R. Iverson

Simulation of a 50GeV PWFA Stage

The plasma afterburner has been proposed as a possible advanced acceleration scheme for a future linear collider. In this concept, a high energy electron(or positron) drive beam from an existing linac such as the SLC will propagate in a plasma section of density about one order of magnitude lower than the peak beam density. The particle beam generates a strong plasma wave wakefield which has a phase velocity equal to the velocity of the beam and this wakefield can be used to accelerate part of the drive beam or a trailing beam. Several issues such as the efficient transfer of energy and the stable propagation of the particle beam in the plasma are critical to the afterburner concept. We investigate the nonlinear beam‐plasma interactions in such scenario using a new 3D particle‐in‐cell code called QuickPIC. Preliminary simulation results for electron acceleration, beam‐loading and hosing instability will be presented.

Measurement of Pressure Dependent Fluorescence Yield of Air: Calibration Factor for UHECR Detectors

In a test experiment at the Final Focus Test Beam of the Stanford Linear Accelerator Center, the fluorescence yield of 28.5 GeV electrons in air and nitrogen was measured. The measured photon yields between 300 and 400 nm at 1 atm and 29 deg C are Y(760 Torr, air) = 4.42 +/- 0.73 and Y(760 Torr, nitrogen) = 29.2 +/- 4.8 photons per electron per meter. Assuming that the fluorescence yield is proportional to the energy deposition of a charged particle traveling through air, good agreement with measurements at lower particle energies is observed.

Comparison of air fluorescence and ionization measurements of EM shower depth profiles: Test of a UHECR detector technique

Measurements are reported on the fluorescence of air as a function of depth in electromagnetic showers initiated by bunches of 28.5 GeV electrons. The light yield is compared with the expected and observed depth profiles of ionization in the showers. It validates the use of atmospheric fluorescence profiles in measuring ultra high energy cosmic rays.

Status and future plans of the PEP-II B-factory

The PEP-II e/sup +/e/sup -/ collider has been operating for two years with the BaBar detector at the energy of the Upsilon 4S resonance. The peak luminosity has reached 3.3 /spl times/ 10/sup 33//cm/sup 2//s with 693 bunches with a positron current of 1.5 A and an electron current of 0.8 A. PEP-II has delivered in excess of 38 fb/sup -1/ of data to BaBar. The beam-beam tune shift limits are approaching 0.05-0.07 horizontally and 0.03-0.05 vertically. The electron cloud instability enlarges the positron beam size at high currents but is reduced by a solenoidal field on the vacuum chambers. The beam currents in PEP-II are being raised to increase the number of bunches and the luminosity. Over the next few years the luminosity goal for PEP-II is 10/sup 34//cm/sup 2//s.

Femtosecond electron bunch lengths in the SLAC FFTB beamline

We describe a new proposal to generate bunches as short as 35 fsec rms with the existing SLAC linac and damping ring. The bunch is compressed in three stages, starting with the existing ring-to-linac compressor. A second stage of compression is added in the form of a simple magnetic chicane installed at the 9-GeV location in the linac. The short bunch after the chicane generates a strong wakefield in the linac which provides a large correlated energy spread allowing a third compression stage in the existing Final Focus Test Beam (FFTB) beamline. Peak currents as high as 30 kA are possible and when passed through a new undulator in the FFTB, can generate hard x-rays of very high peak brightness and pulse duration of 80 fsec FWHM. The short bunch is ideal for plasma and wakefield studies as well as providing abundant R&D for verifying micro-bunch behavior in the future Linac Coherent Light Source (LCLS).

Picosecond bunch length and energy-z correlation measurements at SLAC's A-line and end station A

We report on measurements of picosecond bunch lengths and the energy-z correlation of the bunch with a high energy electron test beam to the A-line and end station A (ESA) facilities at SLAC. The bunch length and the energy-Z correlation of the bunch are measured at the end of the linac using a synchrotron light monitor diagnostic at a high dispersion point in the A-line and a transverse RF deflecting cavity at the end of the linac. Measurements of the bunch length in ESA were made using high frequency diodes (up to 100 GHz) and pyroelectric detectors at a ceramic gap in the beamline. Modelling of the beams longitudinal phase space through the linac and A-line to ESA is done using the 2-dimensional tracking program LiTrack, and LiTrack simulation results are compared with data. High frequency diode and pyroelectric detectors are planned to be used as part of a bunch length feedback system for the LCLS EEL at SLAC. The LCLS also plans precise bunch length and energy-Z correlation measurements using transverse RF deflecting cavities.

Energy measurement in a plasma wakefield accelerator

In the E-167 plasma wakefield acceleration experiment, electrons with an initial energy of 42 GeV are accelerated in a meter-scale lithium plasma. Particles are leaving plasma with a large energy spread. To determine the spectrum of the accelerated particles, a two-plane spectrometer has been set up.

Injection related background due to the transverse feedback

The background in the BaBar detector is especially high during injection, when most components are actually having reduced voltages. The situation is worse for the beam in high energy ring (HER) when the LER beam is present. It was found that the transverse feedback system plays an important role when stacking more charge on top of existing bunches. Lowering the feedback gain helped and it was realized later that the best scenario would be to gate off the feedback for only the one bunch, which got additional charge injected into it. The explanation is that the blown-up, but centered, original HER bunch plus the small injected off-axis bunch (each with half the charge) would stay in the ring if not touched, but the feedback system sees half the offset and wants to correct it, therefore disturbing and scraping the blown-up part.

Parametric exploration of intense positron beam-plasma interactions

A systematic investigation of the longitudinal fields excited in a plasma by a short, dense beam of positrons is carried out using two-dimensional, cylindrical geometry, particle-in-cell code simulations. In particular, we examine the behavior of the accelerating and decelerating fields of the wakefield as a function of beam charge, radius, length, and plasma density. The parameters are chosen to be consistent with those employed in current and future experiments designed to elucidate the physics of positron beam–plasma interactions.

Modeling of Ionization Physics with the PIC Code OSIRIS

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, causing blue shifts in laser spectra, creating and modifying instabilities, etc. Here we describe the addition of an impact ionization package into the 3‐D, object‐oriented, fully parallel PIC code OSIRIS. We apply the simulation tool to simulate the parameters of the upcoming E164 Plasma Wakefield Accelerator experiment at the Stanford Linear Accelerator Center (SLAC). We find that impact ionization is dominated by the plasma electrons moving in the wake rather than the 30 GeV drive beam electrons. Impact ionization leads to a significant number of trapped electrons accelerated from rest in the wake.