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NI Student Design Project: Development and Control of an Electron Beam Polarimeter

Contact Information:

Project of University of Virginia Department of Physics in collaboration with Hall C at Jefferson Lab

Supervisors: Prof. Kent Paschke (UVA), Dave Gaskell (Jefferson Lab)

Team members: Donald Jones(UVA), Brandon Seth Cavness(ASU)

Primary email: jonesdc@jlab.org

Primary telephone: 757-269-6498

Project Information:

Title: Laser Control and Monitoring for an Electron Beam Polarimeter at Jefferson Lab

Hardware: 10 Watt Coherent Verdi Laser, 4 x New Focus 8310 Closed Loop Picomotor™ Actuators, 2 x New Focus 8410 Closed-Loop Picomotor Rotary Stages, 6 x NewFocus 8751-CL Closed loop controllers, 4 x New Focus 8302 Open loop Picomotor Actuators, 3 x New Focus open loop three axis controllers, New Focus 8401 Open loop Picomotor Rotary Stage, New Focus 8816-6 Motorized Stability™ Mount, electro-optical modulator (EOM), MinicircuitsRF amplifier, DigiLock 110 cavity locking hardware module, 4 x quadrant photodiodes,  3 x photodiodes (2 Thorlabs PDA-10, 1 Thorlabs PDA-36), Measurement Computing PCI-DAS6013 16 channel ADC, NI USB-6501 24 channel TTL generator, New Focus 8892 Flipper Mirror Mount.

Software: LabVIEW 9.0, EPICS for Windows, New Focus LabVIEW libraries for motion control, DigiLock 110 Controlyzersoftware for locking and monitoring the optical cavity, Digilock LabVIEW library for communicating with Digilock program, MCC LabVIEW library for communicating with the PCI-DAS6013 ADC, NI TracerDAQ Libraries for use with the TTL generator.

Project Challenge:

Develop the hardware and software for remote control and monitoring of a high power laser optical cavity for an electron beam Compton polarimeter at Jefferson Lab in Newport News, VA.

Electron beam polarization is a statistic that tells how many electrons in a beam are spinning in the same direction. Having this beam parameter accurately measured can, for instance, allow an experiment to access to information about the weak force, the basic physical force that drives nuclear decay in fission processes. A Compton polarimeter measures polarization by colliding an intense beam of photons (laser light) with the electron beam and collecting the scattered electrons and photons. The challenge of this project was to develop a user-friendly set of remote controls and monitors for a high power "laser target" to be used in the new Compton polarimeter in Hall C at Jefferson Lab. The hardware and software design and installation were to be completed in less than one year. Figure 1 shows a schematic of the Compton polarimeter. This laser and optical components which were central to this project are located on the component labeled "Laser Table".

Comptonschematic.png

Figure 1. Schematic of Compton polarimeter. Electron beam shown in red is bent through a chicane by four dipole magnets.

Just above the laser table the electron beam collides with a high power laser beam amplified by a Fabry-Perot optical cavity.

Electrons and photons involved in collisions are collected in detectors. A tiny fraction of electrons in the beam are scattered.

     In January of 2010 a team was working on research for building an optical cavity for the Compton polarimeter; however, there was no design for the layout of the necessary hardware components on the laser table. In fact, most of the hardware was not yet selected or purchased. With a proposed finish installation date of September 2010, there was a great deal of pressure to deliver results quickly and efficiently. From the outset the design goals were clear. The system had to have the following capabilities:

1. Control motorized optics and read back their positions.

2. Remotely monitor and control properties of a high power laser.

3. Communicate with EPICS, the lab's instrumental control framework.

4. Lock and unlock a Fabry-Perot optical cavity used to amplify the laser power

by >100x.

5. Automate certain expert tasks so that the average user could easily perform

them.

6. Read, store and process data (electrical signals).

Add to these the fact that the system was to be attractive, user friendly and operational within a few months. LabVIEW was the obvious choice for a professional and attractive user interface with industrial support from most optical and electronics suppliers. Within a few months, the overall design and location of the hardware components on the table was determined and a schematic of the laser table looked similar to the present layout shown in Figure 2.

tablelayoutwnmbsmall.png

Figure 2. Layout of hardware components on looking down on the laser table of the Compton polarimeter.

The components are listed by number below.  The electron beam pipe running through the center of the

picture is where the laser and electron beams collide.


     1, 19 Periscope (19 closed loop motorized)

     2, 14 Convergent lens F=1000 mm

     3 Electro-optical modulator(EOM)

     4 Diverging lens F=-1000 mm

     5, 7, 11, 12, 17, 21 Dielectric turning mirror

     6, 18, 24, 30 Quadrant Photodiode (QPD)

     8, 20 HWP (8 open loop motorized)

     9, 15 Polarizing cube

     10, 29 Power meter head (high power)

     13 Power meter (low power)

     16 Quarter-wave plate (QWP) (closed loop motorized)

     11a, 23, 31, 33 Integrating sphere and photodiode combination

     25 Motorized open loop steering mirror

     26 Holographic beam sampler (HBS)

     27 CCD camera

     28 Polarizing beam splitter

     32 Glan-laser polarizer on rotating stage.

Central to this project is the concept of a Fabry-Perot optical cavity for light amplification. The concept is simple: set up two mirrors facing each other (this is called an optical cavity) and build up light intensity by reflecting the light back and forth hundreds of times while continuously feeding more light in than is allowed to escape. A little light leaks out the second cavity mirror each consecutive round trip creating a transmitted beam and when this leakage beam is equal (minus loss due to absorption by the mirrors) to the light entering the cavity, an equilibrium is reached. At equilibrium there is a great deal more light inside the cavity than in either the incoming or transmitted beams. Although the concept is simple, implementation is quite complicated. In order to successfully build up light between the mirrors, the peaks and troughs of all the light in the cavity must be aligned. This is the concept of constructive interference. Figure 3 shows a simple diagram of the concept. In order to build up light between the mirrors, the distance between the mirrors must be an integer number of half wavelengths of the light i.e 2L=n*wavelength (shown as Greek lamda in the diagram). If this condition called the "resonance condition" is met, the cavity is said to be "locked". Of course, with light green light from the laser we were using being about 0.5 microns, this is a very stringent condition and small vibrations from sound waves are enough to interrupt a locked state. However, with proper feedback on laser wavelength this condition can be maintained indefinitely. The cavity in this project is locked by the Pound-Drever-Hall technique feeding back on laser wavelength. The feedback is done using a fast digital circuit module called DigiLock 110. This module came with a LabVIEW interface provided by the vendor Toptica Photonics.

cavity.PNG

Figure 3. Diagram of a resonant optical cavity. If twice the distance between the mirrors is equal to

half the wavelength of the light, the light waves will build up between the mirrors. Some light will

leak through the second mirror at each consecutive round trip producing the transmitted beam.

For a cavity with identical mirrors on both ends, the cavity gain is approximately the ratio of the light inside the cavity to that transmitted. The optical cavity in this project has a gain of about 115 storing close to 1000 Watts of power between the mirrors. Figure 4 shows an optical cavity with about 200 Watts of power during testing at UVA.

lockedcavitysmall.PNG

Figure 4. Locked optical cavity shown on an optics bench at UVA during testing.

When the electron beam enters the underground experimental halls at Jefferson Lab, deadly doses of radiation are present. Therefore, all equipment must be remotely controlled and monitored. The controls computer which operates the LabVIEW-based controls program developed for this project is located outside the experimental hall and communicates with components primarily via RS-232. Due to the extensive array of monitors and control variables necessary for this project, it became obvious that the LabVIEW tabs feature would be useful for subdividing various utilities. The controls program evolved to included five tabs in all. These are shown below. 

Optical Component Motion Control:

The cavity mirrors are about 7 mm in diameter and the laser is about 0.2 mm in diameter when it enters the optical cavity. The laser must be aligned to go through the centers of these mirrors. In order to properly lock the cavity the laser must be aligned at the microradian level. Laser alignment and polarization are accomplished by precise motorized optomechanical components via this GUI. Precision laser position readouts are displayed as green dots on crosshair plots.

opticsloop.PNG

ADC Readout:


A 16 channel anaolog to digital converter is read out on this GUI screen. Raw electrical signals from quadrant photodiodes (QPD's) are processed and converted to positions. Laser power both before and after the cavity is measured and displayed.  A calculated power inside the Fabry-Perot cavity based on the transmitted signal times the cavity gain is displayed as cavity power at the top of the screen.

qpdloop.PNG

Optical Cavity Lock Monitor and Control:

This tab of the GUI is used to communicate with the DigiLock 110 module which controls the cavity lock. The GUI shown below reads and sets parameters from DigiLock and automatically cycles the laser on and off so that laser off backgrounds can be measured and subtracted from polarimetry data. Almost all the values displayed on this screen (and throughout the program) are communicated to EPICS, the framework used by Jefferson Lab to monitor, control and archive accelerator hardware parameters.

dgkloop.PNG

Polarization Measurement:

Polarization of the laser must first be determined before it can be used in measuring electron polarization. Laser polarization is measured by rotating a polarizing crystal around the beam axis. As the polarizer rotates, if the light is circularly polarized (we want the light to be 100% circularly polarized) the measured intensity of light through the polarizing crystal should remain constant. If there is any linearly polarized light in the beam, it will produce a sine-like behavior in the transmitted light (see red signal in GUI display below). The linear polarization is calculated to first order as the difference divided by the sum of the maximum and minimum light intensities encountered during the rotation. The red curve shown below is a measure of light intensity as the polarizer is rotated. Degree of circular polarization (DOCP) and degree of linear polarization (DOLP) are displayed as well. The details of this automated process are discussed in detail in the attached technical note.

polloop.PNG

Transfer Function Utility:

This utility is used to precisely measure characteristics of the optical cavity. Although the technical discussion of this utility is not developed here, this section of the GUI is used to measure how the light changes as it travels from inside the cavity to where it is measured in the transmitted analysis region (numbers 31-33 in Figure 2). In minutes, this utility automatically takes a series of measurements that previously took hours to complete manually.  

tfloop.PNG

Conclusion

Thanks to an international team of collaborators, the new Compton polarimeter for Hall C has been installed and commissioned and is now taking data. The laser system discussed in this project, an integral part of the polarimeter, is controlled and monitored by a student-developed, LabVIEW-based program and has proven to be robust, attractive, and user-friendly. This program successfully communicates with Jefferson Lab's EPICS control framework. Furthermore, as improvements and modifcations are required, the program can easily be modifed to meet the new requirements. Many tasks that would have been tedious and "expert only" have been simplifed to the push of a button. With only a few minutes of instruction, the average user can understand the basic functions and controls of the Compton polarimeter GUI.

Team Photo


team.JPG

Figure 5. Laser software project team: (left) Brandon Seth Cavness and (right) Don Jones.

Comments
LPS
NI Employee (retired)
on

Hello there,

 

Thank you so much for your project submission into the NI LabVIEW Student Design Competition. It's great to see your enthusiasm for NI LabVIEW! Make sure you share your project URL with your peers and faculty so you can collect votes for your project and win. Collecting the most "likes" gives you the opportunity to win cash prizes for your project submission. If you or your friends have any questions about how to go about "voting" for your project, tell them to read this brief document (https://forums.ni.com/t5/Student-Projects/How-to-Vote-for-LabVIEW-Student-Design-Projects-doc/ta-p/3...). You have until July 15, 2011 to collect votes!

 

I'm curious to know, what's your favorite part about using LabVIEW and how did you hear about the competition? Great work!!

 

Good Luck, Liz in Austin, TX.

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