This document describes the operation and control of diagnostics attached to the photoinjector beamline. (Control of the beamline elements is in the Components document, and water and vacuum are in a separate Infrastructure document.)
Contents:
The instrumentation on the beamline for beam diagnosis enables measurement of beam transverse profiles using phosphor and YAG screens, integrated beam charge using the current monitor, and total energy using the dipole spectrometer. In addition to these "everyday" measurements, we can measure beam alignment using a set of BPMs, total charge using a Faraday cup after the dipole, and beam longitudinal size using CTR interferometry. There is also a precision-mounted set of slits which can be inserted for emittance measurements.
II. Phosphor Screens and Other Insertion Devices
Currently, there are 9 insertion devices for beam diagnosis on the beamline, including 5 phosphor screens, 2 YAG screens, a foil for coherent transition radiation production, and a mirror for viewing the gun cathode. Each of these is on a separate pneumatic actuator with remote electrical control. (The emittance slits are on a precise stepper motor and discussed separately.) The screens enable transverse beam size measurements as well as being constantly in use for steering and focusing control.
Each actuator consists of a plunger connected to a compressed-air line. The pneumatic pressure is directed outward to keep the plunger in the extracted position. When 120 VAC is placed across two connectors on the body of the actuator, a magnetic solenoid forces the actuator to travel its full extent and stay there (against the air pressure)--thus causing whatever is attached to it to be inserted into the beamline. Consequences of this engineering are that if the power fails, all the plungers will extract; if the compressed air fails, the plungers will insert slowly (under gravity) as the air leaks out.
To insert and extract the actuators, a series of power cables (one connected to each actuator solenoid) is passed into the control room, where they can individually be powered at 120 VAC using a CAMAC relay module connected to a power supply. This means in essence that the operator can choose the device to be inserted by closing its particular relay using the LabView Master Control vi.
Detailed operation of the CAMAC module
The power cords connected to each actuator are run to a breakout and power supply box located to the right of the bunker crate, under the table. The power supply is connected in parallel to each actuator line, with all the lines then running through the CAMAC module, a Kinetic Systems 3076 16-channel relay in slot 9 of the bunker crate, which has 16 normally-open switches, any or all of which can be closed on command. Note that if power to the relay module is cut (e.g. by turning off the crate) all switches will open and all screens will extract.
When the Neptune operator chooses a particular screen for insertion in the Master Control vi, the screen number is passed to the sub-vi "KS3076 1CH Control", which communicates directly with the CAMAC module using GPIB. (The control vi allows only one screen to be inserted at a time, always extracting the previous one.) The bunker crate has GPIB address = 17, and the KS 3076 has N = 9, both of which are set in the front panel of the sub-vi. To control the relay settings, the computer must write a 16-bit binary number to the module, with 1's and 0's representing closed and open relays. In this scheme, the command to close the third relay, for example, is given by writing the number 4 (binary 000000000000100) to the module. To convert the relay number n to the correct binary number, the program evaluates 2n-1. Finally, the "write data" command for the 3076 is F=16, A=0. At the lowest level, communication uses the "Write to CAMAC 16" vi.
To verify the relays, there are red LEDs on the module which indicate that power is being sent to it, that it has been addressed, and which (if any) relays are currently closed. The control vi also reads back the state of the relays and displays the Boolean array on its front panel. The "read data" command for the 3076 is F=0, A=0 and uses the "Read CAMAC 16" vi.
The "Show details" switch on the front panel of the KS3076 vi gives access to an information bundle which sets the appropriate relay channel for each screen or mirror. At one time, inserting a screen would also set the video channel to the corresponding camera. This is no longer true, but the video channels associated with each screen are reprinted here for completeness. For this historical reason, the "Virtual cathode" setting corresponds to actuator "zero"--i.e. no screens are inserted. In practice, this is the way to extract all screens.
| Virtual cathode | 0 | 1 |
| Cathode mirror | 1 | 2 |
| Screen 3 (Before linac) | 2 | 3 |
| Screen 4 (After linac) | 3 | 4 |
| Screen 5 (Before chicane) | 4 | 5 |
| Screen 6 (Beam dump) |
6
|
6 |
| Emittance slits** | 5 | 7 |
| CTR foil | 10 | - |
| Screen 8 (After slits) | 9 | 8 |
| Screen 9 (After folding box) | 8 | 9 |
| Screen 11 (Before plasma) | 7 | 11 |
| Undulator** | 9 | 12 |
| HeNe laser** | 12 | 1 |
**These are currently obsolete and not connected. Channel 3 of the KS3076 is defective and unused.
About phosphor and YAG screens
Intensity vs. charge; saturation; resolution; viewing at 45 deg.
For details of cameras and how to use them for screen images, see the Video document.