C. Perry

Status of the Oxford radiocarbon accelerator

The Oxford accelerator has been used exclusively for 14C measurements, and has progressed to the point where reliable dates can be obtained. Evidence for the present performance is presented, including a histogram of the reproducibility of over 100 wire pairs used in actual dating. The errors of measurement on known-age materials are also shown. Machine improvements which have lead to the present performance are described; two problems in particular remain: a variation of the 13C/12C ratio with beam current, and a variation of beam position and distribution from one target to another. Graphite targets continue to be made on tantalum wires. We have built another source in which the target is rotated during sputtering, but this has not been tested. The greatest age obtained is 62 000 ± 2000 years BP from geological graphite, but this is almost certainly limited by sample contamination. In practice, the background is limited by laboratory contamination (detailed in a companion paper).

Electromagnetic background tests for the ILC interaction-point feedback system

We present results obtained with the T-488 experiment at SLAC Endstation A (ESA). A material model of the ILC extraction-line design was assembled and installed in ESA. The module includes materials representing the mask, beamline calorimeter, and first extraction quadrupole, encompassing a stripline interaction-point feedback system beam position monitor (BPM). The SLAC high-energy electron beam was used to irradiate the module in order to mimic the electromagnetic (EM) backgrounds expected in the ILC interaction region. The impact upon the performance of the feedback BPM was measured, and compared with detailed simulations of its expected response.

The FONT4 ILC intra-train beam-based digital feedback system prototype

We present the design of the FONT4 intra-train beam-based digital position feedback system prototype. The system incorporates a fast analogue beam position monitor front-end signal processor, a digital feedback board, and a fast kicker-driver amplifier. The system latency goal is less than 150 ns. We report preliminary results of beam tests at the accelerator test facility (ATF) at KEK using electron bunches separated by c. 150 ns.

Simulation of ILC feedback bpm signals in an intense background environment

Experiment T-488 at SLAC, End station A recorded distorted BPM voltage signals and an accurate simulation of these signals was performed. Geant simulations provided the energy and momentum spectrum of the incident spray and secondary emissions, and a method via image charges was used to convert particle momenta and number density into BPM stripline currents. Good agreement was achieved between simulated and measured signals. Further simulation of experiment T-488 with incident beam on axis and impinging on a thin radiator predicted minimal impact due to secondary emission. By extension to worst case conditions expected at the ILC, simulations showed that background hits on BPM striplines would have a negligible impact on the accuracy of beam position measurements and hence the operation of the FONT feedback system.

JACoW : Intra-Train Position and Angle Stabilisation at ATF Based on Sub-Micron Resolution Stripline Beam Position Monitors

A low-latency, sub-micron resolution stripline beam position monitoring (BPM) system has been developed and tested with beam at the KEK Accelerator Test Facility (ATF2), where it has been used to drive a beam stabilisation system. The fast analogue front-end signal processor is based on a single-stage radio-frequency down-mixer, with a measured latency of 16 ns and a demonstrated single-pass beam position resolution of below 300 nm using a beam with a bunch charge of approximately 1 nC. The BPM position data are digitised on a digital feedback board which is used to drive a pair of kickers local to the BPMs and nominally orthogonal in phase in closed-loop feedback mode, thus achieving both beam position and angle stabilisation. We report the reduction in jitter as measured at a witness stripline BPM located 30 metres downstream of the feedback system and its propagation to the ATF interaction point.

Development of a Low-latency, Micrometre-level Precision, Intra-train Beam Feedback System based on Cavity Beam Position Monitors

A low-latency, intra-train, beam feedback system utilising a cavity beam position monitor (BPM) has been developed and tested at the final focus of the Accelerator Test Facility (ATF2) at KEK. A low-Q cavity BPM was utilised with custom signal processing electronics, designed for low latency and optimal position resolution, to provide an input beam position signal to the feedback system. A custom stripline kicker and power amplifier, and a digital feedback board, were used to provide beam correction and feedback control, respectively. The system was deployed in single-pass, multi-bunch mode with the aim of demonstrating intra-train beam stabilisation on electron bunches of charge ~1 nC separated in time by c. 220 ns. The system has been used to demonstrate beam stabilisation to below the 75 nm level. Results of the latest beam tests, aimed at even higher performance, will be presented.

First Experimental Results with the CLIC Drive Beam Phase Feedforward Prototype at the CLIC Test Facility CTF3

The two-beam acceleration scheme envisaged for CLIC will require a high degree of phase stability between two beams at the drive beam decelerator sections, to allow efficient acceleration of the main beam. There will be up to 48 such decelerator sections for the full 3 TeV design, and each decelerator section will be instrumented with a feed-forward system to correct the drive beam phase to a precision of 0.2 degrees at 12 GHz relative to the main beam, using a kicker system around a four-bend chicane. A prototype system has been developed and tested at the CLIC Test Facility (CTF3) complex, where the beam phase is measured upstream of the combiner ring and corrected with two kickers in a dog-leg chicane just upstream of the CLEX facility, where the resulting phase change is measured. This prototype is designed to demonstrate correction of a portion of the CTF3 bunch train to the level required for CLIC, with a bandwidth of greater than 30 MHz, and within a latency constraint of 380 ns as set by the beam time-of-flight through the combiner ring complex. A description of the hardware will be given and initial results from the first phase of the experiment will be presented.

Electron bunch profile diagnostics using coherent Smith-Purcell radiation emitted in the terahertz region

We report some preliminary results of the Smith-Purcell radiation produced by the interaction of the 28.5 GeV electron beam with a ruled grating at the Stanford Linear Accelerator Center (SLAC). The objects of the experiment were to investigate the longitudinal (temporal) profile of the beam and to confirm that the radiated power was in agreement with the predictions of our surface current theory of the emission. The results are compared with those from earlier, lower, electron energy experiments and preliminary analysis indicates that our theoretical approach is valid over more than a four order range of electron energy from ~2 MeV to ~30 GeV.