Particle-beam-driven plasma wakefield acceleration (PWFA) enables various novel high-gradient techniques for powering future compact light-source and high-energy physics applications. Here, a driving particle bunch excites a wakefield response in a plasma medium, which may rapidly accelerate a trailing witness beam. In this Letter, we present the measurement of ratios of acceleration of the witness bunch to deceleration of the driver bunch, the so-called transformer ratio, significantly exceeding the fundamental theoretical and thus far experimental limit of 2 in a PWFA. An electron bunch with ramped current profile was utilized to accelerate a witness bunch with a transformer ratio of 4.6_{-0.7}^{+2.2} in a plasma with length ∼10 cm, also demonstrating stable transport of driver bunches with lengths on the order of the plasma wavelength.
We present a new technique for inferring neutral densities in the Martian upper atmosphere from atmospheric absorption of magnetically reflected solar wind electrons. Using electron loss cone measurements from the Magnetometer/Electron Reflectometer (MAG/ER) experiment on board Mars Global Surveyor (MGS), we derive upper thermospheric (∼160–230 km altitude) densities in the southern hemisphere from 160° to 200°E at 2am local time, continuously from 1999 to 2005. We find a mean density of 0.027 kg/km3 at 160 km and observe a latitude‐dependent, repeatable seasonal variation of a factor of 1.8–4, with inter‐annual differences and consistently lower winter densities at 50°–55°S, compared with 0°–30°S. The mean densities, overall seasonal variation and latitude dependence are in general agreement with the Mars Thermosphere Global Circulation Model (MTGCM). These measurements are important for improved understanding of the dynamics of Mars' upper atmosphere and for planning spacecraft aerobraking maneuvers.
Self-modulation of an electron beam in a plasma has been observed. The propagation of a long (several plasma wavelengths) electron bunch in an overdense plasma resulted in the production of multiple bunches via the self-modulation instability. Using a combination of a radio-frequency deflector and a dipole spectrometer, the time and energy structure of the self-modulated beam was measured. The longitudinal phase space measurement showed the modulation of a long electron bunch into three bunches with an approximately 200 keV/c amplitude momentum modulation. Demonstrating this effect is a breakthrough for proton-driven plasma accelerator schemes aiming to utilize the same physical effect.
a b s t r a c tThe self-modulation instability of long particle beams was proposed as a new mechanism to produce driver beams for proton driven plasma wakefield acceleration (PWFA). The PWFA experiment at the Photo Injector Test facility at DESY, Zeuthen site (PITZ) was launched to experimentally demonstrate and study the selfmodulation of long electron beams in plasma. Key aspects for the experiment are the very flexible photocathode laser system, a plasma cell and well-developed beam diagnostics. In this contribution we report about the plasma cell design, preparatory experiments and the results of the first PWFA experiment at PITZ.
In the field of beam driven acceleration of particles in plasma wakefields (PWFA), the source of the plasma medium is a crucial part of the accelerator setup. Gas discharges have proven to be a reliable and simple type of a plasma source in past experiments. Nevertheless, especially in plasma cells that aim for peak density in the range of 1015 cm−3, physical apertures around 10 mm, and lengths of up to several meters, the stability of the discharge ignition and the pulse current waveform is limiting the applicability. We show successful mitigation of these jitters in a 0.1 m argon gas discharge cell, operating at maximum densities of ≤1016 cm−3 by optimisation of the cell design and the discharge current pulse circuit.
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