[5.1.3] Requirements Document


Diagnostic Requirements

The electron beam diagnostic requirements for the FEL driver accelerator are similar in many respects to those for other electron accelerators. There are, however, a few parameters that require additional emphasis, such as the short bunch lengths (<1 psec), low average beam loss (1 nA), dependence on the angle and position of beam entering the wiggler, and the 1/4 megawatt of electron beam power.

Beam Position Monitors

The beam position monitoring (BPM) system will consist of a detector mounted to the beamline and a set of hybrid time domain/frequency domain electronics. This arrangement will enable the operations crew to select and track an individual group of electrons through the accelerator. This type of a system is desired since there are one or two different electron beams in the various parts of the accelerator at any given time.

The power of the machine is controlled by varying the drive laser frequency, and the machine relies on "90% energy recovery of the megawatt of power in the beam. Through the linac there are two different electron beams: the beam on the way to the wiggler and the beam going through energy recovery. The two beams in the linac are separated by 125 nsec. This allows ample time to capture the position of an individual bunch of beam.

The detector will consist of four 1 cm buttons mounted to a vacuum flange. The bipolar signal from each of the electrodes will shock-excite a resonant circuit. An automatic gain control (AGC) amplifier will amplify this signal and pass it to both a synchronized track-and-hold amplifier and a log ratio amplifier. During low-power operation (2 MHz) this voltage will be captured using a track-and-hold amplifier synchronized to the beam bunch selected. When the accelerator is running at full power (40 MHz) the log ratio circuit will provide the position signal of the electron beam.

Bunch Length Monitor

The bunch length in the injector will be measured at injection into the accelerator. The nominal bunch length at this point is 1 psec. Thus, it is desired to have bunch length measured to within 0.1 psec (10%). Since the bunch length will be adjusted only during setup, the range of the measurement should be 0.5 to 5 psec. In the injector, the bunch length will be measured using coherent synchrotron radiation generated by the final bend of the injection staircase. The device uses coherent properties in the far IR to produce measurable power levels. An interferometer and detector (bolometer or Golay cell) will be used to measure the power spectrum of the synchrotron light. The bunch profile can be derived from this spectrum. Using synchrotron light allows the meter to monitor the bunch while running at full beam power. If it is necessary to measure bunch length in the straight section prior to the staircase, coherent transition radiation can be analyzed using the same type of detector. However, the foil that produces the transition radiation cannot remain in the beam path during full-power operation. Measurement of bunch length using coherent transition or synchrotron radiation has been successfully performed [Kung et al.., Phys. Rev. Lett. 73 (7) Aug. 15, 1994, p. 967, and Shibata et al ., Phys. Rev. E 49 (1) Jan. 1994, p. 785]. At CEBAF, tests of a bunch length monitoring system using coherent synchrotron radiation are currently under way. Bunch length in the FEL driver accelerator can be measured using a similar technique.

Beam Current

The beam current will be measured in two different ways: one method to provide information about fast variations ("10 kHz) in the charge per bunch, and a slower ("1 sec), more accurate method to show variation in average current. These systems would be located in the 10 MeV injection line and used to monitor the performance and stability of the photocathode and drive laser.

The fast system will consist of a modified BPM connected to an integrator. The sum signal provided will be proportionate to the charge passing per microsecond. This system will be optimized for relative changes in the charge per bunch. The slow system will use a current transformer with a 1 Hz bandwidth to provide the average current with an absolute accuracy of 10 µA.


The nominal value for normalized rms emittance at the injection point is 4p mm-mrad. The required value is 6p mm-mrad. Hence, it will be necessary to measure emittance to within 30%. at this level. Since space dominates the beam optics in this region, optical transition radiation (OTR) methods will be used [Fiorito and Rule, AIP Conf. Proc No. 319, Beam Instrumentation Workshop 1994, 21]. This type of device uses the OTR interference pattern generated by the 10 MeV electron beam passing through two parallel foils. The rms beam divergence can be determined from the interference rings and the rms beam size can be determined from the OTR image itself.

In the main accelerator, the emittance will be measured by varying a quadrupole magnet and observing the beam profile on a downstream OTR foil.

OTR Viewer Specifications


Table 3 summarizes FEL driver accelerator electron beam diagnostics.

Table 3

Electron Beam Diagnostics Summary

LocationMeasurement Monitor
500 keVBeam profile 1 wire HARP
Setup position Beam viewer
Beam position1 stripline BPM
10 MeVBeam position 7 BPMs
Bunch LengthSR /interferometer
EnergyBeam viewer @ dump
Energy spreadHarp @ dump
Beam currentCurrent transformer
Beam currentResonant cavity
LinacBeam position 59 BPMs
WigglersPosition & angle 8 BPMs

Personnel Safety System

Any safety system requires hardware, procedures, and knowledge to ensure the well-being of personnel. To meet this goal, designs for three systems have been developed to provide protection from the dangers of accelerator operations:

Electrical safety systems to provide protection from high voltage and/or high-current devices.

Radiation systems to protect personnel from exposure to particle beams.

Laser safety systems to prevent exposure to dangerous levels of IR or UV light.

The personnel safety system will be modeled after the existing CEBAF systems.

Inherent in each of these systems is the concept of redundancy. All hardware must be designed in such a way that no single failure will result in the loss of protection. To accomplish this, two separate circuits are used to detect specific conditions. For example, each door that is monitored uses two separate switches to detect the status of the door. Each of these switches in turn is connected to a separate control circuit. Thus if one switch were to fail, the other would still operate, thereby providing the necessary protection. An extension of the redundancy concept is used in controlling radiation safety system critical devices. A critical device is one that prevents beam from entering an area. Two critical devices will be controlled by a radiation safety system. Thus when a possibility exists for personnel to be in an area, two devices are inhibited to provide protection of "fail-safe" circuits. All circuits are designed in such a way that if a circuit fails, the failure would result in a safe condition. For example, if the cable that controls a device were cut, the device could not be enabled. In this way personnel are still safe. Perhaps the most important part of the Personnel Safety System is the searching and securing of an area. This is done to ensure that the area is not occupied before beam or power supplies are enabled. The securing of an area requires that a minimum of two qualified persons thoroughly search an area in a predefined sequence.

The search sequence will be programmed into a Programmable Logic Controller for the FEL Personnel Safety System. The order in which the interlocks are reset has been designed to ensure the area is searched in a logical order, thereby ensuring personnel are not missed by the search team.

The FEL Personnel Safety System is segmented into seven areas: the accelerator hall and the six optical labs. Once the accelerator hall is secured, each of the optical labs must have its two critical devices in place or also be secured. The normal procedure is to have all of the optical labs locked out when the accelerator is in open access, then to secure each lab and retract its critical devices as required once the accelerator has been locked up.

Once an area has been searched and secured, status displays on the outside of each controlled access door and each section gate indicate to individuals that the area is interlocked and that access is forbidden to unauthorized personnel. Immediately before the beam or laser light is brought into an area or power supplies are enabled, a prerecorded message consisting of a siren and verbal announcement will be played so that any personnel who have somehow been missed in the search-a highly unlikely search outcome, but nonetheless necessary to address-can push an emergency crash button and/or leave the area. In addition to the audible warning, strobe lights will flash, giving a visual indication the area is transitioning into a hazardous state while the audio message is played. Emergency crash buttons, audio warning speakers, and visual warning strobes will be located at approximately 50 m increments in the accelerator enclosure and in plain view in each of the six optical labs. Pushing one of these buttons will inhibit beam and power supplies and require that the actuated switch be locally reset before the area can be enabled.

All doors to an area are locked and the keys to open these doors are interlocked and guarded in the FEL Control Room key tree. Distribution of these keys is not taken lightly. Only authorized personnel are allowed access. The type of access determines the authorization level required for the individual.

Control Computers and Software

The function of the control system is to keep the operations crew informed of the current state of the accelerator hardware and to provide an efficient means of starting up the machine, making required minor corrections, and archiving and retrieving data. This collection of hardware and software will be patterned closely after the CEBAF control system.

The EPICS operating system provides a reliable connection from operators on work stations in the control room to the hardware devices in the accelerator. The FEL will take advantage of the software development that has been done at CEBAF and the other labs in the EPICS collaboration. This will save man-years of software effort.

The FEL control system will consist of 12 workstations which will be available for both development of the required additional software and for operation of the accelerator. There will be a server (HP 730 for example) which will interface the workstations to the hardware in the field. This server contains the bootable code for the remote intelligent local input/output controllers (IOCs). The IOCs will contain between 4 and 16 megabytes of memory depending on the complexity of the task.

Each IOC also contains four serial links which can be configured to control the drive laser and the bulk power supplies. This serial link is based on the single ended RS232 format which requires a simple adapter to convert it to the differential RS485 link. The subsystem needs are shown in Table 4.

Table 4

Subsystem Control Requirements

No. of IOCs
RF control
1 systems can be operated from as single IOC; the 1 1/4 + buncher is = 1 1/2
There are 40 BPMs, 28 beam viewers, 7 synchrotron light monitors, and a double slit
DC power
41 dipoles, 86 quads, 36 sextupoles, 35 horizontal trims, 35 vertical trims, plus 6 injector trims
One to interface to CAMAC
Vacuum system
37 ion pumps, 35 gate valves
Machine protection
Monitor only
Personnel safety
Monitor only