Positron production by x -rays emitted from betatron motion in a plasma wiggler.

摘要

A new method for generating positrons has been proposed that uses betatron X-rays emitted by an electron beam in a high-K plasma wiggler. The plasma wiggler is an ion column produced by the head of the beam when the peak beam density exceeds the plasma density. The radial electric field of the beam blows out the plasma electrons transversely, creating an ion column. The focusing electric field of the ion column causes the beam electrons to execute betatron oscillations about the ion column axis. This leads to synchrotron radiation in the 1-50 MeV range, if the beam energy and the plasma density are high enough. A significant amount of electron energy can be lost to radiated X-ray photons. These photons strike a thin (.5Xo), high-Z target and create e+/e - pairs. It is this new method of positron production by X-rays emitted from betatron motion of electrons in a plasma wiggler that is explored in this thesis.;The experiment was performed at the Stanford Linear Accelerator Center (SLAC) where a 28.5 GeV electron beam with sigmar ≈ 10mum and sigmaz ≈ 25mum was propagated through a neutral Lithium vapor (Li). The radial electric field of the dense beam was large enough to field ionize the Li vapor to form a plasma. Since the typical electron beam density of 4 x 1017cm -3 was greater than the plasma the plasma density of 3 10 17cm-3, electrons were completely blown-out forming a pure ion column which led to electron betatron oscillations. The synchrotron radiation spectra from these oscillations had critical energies on the order of 50 MeV, ideal for positron production. The X-rays traveled 40m downstream of the plasma, were collimated and collided with a 1.7mm (.5X o) Tungsten (W) target. The e+/ e- pairs were imaged with a magnetic spectrometer and detected using silicon surface barrier detectors. Positrons were measured in the energy range of 2-30 MeV. The positron yield was measured as a function of plasma density, ion column length and electron beam pulse length. A computational model was written to match the experimental data with theory. The measured positron spectra are in excellent agreement with those expected from the calculated Xray spectral yield from the plasma wiggler. After matching the model with the experimental results, it was used to design a more efficient positron source, giving positron yields of 0.44 e+/ e-, a number that is close to the target goal of 1-2 e+/e- for future positron sources.