The Beam Line

If the detector is the heart of the Hall, then certainly the beam line should be it's main artery. The beamline devices are here to feed the detector with a well defined flux of photon or electron, with known and stable physical characteristics.

To achieve this goal, a lot of apparatus has to be put under control and monitored, ranging from a single photomultiplier connected to a scaler for halo control to hundreds of kilowatts power supplies. All that will ultimately be supported under a single architecture, under some common software reference called EPICS (see chapter 3). Let's have a look at the devices themselves.

The beamline devices

In this part are listed most of the beamline devices. These are used mainly to stir the beam and to monitor the beam qualities. We will go along the beam line, from the arrival of the beam into the Hall until the beam-dump. Some of the devices will be described again with more details in chapter 4. The other devices are presented mainly to give some global perspective over the whole beamline, some little parts will be detailed. A complete picture of the beamline from a Slow Control perspective is displayed on Figure 2.9, and a more geometrical view is given in Figures 2.10, 2.11 and 2.12.

  

Figure 2.9: Beamline devices

  

Figure 2.10: Beamline devices : before the Tagger Magnet

  

Figure 2.11: Beamline devices : between the Tagger Magnet and CLAS

  

Figure 2.12: Beamline devices : After CLAS

Accelerator division devices

Machine Protection System

This is the security device that cuts the electron beam when we lose control on it. It monitors the current of the beam at the injector and in the Halls, and automatically shuts it down if the difference become too large. These devices are not under Hall B control, but are managed by the Accelerator Division.

Vacuum system

The required pumps are installed and already tested. There are two kind of pumps : turbo molecular pumps (that use a very fast turning rotor), and ion pumps (ionizing the molecules and then taking them away with an electric field). The ion pumps are already monitored under EPICS but the second system is not yet implemented. The vacuum should vary from around 10^-4 Pa at the entrance of the hall to 10^-3 Pa at the level of the target.

Beam position monitors

These are needed to track the beam position during runs. There are three devices to perform this task. There is a screen viewer that acts like a TV screen converting electrons to photons and a video camera looking at the screen.

The second system, already installed, consist of four antennas around the beam. By measuring the induced current in it, one can know where the beam is. Another device that has not yet been installed, is a thin sheet of matter that will emit light when crossed by the beam. The interesting thing is that it performs not only a measurement of the position of the beam, but also of it's dispersion. It is controlled by the Accelerator Division.

The rastering Magnet

The CEBAF Hall B beam is not very intense (around 2.5 nA), so the rastering is not a very important problem for us (contrary to Hall A or C). A slow 60 Hz spiral motion of the beam with a 1 mm amplitude at the level of the target is enough for the Hall B.

The Möller polarimeter

The Möller polarimeter is designed to measure the beam polarization it won't be ready until the end of 1997

A Möller polarimeter uses the polarized electrons scattered from a a magnetized foil target to measure the polarization of the beam. The electrons of the beam interact with the target, in a way that is dependent of the polarization of both the target and the beam, as shown in equation 2.4.1.

 

 $$\frac{d\sigma}{d\Omega^{*}} = \frac{d\sigma_{0}}{d\Omega^{*}}(1+\sum_{i,j= \{x,y,z\}} P_{i}^{a}A_{ij}P_{j}^{b})$$

Where :
P_i are the components of the polarization on x,y and z axis of the electron a and b.
A_ij are the asymmetry parameters.
$\frac{d\sigma_{0}}{d\Omega^{*}}$ is the center of mass cross-section for unpolarized electrons.

  

Figure 2.13: Axes and angles in the c.m.s.

With the choice of coordinates shown in Figure 2.13, we have, for the asymmetry parameters and the unpolarized cross-section :

$$\frac{d\sigma_{0}}{d\Omega^{*}} = \frac{\alpha^{2}}{4m^{2}}\gamma^{-2} \frac{(4-\sin^{2}\Theta^{*})^{2}}{\sin^{4}\Theta^{*}}$$

$$A_{zz} = -\frac{(7+\cos^{2}\Theta^{*})sin^{2}\Theta^{*}}{(3+\cos^{2}\Theta^{*})^{2}}$$

$$-A_{xx}=A_{yy}=\frac{\sin^{4}\Theta^{*}}{(3+\cos^{2}\Theta^{*})^{2}}$$

$$A_{zx}=A_{zx}= -\frac{2\sin^{3}\Theta^{*}\cos \Theta^{*}}{\gamma(3+\cos^{2}\Theta^{*})^{2}}$$

A_xy=A_yx=A_zy=A_yz=0

Where $\gamma = \sqrt{\frac{E_{0}+m}{2m}}$, $\alpha$ is the fine structure constant, m is the electron mass and E₀ is the energy of the beam electron in the laboratory system.

If we consider only the electrons scattered at $\Theta^{*}=90^{o}$, where the asymmetry parameters A_ii ($i = \{x,y,z\}$) are maximum, the other asymmetry parameters are vanishing at this angle. So, if we assume the knowledge of the target polarization parameters on the three axis, then we can derivate the polarization of the incident beam.

  

Figure 2.14: The Möller Polarimeter in Hall B

The Möller Polarimeter quadrupole magnets

The magnets will focus the outgoing electrons on the detectors. When the energy of the beam varies, the 90^o angle between the two electrons appears differently in the laboratory reference frame. By applying an adequate field into the magnets, one can always conduct the scattered electrons to the detectors. The choice between a one or two magnets system has been heavily discussed, but only a two magnets system will allow a measurement of the polarization at 6 GeV, an energy that will be reached by the accelerator in the near future.

The Möller Polarimeter detectors

The Möller Polarimeter is not yet completely designed, but it will need some Slow-Control support; high voltage control of the photomultipliers if a scintillator detector is used, and the counting and processing of these detectors will be made by scalers under EPICS control.

As we can see on the picture 2.14, there are two detectors placed symmetrically on the beam axis. This is of course to detect the two scattered electrons, requiring a coincidence that will reduce the background by a large factor.

Devices used for generating and controlling the photon beam

The Harp

The harp is a system of two thin moving wires that measures the beam position with a great accuracy in the x and y directions. It also supports the radiators for the generation of the photon beam. It is located just before the tagger magnet.

The radiator itself is a thin target of around 10^-6 to 10^-3 radiation lengths. It need to be small enough to avoid multiple scattering, and being made of a material with high Z to favorise Bremsstrahlung which cross-section increases roughly in Z² . Some special radiator target will be used for polarized electrons, like an oriented crystal³. But such a device won't be available for the first set of experiments at CLAS. More informations about the Harp and it's control can be found in Section 4.3.

The Tagger Magnet

The magnet provides an uniform magnetic field (1.13 Tesla) which guides the full energy electrons to the beam dump. The quality of this field in homogeneity and stability is crucial for a good measurement of the electron energy.

Its first goal is to remove every charged particle from the beam axis, that is of course the electrons that have (or have not) interacted with the radiator, and some secondary charger particles created by occasional nuclear interactions in the radiator.

It will deflect the electrons so that they will end up on the focal plan detectors with linear relation between energy and position ($12 cm / \%$of the primary beam energy). The usable part of the focal plane is approximately 9.5 meters long, and the full energy electrons will go straight to the beam dump located beneath the detector.

Photon tagger

When CLAS runs with a photon beam, photons will be tagged by collecting the electron that have produced a Bremsstrahlung photon. The measure of the energy of the electron will allow the computation of the photon energy. A time stamping of each photons connects it with a specific events in the CLAS detector.

Collimator system

The collimators are simple holes that will clean up the photon beam from electrons that are not enough collinear to the beamline. For more information about the Collimator and it's control see section 4.5.

Some other important beamline devices

Mini torus

The mini-torus magnet consists of six normal conducting coils ⁴. The maximum current these coils can conduct is 8300A, provided there is sufficient cooling water. The mini-torus magnet resides inside CLAS, surrounding the target and protecting the Region I drift chambers from soft electro-magnetic debris.

Faraday cup

The Faraday Cup is collecting the electron beam in the beam dump, allowing to have a precise measure of the total charges that crossed the detector during a run. This device is very useful to determine the total flux of the beam in the Hall, therefore allowing computation of absolute cross-sections of reactions.

Faraday cup viewer

The purpose of this device is to provide information of the beam location at the Faraday cup position. A fluorescent sheet is placed in the visual field of a video camera, allowing operators to measure the beam position.

Footnotes

...crystal³

Probably a diamond

...coils⁴

ie not superconductor