam becomes unstable with beam currents in the order of 200-300 mA. The longitudinal instability is damped by bringing the positron beam in collision with a high current electron beam (~2A). Besides, we have observed a shift of approx 600Hz in the residual synchrotron sidebands. Precise measurements have been performed by using both a commercial spectrum analyzer and the diagnostics capabilities of the DA{Phi}NE longitudinal bunch-by-bunch feedback. This damping effect has been observed in DA{Phi}NE for the first time during collisions with the crab waist scheme. Our explanation is that beam collisions with a large crossing angle produce a longitudinal tune shift and a longitudinal tune spread, providing Landau damping of synchrotron oscillations.

Optimization of chromatic optics near the half integer in PEP-II

The PEP-II collider has benefited greatly from the correction of the chromatic functions. By optimizing sextupole family strengths, it is possible to correct the non-linear chromaticity, the chromatic beta, and the second order dispersion in both the LER and HER. Having implemented some of these corrections, luminosity was improved in PEP-II by almost 10%.

Positron beam propagation in a meter long plasma channel

Recent experiments and simulations have shown that positron beams propagating in plasmas can be focused and also create wakes with large accelerating gradients. For similar parameters, the wakes driven by positron beams are somewhat smaller compared to the case of an electron beam. Simulations have shown that the wake amplitude can be increased if the positron beam is propagated in a hollow plasma channel. This paper, compares experimentally, the propagation and beam dynamics of a positron beam in a meter scale homogeneous plasma, to a positron beam hollow channel plasma. The results show that positron beams in hollow channels are less prone to distortions and deflections. Hollow channels were observed to guide the positron beam onto the channel axis. Beam energy loss was also observed implying the formation of a large wake amplitude. The experiments were carried out as part of the E-162 plasma wakefield experiments at SLAC.

DAΦNE upgrade: a new magnetic and mechanical layout

The DAΦNE Phi-factory upgrade, foreseen for the SIDDHARTA detector run in 2007, requires a new magnetic and mechanical layout to exploit the large Piwinski angle and crab waist concepts [1]. New permanent quadrupole magnets and aluminium vacuum chamber with thin window have been designed for the new interaction region, with the aim to reuse as far as possible the present magnetic and vacuum chamber components. A vacuum chamber of novel design will allow separating the beams at the second interaction region. The new layout together the new hardware components are presented.

Plasma wakefield acceleration experiments with 28.5 GeV electron and positron beams

Summary form only given, as follows. Large gradient accelerators are necessary to reach the very high energies required at the collision point of future electron/positron colliders In the plasma wakefield accelerator (PWFA), a short electron or positron bunch drives a large amplitude plasma wave or wake. The transverse component of the wake leads to focusing of the particle bunch, while longitudinal components of the wake lead to energy loss and energy gain by particles. The PWFA is an energy transformer in which the energy is transferred from the particles in the core of the bunch in a single bunch scheme, or from a driver bunch in a two bunch scheme, to the particles in the back of the same bunch, or to a trailing witness bunch In the experiments described here, the 28.5 GeV electron or positron beam of the Stanford Linear Accelerator Center Final Focus Test Beam line is sent in a long lithium plasma. The bunch charge density is density is larger than the plasma density and the plasma wake is driven in the non-linear regime. In the case of an electron bunch, the bunch space charge field expels all the plasma electrons from the beam volume. The pure plasma ion column left behind the bunch head acts as an aberration-free plasma lens on the bunch core.

Dynamics of a 28.5 GeV electron or positron beam in a meter-long plasma

Summary form only given, as follows. A plasma wakefield acceleration (PWFA) experiment is presently being conducted at the Stanford linear Accelerator Center (SLAC). In this experiment 28.5 GeV, 2 ps long electron (e/sup -/) or positron (e/sup +/) bunches with /spl ap/2/spl times/10/sup 10/ particles are sent in a 1.4 m long plasma with a density in the 0-2/spl times/10/sup 14/ cm/sup -3/ range. The plasma is obtained by single-photon ionization of a lithium vapor. The focusing of the bunches is observed by imaging on a CCD camera the optical transition radiation emitted by the bunches when traversing thin titanium foils placed /spl ap/1 m upstream and downstream from the plasma. The time resolved dynamics is studied by imaging onto a streak camera the light emitted by the beam when traversing a thin piece of aerogel located /spl ap/25 m downstream from the plasma, after a dispersive magnet. Quadrupoles located between the plasma and the aerogel image the particle beam at the plasma exit onto the aerogel. At the aerogel location, the beam image in the dispersive (vertical) plane is dominated by the beam energy, whereas in the perpendicular, non-dispersive plane it is dominated by the beam spot size. The incoming bunches have a correlated energy spread, and time integrated images of the beam after dispersion in energy also give an insight into the beam transverse dynamics. The beam density is larger than the plasma density, and the experiment can access the non-linear or blow-out regime of the PWFA. The plasma acts as a thick focusing element. Both e/sup -/ and e/sup +/ bunches are focused by the plasma. The focusing dynamics is observed with single bunches. The bunch particles work on the plasma electron to expel them from (e/sup -/), or attract them into (e/sup +/) the bunch volume, and energy loss of the particles in the bunch core is observed. Some of the energy is transferred back from the plasma electrons to the particles in the back of the bunch, and plasma acceleration is observed with e/sup -/ bunches. The experimental set up as well as experimental results will be presented.

E179: A 1.4 meter-long plasma wakefield acceleration experiment

Summary form only given. In the E-157 plasma wakefield experiment conducted at SLAC, a 30 GeV, 2 ps (/spl sigma//sub z/=0.6 mm) electron bunch is sent in a 1.4 m long lithium plasma with a density n/sub c/ in the 1-4/spl times/10/sup 14/ cm/sup -3/ range. The electron bunch density is larger than the plasma density, and the bunch completely expels the plasma electrons (blow-out regime), creating a focusing ion channel. When the plasma electrons rush back into the ion channel, they give rise to a large longitudinal accelerating gradient of the order of 1 GeV/m (with n/sub c/=2.1/spl times/10/sup 14/ cm/sup -3/, and 4/spl times/10/sup 10/ e per bunch). The electrons in the tail of the bunch (/spl sigma//sub z//spl ap//spl lambda//sub p/ the plasma wavelength) experience the accelerating gradient and gain energy. The plasma source consists of a heat-pipe oven producing a 1.4 m long lithium neutral column with a density n/sub 0/ in the 2-5/spl times/10/sup 15/ cm/sup -3/ range. The vapor column length is estimated from temperature profile measurements, and the neutral column density length product (n/sub 0/L) is measured using white light absorption, hook method, and uv absorption.