Design and wakefield performance of the new SLC collimators

The very small transverse beam sizes of the flat SLC bunches are 100-170 {mu}m in the horizontal and 30-50 {mu}m in the vertical near the end of the SLAC linac. Unexpectedly large transverse Wakefield kicks were observed from the collimators in this region during 1995. Upon inspection, it was found that the 20 {mu}m gold plating had melted and formed a line of spherules along the beam path. To refurbish the collimators, an improved design was required. The challenging task was to find a surface material with better conductivity than the titanium core to reduce resistive wakefields. The material must also be able to sustain the mechanical stress and heating from beam losses without damage. Vanadium was first chosen for ease of coating, but later TiN was used because it is more chemically inert. Recent beam tests measured expected values for geometric Wakefield kicks, but the resistive wall Wakefield kicks were four times larger than calculated

Long-range wakefields and split-tune lattice at the SLC

At the SLC, a train consisting of one positron bunch followed by two electron bunches is accelerated in the linac, each separated by about 60 ns. Long-range transverse wakefields from the leading bunch were found to cause up to a factor of three increase in beam jitter for the trailing bunches. Incoming jitter is efficiently damped by BNS damping, but excitations in the middle of the linac from sources such as long-range wakefields can grow in amplitude. To measure the wake function, the time difference between the positron and electron bunches was changed, determining the frequency and strength of the dominant mode contributing to the dipole Wakefield. By splitting the horizontal and vertical phase advance, or {open_quote}tune{close_quote}, of the magnetic lattice, it was possible to decrease the resonant excitation from these wakefields and thereby reduce the jitter of the electron beam by a factor of two

SLC-2000: A luminosity upgrade for the SLC

The authors discuss a possible upgrade to the Stanford Linear Collider (SLC), whose objective is to increase the SLC luminosity by at least a factor 7, to an average Z production rate of more than 35,000 per week. The centerpiece of the upgrade is the installation of a new superconducting final doublet with a field gradient of 240 T/m, which will be placed at a distance of only 70 cm from the interaction point. In addition, several bending magnet in each final focus will be lengthened and two octupole correctors are added. A complementary upgrade of damping rings and bunch compressors will allow optimum use of the modified final focus and can deliver, or exceed, the targeted luminosity. The proposed upgrade will place the SLC physics program in a very competitive position, and will also enable it to pursue its pioneering role as the first and only linear collider

Observation of Target Electron Momentum Effects in Single-Arm M\o ller Polarimetry

In 1992, L.G. Levchuk noted that the asymmetries measured in M\o ller scattering polarimeters could be significantly affected by the intrinsic momenta of the target electrons. This effect is largest in devices with very small acceptance or very high resolution in laboratory scattering angle. We use a high resolution polarimeter in the linac of the polarized SLAC Linear Collider to study this effect. We observe that the inclusion of the effect alters the measured beam polarization by -14% of itself and produces a result that is consistent with measurements from a Compton polarimeter. Additionally, the inclusion of the effect is necessary to correctly simulate the observed shape of the two-body elastic scattering peak.Comment: 29 pages, uuencoded gzip-compressed postscript (351 kb). Uncompressed postscript file (898 kb) available to DECNET users as SLC::USER_DISK_SLC1:[MORRIS]levpre.p

Low Background Micromegas in CAST

Solar axions could be converted into x-rays inside the strong magnetic field of an axion helioscope, triggering the detection of this elusive particle. Low background x-ray detectors are an essential component for the sensitivity of these searches. We report on the latest developments of the Micromegas detectors for the CERN Axion Solar Telescope (CAST), including technological pathfinder activities for the future International Axion Observatory (IAXO). The use of low background techniques and the application of discrimination algorithms based on the high granularity of the readout have led to background levels below 10$^{-6}$ counts/keV/cm$^2$/s, more than a factor 100 lower than the first generation of Micromegas detectors. The best levels achieved at the Canfranc Underground Laboratory (LSC) are as low as 10$^{-7}$ counts/keV/cm$^2$/s, showing good prospects for the application of this technology in IAXO. The current background model, based on underground and surface measurements, is presented, as well as the strategies to further reduce the background level. Finally, we will describe the R&D paths to achieve sub-keV energy thresholds, which could broaden the physics case of axion helioscopes.Comment: 6 pages, 3 figures, Large TPC Conference 2014, Pari

Tau Decays and Chiral Perturbation Theory

In a small window of phase space, chiral perturbation theory can be used to make standard model predictions for tau decays into two and three pions. For $\tau \to 2\pi \nu_\tau$, we give the analytical result for the relevant form factor $F_V$ up to two loops, then calculate the differential spectrum and compare with available data. For $\tau \to 3 \pi \nu_\tau$, we have calculated the hadronic matrix element to one loop. We discuss the decomposition of the three pion states into partition states and we give detailed predictions for the decay in terms of structure functions. We also compare with low energy predictions of meson dominance models. Overall, we find good agreement, but also some interesting discrepancies, which might have consequences beyond the limit of validity of chiral perturbation theory.Comment: 39 pages, Latex, including 8 Postscript figures. The complete paper is also available via anonymous ftp at ftp://www-ttp.physik.uni-karlsruhe.de/ , or via www at http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprint

Scaling of Energy Gain with Plasma Parameters in a Plasma Wakefield Accelerator

We have recently demonstrating the doubling of the energy of particles of the ultra-short, ultra-relativistic electron bunches of the Stanford Linear Accelerator Center [1]. This energy doubling occurred in a plasma only 85 cm-long with a density of {approx} 2.6 x 10{sup 17} e{sup -}/cm{sup -3}. This milestone is the result of systematic measurements that show the scaling of the energy gain with plasma length and density, and show the reproducibility and the stability of the acceleration process. We show that the energy gain increases linearly with plasma length from 13 to 31 cm. These are key steps toward the application of beam-driven plasma accelerators or plasma wakefield accelerators (PWFA) to doubling the energy of a future linear collider without doubling its length

Positron Production by X Rays Emitted By Betatron Motion in a Plasma Wiggler

Positrons in the energy range of 3-30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1-50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra

Beam Head Erosion in Self-Ionized Plasma Wakefield Accelerators

In the recent plasma wakefield accelerator experiments at SLAC, the energy of the particles in the tail of the 42 GeV electron beam were doubled in less than one meter [1]. Simulations suggest that the acceleration length was limited by a new phenomenon--beam head erosion in self-ionized plasmas. In vacuum, a particle beam expands transversely in a distance given by {beta}*. In the blowout regime of a plasma wakefield [2], the majority of the beam is focused by the ion channel, while the beam head slowly spreads since it takes a finite time for the ion channel to form. It is observed that in self-ionized plasmas, the head spreading is exacerbated compared to that in pre-ionized plasmas, causing the ionization front to move backward (erode). A simple theoretical model is used to estimate the upper limit of the erosion rate for a bi-gaussian beam by assuming free expansion of the beam head before the ionization front. Comparison with simulations suggests that half this maximum value can serve as an estimate for the erosion rate. Critical parameters to the erosion rate are discussed

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