Friday, August 16, 2013

Laboratory Magnetospheric Experiments at MIT

This week, August 14-16, 2013, I was part of the LDX Team conducting new experiments with the world's largest laboratory magnetosphere. 

The MIT-Columbia LDX experiment is the U.S.'s only steady state plasma confinement device. Very sophisticated superconducting magnets. A circular, high-field magnet is charged with 1.2 MA, and a carefully controlled "levitation" magnet is used to suspend the dipole magnet for hours of continuous experimentation.  Scientists for today's run: Jay Kesner, Darren Garnier, Phil Michael, Alex Zhukovsky, Rick Lations, Matt Worstell, Paul Woskov, Mark Chilenski, and (my daughter) Marin, shown in photo below:

The first day of running was productive in every respect. Fortyseven "shots" were taken. Cryogenics, control, power, heating, data acquisition systems were flawless. (Congratulations to the LDX Team!)

The new 0.4 mm CH pellet shaker worked well, showing interesting "pinch"/"pump-out" phenomena. New microwave diagnostics showed promising results. New multifrequency ECRH programming showed a clear relationship between resonant zone location, hot electron production, and plasma density.

Key Results

The direction of the particle pinch following pellet drop reverses depending upon the rate of gas fueling and plasma density. Pellet drops with low fueling (low density) result in an inward particle pinch. (See shots 011-013.) At higher fueling, the pellets caused particle "pump-out", an outward pinch. (See shots 025 and 035.)  At intermediate fueling, pellet drops caused no change in the density profile. In all cases, with full-power multi-frequency ECRH, the plasma profile appears "stationary" (and strongly peaked) indicative of the turbulent self-organization we've seen previously. Fast videography indicate that the pellets do not penetrate deeply into the plasma, so these overall "pinch"/"pump-out" effects are dramatic examples of non-locality: fast changes in the edge cause global responses to the plasma density profile.

The programmed timing of the 2.45 GHz changed from our usual "wedding cake" program (early shots < 014) to an "inverted wedding cake", where the 2.45 GHz was on from 2-8 sec. This made the production of trapped, hot/warm electrons by the 2.45 GHz obvious. 4 kW of 2.45 GHz heating produced > 100 J of plasma energy (> 25 msec energy confinement). (Probably, it also broadened the pressure profile and increased the radius of the plasma ring current.) Interesting, the 2.45 GHz did not change the plasma density to the same degree as it changed the plasma energy. In contrast, higher frequency ECRH produced plasma energy less efficiently, but it was much more efficient at producing plasma density.

Reflectometer results showed effects from the plasma cut-off, but further analysis (and signal processing) is required to determine whether a density profile can be obtained. Wed's run scanned only 4-6 GHz, but today's run will scan 4-8 GHz.

Broad-band fluctuations were observed with the 28 GHz homo-dyne receiver, which may become a useful diagnostic on global plasma fluctuation levels.

More Experiments Today!

No comments:

Post a Comment