Measuring the Electron Beam Energy in a Magnetic Bunch ... - DESY

not possible to disentangle which energy change was caused by incoming arrival-time
jitter and which energy change was caused by accelerating gradient and phase jitter
unless the arrival-time jitter generated upstream of the accelerator section has been
measured.
There are two different strategies to deal with this problem. One could use an
accelerator section upstream of each chicane in order to stabilize the arrival-time after
each chicane, regardless of how much the beam energy jitter is increased. One would
then need to stabilize the beam energy using an accelerator section located after the
chicanes. Alternatively, one could use a single accelerator section gradient setpoint to
simultaneously stabilize the beam energy and arrival-time after each chicane. The endresult
of both schemes would, in principle, be the same.
For the sake of machine stability, the author believes that the latter option is
better: a feedback on the first accelerator section should not respond to changes in the
injector jitter and a feedback on the second accelerator section should not respond to
changes in the first accelerator section. To simultaneously stabilize beam energy and
arrival-time after a bunch compressor, one could execute a combination of the following:
• use measurements of the arrival-time jitter upstream of a bunch compressor to
keep the energy/arrival-time feedback from responding to the energy/arrival-time
jitter that it creates.
• stabilize the arrival-time jitter upstream of the bunch compressor before
correcting the energy/arrival-time jitter downstream.
3.3 Beam-Based Feedback Strategy
Two schematics of the synchronization sensitive components in the machine are
shown in Fig. 3.3.1 [27]. An optimal feedback setup is depicted in Fig. 3.3.1(a.) and a
more quickly realizable architecture is depicted in Fig. 3.3.1(b.). The injector laser
(Laser), the injector RF (Gun), the super-conducting accelerator sections (ACC1-7) and
the third-harmonic module (third-) are depicted in block diagram format with arrows
connecting various optical cross-correlators (OCC), chicane beam position monitors
(EBPM), bunch length monitors (EO-1D, THz-1D), and beam arrival-time monitors
(BAM) to the digital processing boards (μTCA or SIMCON-DSP) with which they
would be connected in a beam-based feedback system that controls, on a bunch-to-bunch
basis, the amplitude and phase (A, φ) of a normal-conducting RF cavity (3GHz NRF
cavity), the 1.3 GHz super conducting acceleration cavities (1.3GHz SRF ACC1(2&3))
and the super conducting third- harmonic linearization cavity (3.9GHz SRF).
The reason that the system shown in Fig 3.3.1(b.) will be built before that shown
in Fig 3.3.1(a.) is that the μTCA crate system, shown in blue in the figures below, along
with the corresponding ADC and FPGA boards will not be available in 2010. VME is the
crate system that has been used at FLASH since its inception but will be phased out as
μTCA crates become viable. In the currently available VME crate infrastructure, the
beam arrival-time is calculated on an in-house built Analog Carrier Board (labeled ACB
in the figure) that contains ADCs, delay-chips and FPGAs. Beam arrival-time
information from this board is delivered to the cavity controller via an optical Gigalink.
The cavity controllers reside on VME based SIMCON-DSP boards that each have ADCs,
DSPs, DACs, and an FPGA.
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