Experimental demonstration of novel beam characterization using a polarizable X-band transverse deflection structure.

The PolariX TDS (Polarizable X-Band Transverse Deflection Structure) is an innovative TDS-design operating in the X-band frequency-range. The design gives full control of the streaking plane, which can be tuned in order to characterize the projections of the beam distribution onto arbitrary transverse axes. This novel feature opens up new opportunities for detailed characterization of the electron beam. In this paper we present first measurements of the Polarix TDS at the FLASHForward beamline at DESY, including three-dimensional reconstruction of the charge-density distribution of the bunch and slice emittance measurements in both transverse directions. The experimental results open the path toward novel and more extensive beam characterization in the direction of multi-dimensional-beam-phase-space reconstruction.

Linac4 H⁻ ion sources.

CERN's 160 MeV H(-) linear accelerator (Linac4) is a key constituent of the injector chain upgrade of the Large Hadron Collider that is being installed and commissioned. A cesiated surface ion source prototype is being tested and has delivered a beam intensity of 45 mA within an emittance of 0.3 π ⋅ mm ⋅ mrad. The optimum ratio of the co-extracted electron- to ion-current is below 1 and the best production efficiency, defined as the ratio of the beam current to the 2 MHz RF-power transmitted to the plasma, reached 1.1 mA/kW. The H(-) source prototype and the first tests of the new ion source optics, electron-dump, and front end developed to minimize the beam emittance are presented. A temperature regulated magnetron H(-) source developed by the Brookhaven National Laboratory was built at CERN. The first tests of the magnetron operated at 0.8 Hz repetition rate are described.

Plasma ignition and steady state simulations of the Linac4 H(-) ion source.

The RF heating of the plasma in the Linac4 H(-) ion source has been simulated using a particle-in-cell Monte Carlo collision method. This model is applied to investigate the plasma formation starting from an initial low electron density of 10(12) m(-3) and its stabilization at 10(18) m(-3). The plasma discharge at low electron density is driven by the capacitive coupling with the electric field generated by the antenna, and as the electron density increases the capacitive electric field is shielded by the plasma and induction drives the plasma heating process. Plasma properties such as e(-)/ion densities and energies, sheath formation, and shielding effect are presented and provide insight to the plasma properties of the hydrogen plasma.

Numerical simulation of electromagnetic fields and impedance of CERN LINAC4 H(-) source taking into account the effect of the plasma.

Numerical simulation of the CERN LINAC4 H(-) source 2 MHz RF system has been performed taking into account a realistic geometry from 3D Computer Aided Design model using commercial FEM high frequency simulation code. The effect of the plasma has been added to the model by the approximation of a homogenous electrically conducting medium. Electric and magnetic fields, RF power losses, and impedance of the circuit have been calculated for different values of the plasma conductivity. Three different regimes have been found depending on the plasma conductivity: (1) Zero or low plasma conductivity results in RF electric field induced by the RF antenna being mainly capacitive and has axial direction; (2) Intermediate conductivity results in the expulsion of capacitive electric field from plasma and the RF power coupling, which is increasing linearly with the plasma conductivity, is mainly dominated by the inductive azimuthal electric field; (3) High conductivity results in the shielding of both the electric and magnetic fields from plasma due to the skin effect, which reduces RF power coupling to plasma. From these simulations and measurements of the RF power coupling on the CERN source, a value of the plasma conductivity has been derived. It agrees well with an analytical estimate calculated from the measured plasma parameters. In addition, the simulated and measured impedances with and without plasma show very good agreement as well demonstrating validity of the plasma model used in the RF simulations.

Surface-emission studies in a high-field RF gun based on measurements of field emission and Schottky-enabled photoemission.

We report on investigations into the fundamental surface emission parameters, the geometric field enhancement factor (ß) and the work function (φ), by making both field emission and Schottky-enabled photoemission measurements. The measurements were performed on a copper surface in the Tsinghua University S-band RF gun in two separate experiments. Fitting our data to the models for each experiment indicate that the traditionally assumed high value of ß(≈50-500) does not provide a plausible explanation of the data, but incorporating a low value of φ at some sites does. In addition, direct measurements of the surface conducted after the experiment show that ß is on the order of a few, consistent with our understanding of the electron emission measurements. Thus we conclude that the dominant source of electron emission in high gradient RF cavities is due to low φ sites, as opposed to the conventionally assumed high ß sites. The origin of low φ at these sites is unclear and should be the subject of further investigation.

Numerical analysis of an injection-locked gyrotron backward-wave oscillator with tapered sections.

Injection-locked operation of a gyrotron backward-wave oscillator (gyro-BWO) is investigated by means of our time-dependent self-consistent code. Numerical results for a 100 kW, Ka-band, TE11-mode gyro-BWO developed at NTHU, Hsinchu, Taiwan, are compared to the experimental ones. The results are in good agreement showing, in both cases, the asymmetric form of the locking bandwidth curve. Comparison of these results obtained for the case of injection of the external signal into the upstream port, as has been done in the experiments, to the results for injection into the downstream port is made. The results of the comparison demonstrate that the asymmetry of the locking bandwidth curves must be attributed to the influence of the injection signal on the electron bunching that takes place in the input tapered section. Moreover, in some cases, this influence results also in a significant increase of the locking bandwidth (up to a factor of 5). An increase of the output power of the locked gyro-BWO over the output power of the free running one is also investigated and compared with the experimental data.

Fractal properties of trivelpiece-gould waves in periodic plasma-filled waveguides.

It is shown that dispersion curves describing a spectrum of Trivelpiece-Gould (TG) waves in periodic plasma-filled waveguides have a fractal nature. They are not solid lines as for other types of waves in periodic waveguides but suffer from discontinuities of the first kind at any k(z) = (P/Q) (2m+1)pi/d, where P and Q are integers, d is the period of the corrugation, and m is the transverse index of a mode. The gaps correspond to forbidden bands. The evaluation of the Hausdorf dimension of the dispersion curves is presented. Finally, qualitative consequences of the fractal nature of TG waves for plasma microwave electronics are discussed.