John Dobbins

Record high-average current from a high-brightness photoinjector

High-power, high-brightness electron beams are of interest for many applications, especially as drivers for free electron lasers and energy recovery linac light sources. For these particular applications, photoemission injectors are used in most cases, and the initial beam brightness from the injector sets a limit on the quality of the light generated at the end of the accelerator. At Cornell University, we have built such a high-power injector using a DC photoemission gun followed by a superconducting accelerating module. Recent results will be presented demonstrating record setting performance up to 65 mA average current with beam energies of 4–5 MeV.

Design and construction of the CLEO II drift chamber

Abstract We have constructed a new large cylindrical drift chamber (2 m long and 2 m in diameter) for the CLEO II experiment at the Cornell Electron Storage Ring (CESR). The chamber contains 51 readout layers including 11 stereo layers, with 48480 (12240) total (sense) wires. Segmented cathode strips were used instead of field wires for the innermost and outermost layers to provide two accurate measurements of the longitudinal coordinate. Very low density support structures were developed to hold the cathode strips. Aluminum wire was used for the majority of the field wires to reduce multiple scattering and the tension load on the endplates. A semiautomatic apparatus was developed to string wires with the chamber mounted vertically. With this system, it was also possible to replace any wire in the chamber. A maximum stringing rate of 60 wires/h was achieved. The tension on all wires was measured soon after they were strung; the failure rate due to improper tension was typically less than 2%. Hybrid preamplifiers were developed and mounted directly on the chamber on custom printed circuit boards. Each board also contains a pulser calibration system and a high voltage current monitor. High voltage power is individually supplied to each of the 51 layers under computer control. A new type of hardware track finder was developed for the trigger.

The use of silicon photodiodes in a CsI(Tl) calorimeter

Abstract The properties of silicon photodiodes and low-noise preamplifiers have been studied. The selection of optimal operating parameters for the use of photodiodes in a CsI shower detector is discussed.

Design and tests for CLEO-II silicon vertex detector

We report on the design and prototype studies for a new double-sided silicon vertex detector for the CLEO-II experiment at CESR. To minimize multiple scattering, two new techniques are employed: (1) the detector is supported by low mass, low Z boron carbide foam U-channels; and (2) the preamplifiers are removed entirely from the tracking volume by using a second layer of metallization on the silicon to provide longitudinal readout traces for the transverse sense strips.

Design, conditioning, and performance of a high voltage, high brightness dc photoelectron gun with variable gap

A new high voltage photoemission gun has been constructed at Cornell University which features a segmented insulator and a movable anode, allowing the cathode-anode gap to be adjusted. In this work, we describe the guns overall mechanical and high voltage design, the surface preparation of components, as well as the clean construction methods. We present high voltage conditioning data using a 50 mm cathode-anode gap, in which the conditioning voltage exceeds 500 kV, as well as at smaller gaps. Finally, we present simulated emittance results obtained from a genetic optimization scheme using voltage values based on the conditioning data. These results indicate that for charges up to 100 pC, a 30 mm gap at 400 kV has equal or smaller 100% emittance than a 50 mm gap at 450 kV, and also a smaller core emittance, when placed as the source for the Cornell energy recovery linac photoinjector with bunch length constrained to be <3 ps rms. For 100 pC up to 0.5 nC charges, the 50 mm gap has larger core emittance than the 30 mm gap, but conversely smaller 100% emittance.

Photocathode behavior during high current running in the Cornell energy recovery linac photoinjector

The Cornell University energy recovery linac (ERL) photoinjector has recently demonstrated operation at 20 mA for approximately 8 hours, utilizing a multialkali photocathode deposited on a Si substrate. We describe the recipe for photocathode deposition, and will detail the parameters of the run. Post-run analysis of the photocathode indicates the presence of significant damage to the substrate, perhaps due to ion back-bombardment from the residual beam line gas. While the exact cause of the substrate damage remains unknown, we describe multiple surface characterization techniques (x-ray fluorescence spectroscopy, x-ray diffraction, atomic force, and scanning electron microscopy) used to study the interesting morphological and crystallographic features of the photocathode surface after its use for high current beam production. Finally, we present a simple model of crystal damage due to ion back-bombardment, which agrees qualitatively with the distribution of damage on the substrate surface.

Studies of radiation damage to CAMEX64 preamplifiers exposed to γ-rays at the Cornell high energy synchrotron source

Abstract CAMEX64 preamplifiers, developed at the MPI-Munich for use in the readout of silicon strip detectors, have been exposed to well-measured doses of monochromatic γ-rays from the Cornell high energy synchrotron source (CHESS) and measurements of the resulting radiation damage have been made. Silicon strip detectors with their associated CAMEXes will be installed close to the interaction region in the CLEO-II experiment at Cornell University, where they will be exposed to both charged particle and photon fluences. Powered and unpowered CAMEXes were irradiated and after each exposure measurements of pedestals, gains, coherent and incoherent noise were made, until the devices could no longer be read out. Results are quoted for noise and gain degradation as a function of dose.

Studies of performance and resolution of double sided silicon strip detectors

Double-sided silicon strip detectors have been fabricated and tested in a silicon tracking telescope placed in a high-energy muon beam at Fermi National Accelerator Laboratory. The detectors feature AC coupling to front-end electronics and employ two different techniques for isolating n-strips. The CAMEX 64-channel multiplexing amplifier is used for readout. Preliminary results of measurements are reported on pulse heights, signal-to-noise performance, hit resolution, and impedance characteristics of the detector. Measurements of the bias resistance illustrate a mild dependence on bias voltage and, for the field-effect detectors, a strong dependence on the voltage of the readout strips and field strips. The signal-to-noise ratio is determined to be in the range of 10-12, but is expected to increase with improvements to the electronics. >

CLEOII trigger system

We present a description of the trigger system used in the CLEOII detector currently in operation at the Cornell Electron Storage Ring.

Experience with the New Digital RF Control System at the CESR Storage Ring

A new digital control system has been developed, providing great flexibility, high computational power and low latency for a wide range of control and data acquisition applications. This system is now installed in the CESR storage ring and stabilizes the vector sum field of two of the superconducting CESR 500 MHz cavities and the output power from the driving klystron. The installed control system includes in-house developed digital and RF hardware, very fast feedback and feedforward control, a state machine for automatic start-up and trip recovery, cw and pulsed mode operation, fast quench detection, and cavity frequency control. Several months of continuous operation have proven high reliability of the system. The achieved field stability surpasses requirements.