Abstract

1. Abstract We present initial results from a laser scattering diagnostic viewing scattered light from large particles present in the C-Mod vessel. The light source consists of pulsed Nd:YAG lasers principally used for Thomson scattering. In order to view the YAG laser line (1064 nm) collection fibers and detectors were installed alongside the existing Thomson scattering diagnostic. Results from this diagnostic are presented. Additionally, we present designs for a new Dust Scattering Diagnostic to view the dynamics of injected dust in the scrape off layer. Boron dust particles will be injected into the vessel via a gas puff during a shot. The particles will be illuminated by 532 nm laser light and the images will be captured with a fast-framing CCD camera. This diagnostic is scheduled to be operational in the next campaign.

2. Outline • Thomson Scattering Experiment • Experimental Setup • Results • Future plans • Dust Injection Experiment • Overview to the Experiment • Lab Setups • The future

3. Thomson Overview • The Thomson Scattering system on Alacator C-Mod uses 2 YAG lasers at 1064 nm wavelength. Light is scattered off of electrons, giving temperature and density information. Each laser is run at 30 Hz, and each pulse is about 8 ns. The scattered light gets focused through a lens on G-Port onto a plate which contains viewing fibers. These fibers all have low-pass optical filters on them to block out all signals at 1064 nm and above. The system used in 2006 has 11 channels viewing the core and 21 channels viewing the edge in the upper divertor region. The laser is focused at the center of the plasma, and when it reaches the lower divertor it has a diameter of ~2cm.

4. Thomson Viewer Setup • 3 optically filtered fibers view the laser line • The light exiting each fiber is focused onto an Avalanche Photo Diode (APD) • The fibers are placed on the top of the Thomson plate. Therefore, they view the lower divertor region. • There is space in the fiber holder for 17 fibers, however we currently only have enough detectors for three. Also, some of the fibers near the very top have views which terminate on the shelf without plasma interaction • The fibers are digitized using spare Thomson channels, this means that during certain runs we cannot digitize, as Thomson scattering requires those channels. The fiber holder was adapted from the design used for the upper divertor edge fibers The fibers rest flush to the surface of the holder. It is designed so that the lens focuses the light onto the fibers

5. Detector Set-up Fiber Holders Focusing Lenses The design for the box is adapted from the design for the Thomson alignment fibers made by K. Zhurovich. At any point in time we can monitor three separate chords by focusing three different fibers. We can also swap fibers to view different chords. Avalanche Photo Diodes

6. Sample Traces 2 sample traces. The left trace is a disrupted plasma with dust activityafter the disruption. The right is a generic full-length plasma with no dust activity. Also included are two raw signals from the Core Thomson system for comparison. Dust- viewing channel 1 Dust- viewing channel 2 Thomsonchannel 1 Thomson channel 2

7. Spike Height Analysis The height of a spike depends on the size of the particle and how much of the beam it intersects. The histogram to the right shows the size distribution of particles during the run of June 22, 2006. This was a disruption mitigation run which produced many particles, due to disruptions on nearly ever shot. The signal saturates at 2048.

8. Timing of Dust Particles Almost all of the dust events occurred after plasma termination. Most events occur close to the disruption, however some events are seen up to 1s after the disruption. The three events recorded before the disruption were low amplitude events and are therefore subject.

9. Future Plans for Thomson Scattering System In this past campaign, we were limited by available digitizer channels. Also we did not have any way of absolutely calibrating the system. This clearly limited the amount of data we were able to get. However, for the next campaign we hope to implement the following. • The Thomson upgrade will have extra digitizer channels, so we will be able to gather data over extended periods of time spanning many run days. • The detectors will be online during Thomson calibrations, allowing for proper setting of the detector voltages. This will allow us to make signal amplitude comparisons between detectors. • There will be separate voltage supplies for each detector, unlike the current setup which only has two separate voltage sources.

10. Intro to Dust Injection Experiment The Dust Injection Experiment is an experiment to monitor the motion of dust particles. The particles will be injected with a puff of gas in the SOL near the outer midplane. They will be guided through a capillary from a lower vertical port. The dust will be illuminated with a laser and images will be gathered with a fast-framing CCD camera. Dust particles will have diameters in the tens of microns. Dust Cloud Laser Illumination Capillary

11. Experimental Goals and Concerns • In this experiment we hope to successfully implement the following: • Reliable injection of boron dust particles into the plasma scrape-off layer • Digital imagery of particles with time resolution • Non-perturbative interaction with the plasma • Comparisons to output generated by numerical codes such as DUSTT • The following concerns need to be addressed • Can we put enough dust particles to make measurements with a small enough amount of gas so that we are non-perturbative? • Will we be able to see the dust particles over the plasma background? • Will the valves and gas puffing apparatus become leaky in the presence of the dust? Is there a way to minimize this effect?

12. Lab Experiment • Lab experiments are currently underway to address two of the three main concerns, namely: • We will determine the inventory and pressure of gas that we must use in order to push boron through the appropriate capillary length. • We will ensure that any system installed on C-Mod will be robust to boron particles. Furthermore, if valves start leaking, we will have a manner of cleaning them with as little effect on the plasma vacuum as possible. SaltShaker Gate valve Funnel Pneumatic T Swagelok T

13. Experimental Setup Boron “Saltshaker” Boron particles are deposited by the “saltshaker” through a gate valve and a funnel to a ¼” VCR Tee. After the particles build up the gate valve is closed and a puff of gas, nominally at 40 PSI moves the particles through a hand operated ball valve. The particles are deposited out of the far end of a long capillary tube. The current lab setup on the previous slide does not include the ball valve. Gas Supply Gate Valve Ball Valve To Vacuum Feedthrough and capillary Pneumatic valve Swagelok T

14. Boron “Saltshaker” • The saltshaker was designed by J. Irby to deposit boron flakes through a upper vertical port. • The solenoid is pulsed, pulling up a plug which allows particles to fall from the hopper into the piping below. • The solenoid is driven at 100V typically at speeds between 1-30 Hz. • Boron particles may range in size but are typically on the order of 10s of microns. • Roughly 1 mg of boron particles are discharged per pulse at 10 Hz. • An additional laser scattering apparatus may be attached below the boron shaker to ensure particles are being dropped. Picture courtesy of J. Irby APS Oct 28,2003

15. Lab Results • Here are some results of lab tests and for other tests still on the horizon. • 40 Psi was sufficient to push boron particles through ~3 meters of coiled capillary. This experiment was done at air. • The particles took significant time to make it through the capillary, though we expect this to drop under vacuum. • The vacuum setup is almost complete and we expect to be able to test the system under vacuum in the upcoming weeks. • For now we will only be able to test whether boron particles make it through on a given puff by viewing an adhesive surface attached to a window on the far side of the chamber Coiled capillary inside Feedthrough Hovac pump connection The vacuum chamber for lab tests

16. In-Machine Proposal • The gas puffing system will be the same that was used to fire the “burping” probe on F-Port that has been removed. • The capillary will extend upward from a feedthrough at A-Port bottom. • The laser will come into the vessel via fiber feedthrough at B-Port side. The laser will illuminate a region by the end of the disruption mitigation gas puffing tube. • The CCD camera will be the same camera that views the disruption mitigation tube. Disruption Mitigration Feedthrough Laser Feedthrough

17. Conclusions • We detected many dust particles post-disruption using laser-line scattering from the Thomson laser beam. • We did not detect (m)any particles during plasma operation, although we only have a small subset of run days to choose from. • We expect to have continual dust data from Thomson Scattering in the next campaign. • Lab tests are still in progress to test the feasibility of a dust injection system using gas puffing. • We expect to finish the lab tests and finish installation of the dust injection system (provided feasibility) by the start of the next campaign.