FEL shifts 7th/8th August 2012
David Dunning, 14th August 2012
There have been two previous 2-day blocks of FEL shifts in the current running period which were
used to re-establish and optimise a standard lasing setup, plus investigating instability etc. This was
the first block with a good setup already established so we were able to significantly progress the
programme on two main fronts: FEL operation at longer wavelengths, and gain measurements.
1. FEL Operation at Longer Wavelengths
1.1. Detectors
The response of the MCT detector that we usually use to observe the spontaneous signal is expected
to drop off at longer wavelengths than our usual 5-9 μm operating range. Mark Surman has
previously sourced and installed a bolometer which is expected to be more responsive than the MCT
at 20 μm (but less responsive at 9 μm). This had been removed, so was re-installed in the first shift.
1.2. Electron beam setup at 17.5 MeV
The standard FEL setup was restored easily using the good setup from the recent FELIS shifts (13:58
1/8/12, #3063, 1120801_140858). The bolometer responded strongly to the lasing signal at 9 μm,
but there was only a hint of spontaneous signal.
We used James Jones’ magnet scaling program to first scale the injector from 6.5 MeV to 4.5 MeV
(quite straightforward), then energy after the linac down to 17.5 MeV (some difficulties with the
program in #3078 but worked well in #3083). We didn’t tune the linac but no problems.
A reasonable setup was established with 40 pC round to the PID, with the beam nicely steered and
focussed through the undulator (and ok everywhere else as well). There was a very small signal on
both the bolometer and the MCT. However, some signal remained even with the FEL line vacuum
valve closed, indicating the detectors were responding to the electron beam in some way, rather
than to spontaneous undulator radiation.
1.3. Spontaneous signal
On the second attempt (#3083), we were convinced that there was a response to the spontaneous
signal on the MCT at 20 μm. It was weaker than at 9 μm (~10-20 mV on scope rather than ~50) but
didn’t need too much averaging (~16) so it was useable for trying to establish lasing. The bolometer
signal was less convincing.
1.4. Attempting to establish lasing
We had made two lengthy attempts to enhance the spontaneous signal, looking at the bolometer
(first attempt) and both detectors (second attempt). There was no obvious enhancement despite
adjusting almost everything. One problem was that the detectors were roughly as sensitive to the
electron beam as they were to the spontaneous signal so it was quite misleading when trying to
optimise.
1.5. Incrementally reducing electron beam energy
We tried an alternative approach to get lasing at longer wavelengths by incrementally stepping
down the electron beam energy, and restoring lasing at each step. The energy was reduced from 26
to 23 (injector remained at 6.5MeV), and lasing was restored quite easily. This corresponds to a
wavelength of 11 μm (at minimum undulator gap – opening the gap gives shorter wavelength).
The energy was reduced to 20 MeV (injector remained at 6.5MeV) but only a limited amount of time
was spent at this energy. Initial indications were that energy recovery was not as good (might need
to start scaling injector here). Spontaneous signal was observed (~15 μm) but no time to try lasing.
Figure 1: FEL wavelength as a function of electron beam energy (at minimum undulator gap). The longest wavelength
lasing established was at 23 MeV, corresponding to ~11 μm.
1.6.Future work
Based on the experience so far it seems that incrementally reducing the electron beam energy is a
more promising approach for getting lasing out to longer wavelengths than going straight to
17.5 MeV. Making the step from 26 MeV to 23 MeV was relatively straightforward since the injector
was not scaled. It seems that 20 MeV could be possible without scaling the injector (this would reach
~15 μm which would be a significant step). Maybe smaller increments would help in retaining lasing.
Further work on getting the bolometer to respond to the spontaneous signal would also be useful.
2. Gain measurements
2.1. Detector
Gain measurements involve looking at the FEL output at the start of the train and measuring the
fractional increase from pulse to pulse. We have previously used MCT detectors for this. The preamp must be bypassed as this limits the response time (and sets an artificial limit to the gain of
~6%). The MCT response is not quite fast enough to resolve individual pulses (there is a slightly
ripple corresponding to the pulse structure) but it should be fast enough to measure the gain (up to
~20% has been recorded (** but what is the limit?? **). A new detector (PEM-10.6 from Vigo
System, works using photoelectromagnetic effect) with sufficiently fast response to resolve
individual pulses has been sourced, so we want to try using this for gain measurements.
The fast-response detector has previously been installed in the accelerator hall (5th July) (opposite
local MCT, need to retract ‘flip mirror’ before MCT to send light to fast-response detector). The fastresponse detector is mounted on a lab jack and moved out of the direct path of the beam to avoid
damaging it. The detector is moved carefully into the lasing beam until a signal is detected (normally
a few mm from the direct beam path - as determined with HeNe alignment laser).
2.2.Software
When using the MCT the pulse structure was not resolved, so a simple program taking two cursor
readings from the scope could be used. This cannot be used with the fast detector output. Andy
Wolski has written a program to grab the data from the scope and make a fit to the envelope to
calculate the gain. Work to commission the program was carried out in shift #3084. Further work to
commission the fast detector + software as a representative online measure of the gain is required.
Figure 2: Example screenshot from the new gain meter program. The fast detector resolves individual pulses so the
software fits the envelope to calculate the gain.
2.3.Measurements
The FEL gain (and power) were measured as a function of cavity length. The expected behaviour was
observed, i.e. asymmetric power curve and symmetric gain curve. The peak gain of ~9% is lower than
previous highest (~20% using MCT), probably due to a non-optimum setup.
Figure 3: Detuning curves (FEL power and gain as a function of cavity length detuning), using the fast-response detector
+ new software.
3. Other progress
3.1. Cavity mirror angles
Adjusting the cavity mirror angles is part of optimising the FEL power, but had not been done during
the previous FELIS run in case it moved the focussed beam position at the SNOM.
In shift #3081 the downstream cavity mirror angle was adjusted to optimise the FEL output power
(the upstream cavity mirror cannot currently be moved as the motor belt drives are disconnected
following some problems). This resulted in a significant increase in the power (10 mW -> 17.5 mW at
14 mm gap). Using this cavity alignment, then next shift was able to get up to 24 mW with increased
bunch charge (60 to 80 pC), approximately double the typical power during the last FELIS run.
It was not established how much the beam moved in the diagnostics room, but the power on the
detectors in the diagnostics room increased proportionally to the increase measured on the power
meter in the accelerator hall, suggesting the steering was not changed significantly.