DCM - User Guide
Table of Contents
- 1. Hardware
- 2. Setup Procedure
- 3. Power Off Procedure
- 4. Bliss
This report is also available as a pdf.
id21 |
id24 |
|
|---|---|---|
| Ring | 311 | 111 |
| Hall | 111 | 311 |
1. Hardware
1.1. Interferometers
Each interferometer “box” (either Attocube or QuDIS) has 3 measurement channels. As 15 distances have to be measured, 5 interferometer “boxes” are used.
| Description | id21 |
id24 |
|---|---|---|
| Xtal 1 hall | attodcm4 |
attoid24dcm1 |
| Xtal 1 ring | attodcm1 |
attoid24dcm2 |
| Xtal 2 hall | QuDIS |
attoid24dcm3 |
| Xtal 2 ring | attodcm3 |
attonass2 |
| Metrology | attodcm5 |
attodcm2 |
1.2. PEPU
2. Setup Procedure
2.1. Turning on the Hardware
- Interferometers
- PEPU
- IcePAP
- Speedgoat
Verify that everything is working.
2.2. Turning on the Speedgoat Server
Connect to the Speedgoat server computer (see Section 1.7) and run the BLISS SPEEDGOAT SERVER program on the desktop.
A terminal window should be displayed, with the last line being:
Serving XPC speedgoat on tcp://0.0.0.0:8200
This means the server is correctly launched.
2.3. Running the Speedgoat program
2.4. First interferometer reset to be able to use mode B and C
After homing all fast jacks, perform an interferometer reset such that \(r_x = 0\), \(r_y = 0\) and \(d_z = \frac{d_{\text{off}}}{2 \cos \theta}\) (i.e. the distance between the crystals is following the theoretical value).
Then, make a LUT over the full stroke.
And verify that the feedback regulator is working over the full stroke.
2.5. Metrology Frame Deformations - Calibration
Hardware used:
- Position sensor far way from the DCM. An angular sensor may be used instead. The sensor should have high bandwidth
Using a position sensor far away from the DCM (ideally a quadrant photodiode with high measurement bandwidth).
2.6. Ensure crystal relative pose
Interferometers are only measuring relative displacement. Therefore, it is important to correctly initialize them.
They should be initialize in such a way that:
- \(r_x = 0\) and \(r_y = 0\) when the two crystallographic planes are parallel
- measured \(d_z\) is indeed equal to the distance between the crystallographic planes.
The main issue is therefore to determine when the planes are parallel and to know the distance between the crystals with an external metrology.
Note that the interferometers should not be reset the same way for the two pairs of crystals as their crystallographic planes are not parallel.
All the following should be performed in mode C for better results.
2.6.1. Rocking curve to find \(y\) crystal parallelism
Perform a rocking curve to find the maximum output beam intensity. At the maximum intensity, the two crystals are parallel and the interferometers can be reset in such a way that \(r_y = 0\).
2.6.2. Bragg scan to find \(x\) crystal parallelism
Measure the rotation of the output beam along the \(z\) axis. This can be performed by using a position sensor positioned away from the DCM or using an angular metrology (lens + position sensor at the focal plane).
Perform a scan in mode C (i.e. closed loop, \(r_x \approx 0\) during the scan), and measure simultaneously the \(R_z\) motion of the output beam.
The \(r_x\) offset can be estimated from the data. This offset is then included in the interferometer data as an offset.
2.6.3. Bragg scan to find the distance between the crystals
Consider:
- \(\epsilon_{d_z}\) an error in the distance estimation between the crystals
- \(\theta\) the Bragg angle
- \(\epsilon_z(\theta)\) the vertical motion of the output
It can be shown that:
\begin{equation} \boxed{\epsilon_z(\theta) = \epsilon_{d_z} \cdot 2 \cos \theta} \end{equation}e_dz = 0.1e-3; e_dz = 1; thetas = 0:1:90; e_z = e_dz*2*cos(thetas*pi/180);
figure; plot(thetas, e_z) xlabel('Bragg Angle [deg]'); ylabel('Beam Z-offset [$\mu$m]');
The cosine function can be fitted from the data and the distance offset can be estimated.
The accuracy of the results depends on:
- how well the metrology deformations are calibrated
- How close the sensor is from the DCM and how well is the y parallelism between the crystals.
- How sensitive and accurate is the sensor
If a quadrant photodiode is used, a feedback loop may be performed between the measured \(z\) motion by the photodiode and the vertical \(z\) motion of the piezoelectric actuators. This means that:
- \(r_x\) and \(r_y\) are regulated from the interferometers
- \(d_z\) is regulated from the photodiodes
Then, by plotting the measured \(z\) motion of the crystals by the interferometers as a function of the Bragg angle, it should be possible to estimate the offset between the crystals.