SLAC, Menlo Park, California
During Run 5, PEP-II has been plagued by beam instabilities causing beam aborts due to radiation in the BaBar detector or due to fast beam loss triggering the dI/dt interlock. The latest of such instabilities occurred in the High Energy Ring (HER), severely curtailing the maximum beam current achievable during physics running. Techniques used in tracking down this instability included fast monitoring of background radiation, temperatures and vacuum pressure. In this way, the origin of the instability was localized and inspection of the vacuum system revealed several damaged bellows shields. Replacing these units significantly reduced the incident rate but did not eliminate it fully. After the end of the run, a number of damaged rf seals were found, possibly having caused the remaining incidents of instability. In this paper we will outline the steps taken to diagnose and remedy the issue and also compare the different signatures of vacuum-induced instabilities we have seen in both rings of PEP-II during the run.
SLAC, Menlo Park, California
INFN/LNF, Frascati (Roma)
SLAC, Menlo Park, California
UCLA, Los Angeles, California
USC, Los Angeles, California
The costs and the time scales of colliders intended to reach the energy frontier are such that it is 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 ultra-high accelerating fields over a substantial length to achieve a significant energy gain. More than 42 GeV energy gain was achieved in an 85 cm long plasma wakefield accelerator driven by a 42 GeV electron drive beam at the Stanford Linear Accelerator Center (SLAC). 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. 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.
UCLA, Los Angeles, California
SLAC, Menlo Park, California
Euclid TechLabs, LLC, Solon, Ohio
USC, Los Angeles, California
LLNL, Livermore, California
Electron bunches with the unparalleled combination of high charge, low emittances, and short time duration, as first produced at the SLAC FFTB, are foreseen to be produced soon at the SABER facility. These types of bunches have enabled wakefield driven accelerating schemes of >10 GV/m. In the context of the Dielectric Wakefield Accelerators (DWA) such beams, having rms bunch length as short as 20 um, have been used to drive 100 μm and 200 μm ID hollow tubes above 20 GV/m surface fields. These FFTB tests enabled the measurement of a breakdown threshold in excess of 4 GV/m (2 GV/m accelerating field) in fused silica. With the construction and commissioning of the SABER facility at SLAC, new experiments are made possible to test further aspects of DWAs including materials, tube geometrical variations, direct measurements of the Cerenkov fields, and proof of acceleration in tubes >10 cm in length. The E169 collaboration will investigate breakdown thresholds and accelerating fields in new materials including CVD diamond. Here we describe the experimental plans, beam parameters, simulations, and progress to date as well as future prospects for machines based of DWA structures.
UCLA, Los Angeles, California
SLAC, Menlo Park, California
USC, Los Angeles, California
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.
[1] I. Blumenfeld et al., Nature 445, 741(2007)[2] J. B. Rosenzweig et al., Phys. Rev. A 44, R6189 (1991)
USC, Los Angeles, California
SLAC, Menlo Park, California
UCLA, Los Angeles, California
Systematic measurements of energy gain as a function of plasma parameters in the SLAC electron beam-driven plasma wakefield acceleration (PWFA) experiments lead to very important understanding of the beam-plasma interaction. In particular, measurements as a function of the plasma length Lp show that the energy gain increases linearly with Lp in the 10 to 30 cm range. Based on this scaling, the plasma was subsequently lengthened to Lp=90cm, resulting in the first demonstration of the doubling of the energy of a fraction of the incoming 42GeV electrons*. The peak accelerating gradient is larger than 40GV/m and is sustained over meter-scale plasma lengths. These measurements also reveal that the optimum plasma density for acceleration is about 2.7·1017/cc, larger than the value predicted by the linear theory for the approximately 20 microns bunch length, confirming that the experiment is conducted in the non-linear regime of the PWFA. Detailed experimental results will be presented.
* "Energy doubling of 42 GeV electrons in a meter scale plasma wakefield accelerator", I. Blumenfeld et. al., Nature, 2006, accepted
SLAC, Menlo Park, California
UCLA, Los Angeles, California
USC, Los Angeles, California
Recent experiments at SLAC have shown that high gradient acceleration of electrons is achievable in meter scale plasmas. Results from these experiments show that the wakefield is sensitive to parameters in the electron beam which drives it. In the experiment the bunch length and beam waist location were varied systematically at constant charge. Here we investigate the correlation of peak beam current to the decelerating gradient. Limits on the transformer ratio will also be discussed. The results are compared to simulation.
SLAC, Menlo Park, California
Orbit bumps in sextupoles are routinely used for tuning the luminosity in the PEP-II B-Factory. Anti-symmetric bumps in a sextupole pair generate dispersion, while symmetric bumps induce a tune shift and beta beat. By coming two of these symmetric bumps with opposite signs where the second pair is 90 degree away, the tune shift cancels and the beta beat doubles. In the low energy ring (LER) we have four sextupole pairs per arc, where pair 1 and 3 are at the same betatron phase and pair 2 and 4are 90 degree away. By making two symmetric bumps with opposite sign in pair 1 and 3 the tune shift and the beta beat outside this region cancel, BUT the LER lifetime improved by a factor of three, losses by a factor of five, and the beam-beam background in the drift chamber of the BaBar detector by 20%. Simulations showed that the phase change at the second sextupole pair introduced by the beta beat can completely cancel the third order chromaticity.
SLAC, Menlo Park, California
The low energy ring (LER) in PEP-II has a design emittance of 0.5 nm-rad in the vertical, compared to nearly 0.1 nm-rad for the HER ring. This was thought to come from the "vertical step" of about 1 m in the interaction straight, where the LER beam after horizontal separation gets bend vertical so it sits on top of the HER in the rest of the ring. Since the program MAD does not easily reveal the location of the major emittance contribution, a program was written to calculate the coupled "curly H" parameter of mode 2 (mainly vertical) along z. Weighting it with the magnet bending revealed that the weak long bends inside the "vertical step" did less than 20% of the emittance growth. More than 80% comes from the ends of the adjacent arcs with strong bends. This is caused by the coupling cancellation of the solenoid starting already there with the skew quadrupoles SK5 and 6. By introducing additional skews in the straight instead of SK5 and 6 the emittance could be reduced by a factor of ten in simulations, but with very strong skews. Reasonable strong magnets might generate a workable compromise, since a factor of two in emittance promises 50% more luminosity in beam-beam simulations.
SLAC, Menlo Park, California
For optics measurement and modeling of the PEP-II electron (HER) and position (LER) storage rings, we have been doing well with MIA* which requires analyzing turn-by-turn Beam Position Monitor (BPM) data that are resonantly excited at the horizontal, vertical, and longitudinal tunes respectively. However, in anticipating that certain BPM buttons or even pins in the PEP-II IR region will be missing for the next run starting in January 2007, we have been developing a data validation process, hoping to reduce the effect due to the reduced BPM data accuracy on PEP-II optics measurement and modeling. Besides the routine process for ranking BPM noise level through data correlation among BPMs, allowing BPMs to have linear gains and linear cross couplings, we can also check BPM data symplecticity by comparing the invariant ratios. We may also work out nonlinear BPM data correction if needed. Results on PEP-II measurement will be presented.
* Y. T. Yan, et. al. EPAC06 Proceedings, WEPCH062, (2006)
SLAC, Menlo Park, California
UCLA, Los Angeles, California
USC, Los Angeles, California
Particles are leaving the meter-long plasma wakefield accelerator with a large energy spread. To determine the spectrum of these particles, four diagnostics have been set up. These were used to determine energies of the particles that gain energy in the plasma, those that lose energy by driving the wake and the self-injected particles that are accelerated from rest.
SLAC, Menlo Park, California
UCLA, Los Angeles, California
USC, Los Angeles, California
Recent electron beam driven plasma wakefield accelerator experiments carried out at SLAC showed trapping of plasma electrons. These trapped electrons appeared on an energy spectrometer with smaller transverse size than the beam driving the wake. A connection is made between transverse size and emittance; due to the spectrometer?s resolution, this connection allows for placing an upper limit on the trapped electron emittance. The upper limit for the lowest normalized emittance measured in the experiment is 1 mm·mrad.