Beam jumping waveform and gating signals are generated by a set of CAMAC modules designed and manufactured by NEC. Below is a description of each of the modules which make up the system. Please refer to the figures titled "AMS data collection System block diagram" and "AMS carbon data acquistion waveforms".
The "Sequence Controller" CAMAC module generates the timing information. A single jumping cycle may be as long as 4 seconds but is more typically about 100ms. A 4MHz clock and 24 bit programming registers provide a resolution of 0.25us/step.
The Sequence Controller may be programmed to free run, meaning that the next jumping cycle starts immediately after the previous one ends, or may be line synced, meaning that the next jumping cycle is started at the next zero crossing of the AC power line. The choice of free run or line sync and the polarity of the zero crossing may be set by the user.
Jumping cycles are counted by the computer. At the end of each jumping cycle a LAM (Look At Me) signal is generated by the Sequence Controller, which then pauses until the LAM is acknowledged by the computer. LAM processing may be disabled by the user for diagnostic purposes.
A ribbon cable connects the Sequence Controller to the other CAMAC modules making up the jumping system. This cable carries the timing data to the other modules.
One jumping cycle is made up of several "states". The Sequence Controller outputs a state number which is used by the rest of modules.
The Sequence Controller contains 16 timing comparator registers. There is one comparator register for each state.
At the beginning of each jumping cycle a counter register is initialized to zero. This register is incremented by the 4MHz clock.
The comparator registers are programmed to increment the state number at the appropriate delta T for each state. The value of the comparator register for the current state is compared to the counter register. When the value of counter register is greater than the value of the comparator register the state number is incremented.
The maximum number of states used in a jumping cycle is programmable. The jumping cycle ends when the value of comparator register for the maximum state number to use is exceeded.
Typical carbon AMS is done using 7 states in the following way. See "AMS system components" for more information.
This is the rest state. When the sequencer is not performing a jumping cycle. i.e. it is waiting for the next AC line trigger or for the LAM to be acknowledged it is in this state.
This state may be used to provide a predelay, for example, if line sync is in use one can delay the start of the actual jumping cycle for some amount of time after the trigger.
Power supply voltages slew to the correct value for injection of the first abundant isotope.
High energy side measurement of the first abundant isotope.
Power supply voltages slew to the correct value for injection of the second abundant isotope.
High energy side measurement of the second abundant isotope.
Power supply voltages slew to the correct value for injection of the rare isotope (14C). Usually for carbon AMS this means 0 volts on the magnet chamber, other species such as aluminium may require different settings.
Rare Isotope data collection and low energy side measurement of both abundant isotopes.
Not used.
A Sequenced D/A converter CAMAC module provides the waveforms needed to drive the jumping power supplies.
Usually three Sequenced D/A Converters are used in a system. One converter drives the injection magnet chamber bias power supply. The other two are used to drive a set of power supplies connected to steerers installed in the beamline after the injector bending magnet.
The Sequenced D/A Converter contains four analog value registers. Another set of registers associates a value register with the state number provided by the Sequence Controller.
If the value registers are assigned in this way:
Abundant isotope #1
Abundant isotope #2
Rare isotope. Usually this value is zero for the magnet chamber power supply but may be something else for the steerers.
Not used.
Then the state numbers from the Sequence Controller would be assigned as follows:
VCreg2
VCreg0
VCreg0
VCreg1
VCreg1
VCreg2
VCreg2
Not used.
The Gate Generator CAMAC module provides various signals used to control the AMS data collection system. The module provides 8 separate outputs. Each output may be individually programmed to be on or off during any state. The outputs are differential to allow them to drive long cables. Usually the outputs are connected to the "Quad Receiver" described below at the destination end of the signal.
Typically two channels are used to clock the low energy transient recorders, two channels clock the high energy transient recorders, and one channel is the rare isotope gate.
See the sections which follow for more information.
The Quad Receiver CAMAC module is a simple module containing four channels of differential receivers and two TTL level outputs for each receiver. The differential receivers are connected to the gate generator outputs. The outputs may be individually programmed by DIP switches inside of the CAMAC module to provide a high true or low true output.