Search Results (6 total)

In a particle accelerator with a periodic structure beam space charge force may excite resonant beam emittance growth if the particle’s transverse phase advance approaches 90°. A recent simulation study with the PARMILA code [D. Jeon , Phys. Rev. ST Accel. Beams 12, 054204 (2009)] has shown the feasibility of measuring the stop band of this fourth order resonance in the GSI Universal Linear Accelerator UNILAC and proposed its experimental verification, which is reported here. Measurements of transverse phase space distributions behind a periodically focusing structure reveal a fourfold symmetry characteristic of fourth order resonances as well as a resonance stop band above σ₀=90° per focusing cell. These experimental findings agree with results from three different beam dynamics simulation codes, i.e., DYNAMION, PARMILA, and TRACEWIN.

It is discovered that, for a high intensity beam, the 4σ=360° (or 4ν=1) resonance of a linear accelerator is manifested through the octupolar term of space charge potential when the depressed phase advance per cell σ is close to and below 90° but no resonance effect is observed when σ is just above 90°. To verify that this is a resonance, a frequency analysis is performed and a study of resonance crossing from above and from below the resonance is conducted. It is observed that this fourth order resonance is dominating over the better known envelope instability and practically replacing it. The simulation study shows a clear emittance growth by this resonance and its stop band. A proposal to GSI was made to perform an experiment to measure the stop band of this resonance using the UNILAC. The experiment confirmed this resonance and will be published in a separate paper.

Transverse emittance growth along the Alvarez drift tube linac (DTL) section is a major concern with respect to the preservation of beam quality of high current beams at the GSI UNILAC. In order to define measures to reduce this growth, appropriate tools to simulate the beam dynamics are indispensable. This paper is about the benchmarking of three beam dynamics simulation codes, i.e. DYNAMION, PARMILA, and PARTRAN against systematic measurements of beam emittances for different transverse phase advances along the DTL. Special emphasis is put on the modeling of the initial distribution for the simulations. The concept of rms equivalence is expanded from full intensity to fractions of less than 100% of the beam. The experimental setup, data reduction, preparation of the simulations, and the evaluation of the simulations are described. In the experiments and in the simulations, a minimum of the rms-emittance growth was observed at zero current phase advances of about 60°. In general, good agreement was found between simulations and experiment for the mean values of horizontal and vertical emittances at the DTL exit.

Electron bunches of high charge (up to 10 nC) are compressed in length in the Compact Linear Collider Test Facility magnetic chicane to less than 0.4 mm rms. The short bunches radiate coherently in the chicane magnetic field, and the horizontal and longitudinal phase space density distributions are affected. This paper reports the results of beam emittance and momentum measurements. Horizontal and vertical emittances and momentum spectra were measured for different bunch compression factors and bunch charges. In particular, for 10 nC bunches, the mean beam momentum decreased by about 5% while the rms momentum spread increased from 2% to 8%. The experimental results are compared with simulations made with the code TraFiC4.

Measurements have been performed at the superconducting Darmstadt electron linear accelerator (S-DALINAC) to investigate systematically channeling radiation produced by bombarding natural diamond crystals with thicknesses of 13, 20, 30, and 55 μm with electrons at 5.2 and 9.0 MeV. Planar channeling from the (110) and (111) planes was studied for a variety of transitions with respect to their energy, intensity, and linewidth. Axial channeling from the 〈110〉 axis could be detected as well. It was found that the intensity increases as a function of the crystal thickness, and values up to 7.7×10^-2 photons/esr could be obtained, which is the highest intensity at low electron energies achieved so far. The intensity increases with electron energy as γ^5/2. The 1/e occupation length deduced from the photon yield as a function of the crystal thickness was found to be l_occ≈29 and 85 μm for planar and for axial channeling, respectively. These values are by far the largest ever observed. Comparison with a quantum mechanical theory of channeling radiation exhibits fairly good agreement for the intensity and linewidth provided that contributions caused by electronic scattering and Bloch wave broadening, which actually are largest for diamond, are properly taken into account. It turns out that multiple scattering dominates in the planar case and single scattering for the axial channeling. The coherence length could be deduced to be of the order of 0.7 μm, which is about a factor of 2 larger than observed before in silicon.

Parametric x-ray radiation of type B has been produced with an electron beam of energies between 3.5 and 9.1 MeV from the superconducting accelerator S-DALINAC and diamond of thickness 55 μm. The photon intensity and its energy dependence were determined as a function of the tilt angle of the crystal. The intensity maximum varies with γ² and is about 3 orders of magnitude smaller than channeling radiation. Comparison with theoretical predictions exhibits very good agreement after taking into account effects caused by multiple scattering of the electrons in the crystal.