Specification for 12 GeV electron beam and associated instrumentation
Notice: this document is currently in draft form, and is expected to change over the coming weeks - Richard Jones, 2/7/2008
The GlueX collaboration has developed this specification as a match between the physics requirements of the GlueX experiment outlined in the Conceptual Design Report  and the expected capabilities of the 12 GeV CEBAF accelerator, as estimated by the Jefferson Lab beam physics group CASA. An earlier draft  of this specification was provided as input to the 12 GeV Upgrade Project Team which has responsibility for setting the official performance benchmarks for the upgrade, including the electron beams delivered to each 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 achieved 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 in order 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 presented in Ref.  are summarized in Appendix I below.
These formal requirements are not very useful to the GlueX physicist who needs 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. Ref.  was written in answer to need 1. The essential results from  are shown below in Tables 1 and 2. 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 Ref.  are summarized in Appendix II.
The models upon which the Expectations 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. On the other hand, the GlueX experiment needs a fixed target for electron beam properties upon which to base its design. This specification provides that design target, incorporating enough of a margin over current CASA expectations to be reasonably insensitive to refinements of the machine design.
Table 1: Electron beam properties specification for the GlueX experiment. Provision of an electron beam to Hall D that satisfies this specification will allow the experiment to achieve the physics goals set forth in the CDR in an optimal fashion.
|first 6 months||months 6-12||year 2|
|minimum energy||10 GeV||11 GeV||12 GeV|
|maximum current||3 μA||3 μA||3 μA|
|minimum current||1 nA||1 nA||1 nA|
|maximum emittance||50 nm-rad||20 nm-rad||10 nm-rad|
|maximum energy spread||< 0.5%||< 0.5%||< 0.5%|
|maximum halo fraction||10-4||10-4||5·10-5|
|maximum e- polarization||unspecified||unspecified||1%|
Table 2: The following parameters describe the size, stability and range of motion of the virtual electron beam spot projected forward from the radiator to the primary photon beam collimator in Hall D. This spot is what the electron beam intensity pattern would look like if there were no magnetic fields between the radiator and the collimator.
|first 6 months||months 6-12||year 2|
|maximum x spot size||2 mm RMS||1 mm RMS||0.5 mm RMS|
|maximum y spot size||2 mm RMS||1 mm RMS||0.5 mm RMS|
|x and y position stability||1 mm RMS||0.5 mm RMS||0.2 mm RMS|
|position stabilization bandwidth @ 300 nA||1 Hz||60 Hz||120 Hz|
|x and y range of motion1 of virtual spot at coll.||±25 mm||±25 mm||±25 mm|
|x and y centering2 of real spot at radiator||±1.0 mm||±0.5 mm||±0.5 mm|
|virtual spot placement3||±5 mm||±5 mm||±5 mm|
1Refers to ability to move the virtual spot over a certain range on the collimator entrance plane, while independently keeping the real spot centered on the radiator, possibly through a sequence of steps. This has implications for the size of the electron beam dump and beam line leading to it.
2Refers to ability to reproducibly center the real electron beam spot on a fixed location at the radiator position, while independently moving the virtual spot on the collimator, possibly through a sequence of steps. Spot moves on the diamond surface are accomplished by translations of the diamond goniometer mount.
3Refers to ability to place the virtual spot within the active collimator acceptance during initial beam tune-up using electron beam instrumentation alone, before the active collimator is switched on.
The electron beam specification for GlueX is derived indirectly from the GlueX physics requirements, through the implications that the photon beam properties have for each. 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 30% would result in higher systematic errors in polarization observables and require more run time to achieve the same statistical precision, but it would not make the difference between success and failure. In cases like this, it is important to give quantitative measures of how much impact on the physics reach of GlueX is entailed when one of the critical beam parameters slides below the specification. 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 Ref. .
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|