Specification for 12 GeV electron beam and associated instrumentation
The following specification was developed by the GlueX collaboration as a match between the physics requirements of the GlueX experiment and the anticipated capabilities of the 12 GeV CEBAF accelerator based upon input from the Jefferson Lab beam physics group CASA. An early draft of this specification was provided as input to the 12 GeV Upgrade Project Team. That team has responsibility for setting the official performance benchmarks for the machine upgrade, including the electron beam delivered to all of the experimental end stations. The official benchmarks for the upgrade are referred to below under the heading CD4 Requirements.
The CD4 Requirements establish a performance baseline that must be met by the upgraded accelerator complex by the target date set for the end of construction in order to receive high marks as a successful DOE project. The requirements are set deliberately loose to allow for an on-schedule transition from construction to startup of physics, even if everything does not go exactly as planned. The CD4 Requirements as outlined in gluex-doc-965 are summarized in Appendix I below.
These formal requirements are not very useful to the GlueX physicist who wants to know (1) what is required in order to enable the GlueX experiment to achieve the physics goals stated in the Conceptual Design Report, and (2) what beam parameters are realistically achievable. The present document is provided in answer to need 1.
In answer to need 2, the CASA group has developed a set of Expectations: parameters that describe what the current simulation of the upgrade design predicts for the electron beam properties. Expectations presented by the CASA group to the GlueX collaboration in gluex-doc-965 are summarized in Appendix II. The models upon which the Expections are based are extremely thorough. They take into account synchrotron radiation in the arcs, beam orbit errors resulting from mechanical misalignment and higher moments in magnetic elements, and the number and placement of beam monitors upon which orbit corrections are based. They are not perfect, however, and are subject to change as the design matures and simulations improve.
The physics requirements of the GlueX experiment contain some flexibility in terms of photon beam properties, some properties more than others. For example, a decrease in the photon linear polarization from 40% at 9 GeV to 35% would result in higher systematic errors and require more run time to achieve the same statistical precision in polarization observables, but it would not make the difference between success and failure. Nevertheless, for the design of the tagger and the rest of the experimental apparatus it is necessary to specify a set of electron beam properties that are both consistent with machine expectations and optimized for achieving the physics goals outlined in the GlueX CDR. That is what is provided in the following specification. For more details on how these numbers were derived and how sensitive the experiment is to their variation, see gluex-doc-646.
Appendix 1: CD4 Requirements for Hall D Electron Beam
|x emittance||20 nm-rad|
|y emittance||20 nm-rad|
Appendix 2: CASA Expectations for Hall D Electron Beam
|x emittance||< 7 nm-rad|
|y emittance||< 2 nm-rad|
|δp/p||< 0.02% RMS|