Abstract
The Advanced Wakefield Experiment (AWAKE) develops the first plasma wakefield accelerator with a high-energy proton bunch as driver. The 400 GeV bunch from CERN Super Proton Synchrotron (SPS) propagates through a 10 m long rubidium plasma, ionized by a 4TW laser pulse co-propagating with the proton bunch. The relativistic ionization front seeds a self-modulation process. The seeded self-modulation transforms the bunch into a train of micro-bunches resonantly driving wakefields. We measure the density modulation of the bunch, in time, with a streak camera with picosecond resolution. The observed effect corresponds to alternating focusing and defocusing fields. We present a procedure recovering the charge of the bunch from the experimental streak camera images containing the charge density. These studies are important to determine the charge per micro-bunch along the modulated proton bunch and to understand the wakefields driven by the modulated bunch.
Highlights
Rb plasma r ζ modulated proton bunch unmodulated proton bunch captured electrons ionizing laser pulse Rb vaporAWAKE uses the CERN Super Proton Synchrotron (SPS) proton bunch as a plasma wakefield driver
With the minimum bunch diameter (780 μm at the optical transition radiation (OTR) screen from Figure 10, corresponding to 220 μm at the streak camera slit due to the de-magnification by the OTR light transport) being larger than the slit width (20 μm), the streak camera image profile at each time (Figure 8) can be interpreted as a measurement of the bunch charge density as a function of time n(r, t) or n(x, t) on the images
It retains some of the modulation in the charge density and the recovered charge decreases along the bunch when compared to the incoming bunch charge
Summary
AWAKE uses the CERN SPS proton bunch as a plasma wakefield driver. The bunch propagates through 10 m of plasma, created by laser ionization of a rubidium (Rb) vapor. The laser pulse co-propagates with the proton bunch, seeding the self-modulation with the relativistic ionization front, i.e. with the abrupt beam plasma interaction within the bunch [1]. The micro-bunches resonantly drive wakefields in the plasma. In the following we present a method to determine the charge in each micro-bunch from timeresolved images of the proton bunch transverse distribution. The images are produced by a streak camera. 2. Method we explain the analysis of the streak camera images [5] applied for the determination of the charge per micro-bunch
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