Optical Diffraction Radiation from a Beam Off a Circular Target PDF Download

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Languages : en
Pages : 29

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Book Description
The use of optical diffraction radiation (ODR) as a diagnostic tool has increased in recent years. The potential of this technique has been demonstrated in several experiments at KEK [1], APS [2], FLASH [3] and possibly other facilities. These experiments were performed in extraction beam lines of lepton machines. However this technique can also be applied to high energy hadron beams. In this report we consider the ODR produced by such beams with the target as a round hole and apply the results to the Tevatron. This radiation is produced when a beam passes in the vicinity of a conducting target. The electro-magnetic fields due to the beam induce currents on the target and as the beam propagates, the currents change in time producing radiation both in the direction of beam propagation and along the direction of specular reflection from the target. This latter radiation, also termed backward diffraction radiation (BDR), is more useful for diagnostics since it can be directed out at the same longitudinal location as the target. This radiation is different from optical transition radiation (OTR) in which the beam passes through a metal target. Transition radiation is not suitable for continuous monitoring of a beam in a collider due to the beam energy loss and emittance growth and the fact that the target may be damaged. However the techniques for analyzing ODR are similar in many respects to those for OTR. Measurements of the radiation intensity either in the near field or far-field have been used to determine beam positions and sizes. For example, the beam size and beam position of a 1.28 GeV electron beam were measured in an extraction beam line at KEK [1] using the far-field angular distribution of the radiation. The near-field image was used to monitor the relative beam size of a 7 GeV electron beam in the extraction line at APS [2]. In principle, measurements of the beam divergence are also possible using the interference of ODR between two targets, as has been done with OTR. This paper is motivated by the desire to use this technique in colliders, especially for the LHC and possibly for future colliders envisaged such as the muon collider. A brief report on these prospects was presented earlier [4]. If the technique yields beam measurements with sufficient accuracy and reliability then the non-invasive nature would allow continuous monitoring during the length of a luminosity run. This would be valuable if the beam can be imaged close to the interaction points. Synchrotron radiation is already used as a non-invasive diagnostic tool in the Tevatron and will also be used in the LHC. The principal advantage of ODR is that it can be generated in a straight section and therefore used for imaging in an experimental insertion. The disadvantage is that the ODR flux is less copious than synchrotron radiation (OSR) and imaging will take longer than with OSR. In Section 2 we briefly discuss the parameters of different hadron colliders. In Section 3 we derive the basic results for the angular differential spectrum of ODR from a round hole due to a bunch. We apply these results in Section 4 to find the sensitivity of the spectrum to beam size and offset changes. In Section 5, we calculate the expected photon yield from a bunch per turn as a function of frequency and we use this to find the frequency range where a sufficiently strong ODR signal can be obtained. In Section 6 we do a brief comparison of the ODR spectrum with the OTR spectrum. We briefly list in Section 7 the experimental issues associated with measuring ODR when two beams are present. We end with our conclusions in Section 8. We will use CGS units throughout.