Procedure FEL2 on 3rd harmonic

To supply light of 3–5 µm to the users, FEL2 can be operated on the 3rd harmonic wavelength. In order to get lasing only on the 3rd harmonic, you have to suppress the fundamental wavelength. There are currently two possibilities: either use mirrors that have a coating that reflect in the preferred wavelength range, or insert a calcium fluoride slab (under Brewster's angle) that acts as a high-pass filter (i.e. rejects long wavelengths) up to around 6 µm.

Preparation

1. Use 44 MeV settings.
2. Put undulator at 13.5 µm (fundamental wavelength).
3. Align properly with using normal cavity mirrors (1.4 mm hole upstream).
4. Reach 60+ mJ at 13.5 µm and 9 µs pulse.

Choose one of the following methods

Switch to 3rd harmonic with cavity mirrors

1. Turn off cavity length tracking
2. Turn off spectrum analyzer tracking
3. Change upstream mirror to 0.9 mm hole, align properly. Do not let cavity lase or you might damage mirrors (FYI: someone accidentally did and reported 35 mJ).
4. Change undulator offset from 0.6 mm to ~0.4 mm (0.3 worked best on 20/06/2018)
5. Change downstream mirror to 3.9-4.9 µ mirror, align. Do not touch the upstream mirror.
6. Scan cavity length down to find the cavity zero position. (-191 on 16/02/2018).
7. Set cavity detuning to 0.6 λ.
8. Slightly tune cavity mirrors (do not "send them to the end of the world")
9. Optimize machine as before, quite sensitive to quadrupole focussing (Q40, Q50, Q60, 2Q1060, 2Q2050, 2Q3040).

Switch to 3rd harmonic with calcium fluoride slab

The GUI used to control the position of the CaF₂ slab.

1. Turn off cavity length tracking

Method 1

1. Change upstream mirror to 0.9 mm hole, align properly
2. Change undulator offset from 0.6 mm to ~0.35 (0.25?) mm
3. Insert the slab using the "3e Harmonische Filter" window in the optical cavity program (under Tools menu, or press CTRL-SHIFT-H)
4. Scan cavity length down to find the cavity zero position. (-1200 on 05/04/2018)
5. Set cavity zero to (10–12λ multiplied by the fundamental wavelength)
6. Set cavity detuning around 10–12 λ
7. Slightly tune cavity mirrors (do not "send them to the end of the world")
8. Optimize machine as before.

Example: cavity zero found at -1200um at 4um a.k.a 12um undulator
12detuning * 12um = 144um
set Cavity zero to -1200+144= -1056 um and put in 12detuning.

Method 2

The FELIX optical cavity GUI with position of the detuning controls indicated. Use the GUI DTCL-button to open the table editor.

Detuning table editor. Add good lasing positions with the Copy To Array-button. Then select e.g. CL_table_linear in CL_calcMode to enable linear interpolation between the points in the table. Then click WriteToOC to apply.

1. Move undulator to produce 5.5 Âµm on the fundamental
2. Optimize
3. Change upstream mirror to 0.9 mm hole
4. Scan cavity length down to find the cavity zero position. (~-45 on 07/12/2018)
5. Insert the slab using the "3e Harmonische Filter" window in the optical cavity program (under Tools menu, or press CTRL-SHIFT-H)
6. Realign the 0.9 mm mirror to correct for any translations by the slab
7. Scan cavity length down to find a lasing position. (a bit below ~-1245 on 07/12/2018)
8. You are now lasing on the fundamental with the slab inserted
9. Now move the undulator to 5.5 Âµm with 0.35 (0.25?) mm offset (undulator program should have switched to 3rd harmonic mode automatically)
10. Now it should lase on the 3rd harmonic
11. Optimize as before
12. Find good lasing cavity lengths in the range that you are interested in and save them with the new detuning program
13. Remember that you will need to put the spectrometer to 2nd order when you go below 4 Âµm, and 3rd order below 3 Âµm

Instruct the user

1. The undulator has to be set to 3x the desired wavelength (because you want the 3rd harmonic of that fundamental wavelength).
2. The spectrum analyzer has to be set to 2x the desired wavelength (because we use the 2nd order diffraction off of the grating).

Free-Electron Laser for Infrared eXperiments