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Papers by kevin jordan

Physical Review Special Topics-accelerators and Beams, 2005

In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum curre... more In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current can been limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovered pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting RF technology. Experimental observations of the instability at the Jefferson Laboratory 10 kW Free-Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are presented and compared to the predictions of several BBU simulation codes. With BBU posing a threat to high current beam operation in the FEL Driver, several suppression schemes were developed. These include direct damping of the dangerous HOMs and appropriately modifying the electron beam optics. Preliminary results of their effectiveness in raising the threshold current for stability are presented.

Perception, 1990

Noting the similarity between the illusion decrement and selective adaptation paradigms, Long has... more Noting the similarity between the illusion decrement and selective adaptation paradigms, Long has challenged the view that illusion decrement effects reflect a strategic--as opposed to a structural--underlying mechanism, and has called for further research on this issue. To investigate the confound between prolonged free inspection and repeated trials in the standard decrement procedure, the effects of three inspection conditions (continuous, intermittent, and immediate) on the magnitude of the overestimation Mueller-Lyer illusion have been assessed under two levels of trials (a total of two or six judgments). Significant illusion decline was found only under conditions of repeated trials, with either continuous or intermittent inspection. These findings do not support the predictions of purely structural theories (including neural adaptation and efferent readiness theories), according to which degree of decrement should be determined solely by viewing time. Instead, the data demonstrate that illusion decrement is a product of practice, providing converging evidence for the view of decrement as involving a cognitive 'recalibration' or learning process.

Attention Perception & Psychophysics, 1986

The distortion of perceived line length produced by the parallel-lines configuration reversed fro... more The distortion of perceived line length produced by the parallel-lines configuration reversed fromassimilation to contrast with large suprafoveal spatial separation of the contextual and test lines (Experiment 1). This new contrast effect was predicted by a pool-and-store model of size distortion (Girgus & Coren, 1982). However, contrary to the predictions of the model, the amount of spatial separation needed to produce the reversal did not depend on an absolute visual-angle (foveal) measure of separation. Rather, the reversal was determined by the relative spatial separation of contextual and test lines (Experiment 2); that is, small test lines required a smaller spatial separation from a contextual line to produce contrast than did larger test lines. On the basis of these findings, a revised pool-and-store model of length distortion is proposed.

Lasers in Surgery and Medicine, 2006

Physical Review Special Topics-accelerators and Beams, 2005

In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum curre... more In recirculating accelerators, and in particular energy recovery linacs (ERLs), the maximum current can been limited by multipass, multibunch beam breakup (BBU), which occurs when the electron beam interacts with the higher-order modes (HOMs) of an accelerating cavity on the accelerating pass and again on the energy recovered pass. This effect is of particular concern in the design of modern high average current energy recovery accelerators utilizing superconducting RF technology. Experimental observations of the instability at the Jefferson Laboratory 10 kW Free-Electron Laser (FEL) are presented. Measurements of the threshold current for the instability are presented and compared to the predictions of several BBU simulation codes. With BBU posing a threat to high current beam operation in the FEL Driver, several suppression schemes were developed. These include direct damping of the dangerous HOMs and appropriately modifying the electron beam optics. Preliminary results of their effectiveness in raising the threshold current for stability are presented.

Perception, 1990

Noting the similarity between the illusion decrement and selective adaptation paradigms, Long has... more Noting the similarity between the illusion decrement and selective adaptation paradigms, Long has challenged the view that illusion decrement effects reflect a strategic--as opposed to a structural--underlying mechanism, and has called for further research on this issue. To investigate the confound between prolonged free inspection and repeated trials in the standard decrement procedure, the effects of three inspection conditions (continuous, intermittent, and immediate) on the magnitude of the overestimation Mueller-Lyer illusion have been assessed under two levels of trials (a total of two or six judgments). Significant illusion decline was found only under conditions of repeated trials, with either continuous or intermittent inspection. These findings do not support the predictions of purely structural theories (including neural adaptation and efferent readiness theories), according to which degree of decrement should be determined solely by viewing time. Instead, the data demonstrate that illusion decrement is a product of practice, providing converging evidence for the view of decrement as involving a cognitive 'recalibration' or learning process.

Attention Perception & Psychophysics, 1986

The distortion of perceived line length produced by the parallel-lines configuration reversed fro... more The distortion of perceived line length produced by the parallel-lines configuration reversed fromassimilation to contrast with large suprafoveal spatial separation of the contextual and test lines (Experiment 1). This new contrast effect was predicted by a pool-and-store model of size distortion (Girgus & Coren, 1982). However, contrary to the predictions of the model, the amount of spatial separation needed to produce the reversal did not depend on an absolute visual-angle (foveal) measure of separation. Rather, the reversal was determined by the relative spatial separation of contextual and test lines (Experiment 2); that is, small test lines required a smaller spatial separation from a contextual line to produce contrast than did larger test lines. On the basis of these findings, a revised pool-and-store model of length distortion is proposed.

Lasers in Surgery and Medicine, 2006

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