R. M. Jones

The degradation of quantum efficiency in negative electron affinity GaAs photocathodes under gas exposure

The influence of O2, CO2, CO, N2, H2 and CH4 on the stability of the quantum efficiency (QE) of a negative electron affinity gallium arsenide (GaAs) photocathode activated with caesium (Cs) and oxygen (O) has been demonstrated for the first time under an extremely high vacuum condition, a base pressure of 1.5???10?11?mbar, where the influence of the background gas is minimized. It was found that exposure of a GaAs photocathode to N2, H2 and CH4 does not affect the QE, whereas exposure to O2, CO2 and CO leads to a substantial reduction in photocathode QE. It was also found that the QE of photocathodes which have been degraded under O2 exposure can be recovered to 95% of their initial QE level by the re-caesiation process, while those which have been degraded under exposure to CO and CO2 can only be partly restored to 60?70% of their initial QE levels.

A study of beam position diagnostics using beam-excited dipole modes in third harmonic superconducting accelerating cavities at a free-electron laser.

We investigate the feasibility of beam position diagnostics using higher order mode (HOM) signals excited by an electron beam in the third harmonic 3.9 GHz superconducting accelerating cavities at FLASH. After careful theoretical and experimental assessment of the HOM spectrum, three modal choices have been narrowed down to fulfill different diagnostics requirements. These are localized dipole beam-pipe modes, trapped cavity modes from the fifth dipole band, and propagating modes from the first two dipole bands. These modes are treated with various data analysis techniques: modal identification, direct linear regression (DLR), and singular value decomposition (SVD). Promising options for beam diagnostics are found from all three modal choices. This constitutes the first prediction, subsequently confirmed by experiments, of trapped HOMs in third harmonic cavities, and also the first direct comparison of DLR and SVD in the analysis of HOM-based beam diagnostics.

High stability of negative electron affinity gallium arsenide photocathodes activated with Cs and NF3

In this paper we report the first time demonstration under extremely high vacuum (XHV) conditions of the influence of O2, CO2, CO, N2, H2 and CH4 on the quantum efficiency (QE) of negative electron affinity (NEA) gallium arsenide (GaAs) photocathodes activated with Cs and NF3. The photocathodes were exposed to a small quantity (<0.25 Langmuirs) of each gas species under test in a vacuum chamber with a typical base pressure of mbar, thereby minimising the influence of the residual gas in the photocathode response. It was found that exposure to N2, H2 and CH4 does not affect the QE of the photocathodes, whereas exposure to O2 and CO2 lead to a substantial reduction in the QE of the photocathodes. Only small degradation in the QE under CO exposure was observed. Compared to those activated with Cs and O2 in our previous study [1], photocathodes activated with Cs and NF3 are more stable, especially under exposure to CO.

Resolution study of higher-order-mode-based beam position diagnostics using custom-built electronics in strongly coupled 3.9-GHz multi-cavity accelerating module

Beam-excited higher order modes (HOMs) can provide remote diagnostics information of the beam position and cavity misalignment. In this paper we report on recent studies on the resolution with specially selected series of modes with custom-built electronics. This constitutes the first report of measurements of these cavities in which we obtained a resolution of 20 micron in beam offset. Details of the setup of the electronics and HOM measurements are provided.

COLDDIAG: A Cold Vacuum Chamber for Diagnostics

One of the still open issues for the development of superconducting insertion devices is the understanding of the heat load induced by the beam passage. With the aim of measuring the beam heat load to a cold bore and in order to gain a deeper understanding in the beam heat load mechanisms, a cold vacuum chamber for diagnostics is under construction. We plan to have access with the same set-up to a number of different diagnostics, so we are implementing: i) retarding field analysers to measure the electron flux, ii) temperature sensors to measure the total heat load, iii) pressure gauges, iv) and mass spectrometers to measure the gas content. The inner vacuum chamber will be removable in order to test different geometries and materials. COLDDIAG is built to fit in a short straight section at ANKA, but we are proposing its installation in different synchrotron light sources with different energies and beam characteristics. A first installation in DIAMOND is planned in June 2011. Here we describe the technical design report of this device and the planned measurements with beam.

Influence of fabrication errors on wake function suppression in NC X-band accelerating structures for linear colliders

Wake function suppression is effected by ensuring that the mode frequencies of an X-band normal conducting (NC) accelerating structure of multiple cells are detuned and moderately damped by waveguide manifolds attached to the outer wall of the accelerator. We report on the dilution in the wake function suppression that occurs due to errors resulting from the fabrication process. After diffusion bonding 206 cells a non-uniform expansion in the cell geometry forces a substantial shift in the frequencies of select cells. We remap all circuit parameters to these shifted cell frequencies to predict the wake function. Experiments performed on the SLC at the SLAC National Accelerator Laboratory indicate that the wake function is well predicted by the circuit model.

Simulations of stretched wire measurements of 3.9GHZ cavities for the ILC

We present simulations on the transverse modes in crab cavities required in the beam delivery system of the ILC to maximise the luminosity of the beam. We utilize both finite difference and finite element computer codes to simulate the electromagnetic fields in these cavities in the presence of a stretched wire. This study is intended to both predict the wake-field in experiments on the modal characterisation of 3.9 GHz cavities in progress at the Cockcroft Institute. A scattering matrix approach is used on multi-cell cavities and compared with experimental data. Results are presented on the modes in crab cavity structures subjected to infinitely periodic boundary conditions.

Impedance measurements on a test bench model of the ILC crab cavity

In order to verify detailed impedance simulations, the resonant modes in an aluminium model of the ILC crab cavity were investigated using a bead-pulling technique as well as a stretched-wire frequency domain measurement. The combination of these techniques allow for a comprehensive study of the modes of interest. For the wire measurement, a transverse alignment system was fabricated and RF components were carefully designed to minimize any potential impedance mismatches. The measurements are compared with direct simulations of the stretched-wire experiments using numerical electromagnetic field codes. High impedance modes of particular relevance to the ILC crab cavity are identified and characterized.

Simultaneous Tune and Coupling Feedback during RHIC Run 6

The Relativistic Heavy Ion Collider (RHIC) is a versatile facility, capable of operating with both polarized protons and a variety of ion species over a broad range of energies. Acceleration ramp requirements change frequently to meet the needs of the physics experiments. There has been an ongoing effort at RHIC to implement reliable betatron tune feedback as a tool for ramp development, and possibly to control tune during routine operations. Vertical orbit fluctuations in the chromaticity correction sextupoles during ramping are sufficient to fully couple the tunes. This complicates tune control, and with tune feedback causes the basis in which tune errors are reported to not match the basis used to correct the quadrupole currents, leading to feedback instability. This problem has been overcome by using realtime measurements of the coupling projections of both betatron eigenmodes to close feedback loops onto two skew quadrupole families. This minimizes coupling and matches the tune and coupling reporting basis to the correction basis. We report on successful simultaneous tune and coupling feedback during RHIC ramp development and implications of this effort for the LHC.

Beam phase retrieval based on higher order modes in cylindrical superconducting radio frequency cavities

The control of beam phase relative to the accelerating RF field within a superconducting cavity is important in many accelerator applications and is of particular importance for a free electron laser facility. As standard practice, the phase is usually inferred from the beam-induced transient field with respect to a timing reference. We report here on an alternative and novel means of beam phase determination based on beam-excited higher order electromagnetic modes and the accelerating electromagnetic mode, which are conveniently available from the same coupler. The monopole modes are immune to the electron beam offset and therefore are best suited for the task. A coupled circuit model is used to assist the development and to rapidly assess the facility of the method. Simulations based on the circuit model indicate that the resolution of this system depends critically on the signal to noise ratio. Beam-based measurements with a test setup were carried out at the European X-ray Free Electron Laser (XFEL), Germany. Based on this new method, we have routinely obtained a resolution of 0.1°. The best resolution observed with the current setup was 0.03°. These results agree very well with the predictions from those predicted by a circuit model. The system investigated here can be used to provide diagnostic information for the current low level RF system employed in the European XFEL. To this end, the associated electronics are under development. This monitor is the first of its kind that can deliver direct and online measurements of the beam phase with respect to the RF field.