alpha
alpha
The Auburn Large-area Plasma Helicon Array (alpha) is the forerunner of the HelCat laboratory plasma physics experiment at the University of New Mexico. It was used to study the possibility of making a large-area, high density plasma source using multiplehelicon sources. In the case of alpha, seven helicon sources create and array used to generate a large area plasma.
The design of alpha was motivated by several considerations with regards to the intended studies of Alfvén waves and magnetic reconnection. Alfvén wave studies constrain laboratory plasmas to be large and/or dense. This is because the nominal Alfvén wavelength, vA / wci scales as 1/√n. alpha is unique in using helicon sources to generate a high-density, highly ionized, steady state, quiescent plasma discharge for these Alfvén wave studies. Importantly, in contrast all previous Alfvén wave studies, the discharge is inherently current-free. A prototype of the facility, ALESPI, using a single helicon source is in operation at Auburn University.
Details of the alpha facility are provided in a recently submitted paper. You can learn about the construction of alpha. The device consists of a 4.0 m long cylindrical vacuum chamber 50 cm in diameter. A large number of both large and small ports allows for easy diagnostic access. The chamber is surrounded by a magnetic solenoid of 13 magnets, each with a nominal diameter of 82 cm and consisting of 140 turns. These create a maximum field on axis of 0.22 T. The field strength and large diameter are sufficient to insure that the ions are magnetized; at a magnetic field of 0.1 T the ion gyroradius is ~2 mm for He in our plasmas. The measured magnetic field ripple is less than 1% along the machine axis, and the uniformity is better than 3% across the plasma cross-section.
At one end of the chamber is located a 1000 l/s turbo/molecular drag pump which maintains a nominal base pressure of 2x10-7 torr. During plasma operation a computer controlled butterfly valve in front of the pump regulates the chamber pressure, allowing adequate gas flow to maintain a “clean” plasma during steady-state operation, while not choking the pump with too much gas. The gas input is located near the pump to minimize any potential bulk flow. The helicon plasma sources are located at the opposite end of the chamber.
Current diagnostics include an array of RF-compensated radial Langmuir probes and double probes and an axial probe that rides along a servomotor driven slide. These measure Te and ne. A single chord microwave interferometer operating at 65 GHz (to be upgraded to a 94 GHz system) is used to corroborate probe density measurements. Magnetic loops are used to measure the fluctuating magnetic field of the Alfvén waves. A 0.5 m Ebert spectrometer has been used to measure the ion temperature via Doppler broadening. However, the ions are relatively cold, with a Doppler width the order of the spectrometer width, and results were inconclusive. An LIF system has just been installed for ion velocity distribution measurements in both argon (Ar II) and helium (He I) plasmas.
As mentioned above, alpha uses helicon sources to generate the background plasma. One problem with using helicon sources is that the achievable density decreases as the diameter of the helicon antenna increases. Thus, for a large plasma area helicons are generally not appropriate. However, overcome this difficulty through the use of a "Gatling gun" approach. The idea is to use a collection of (articulated) helicon sources acting in concert to create a large volume, high-density plasma. A system of seven 13 cm diameter, hexagonally packed helicon antennae act as a single source to create a 40 cm diameter uniform plasma. Each individual helicon antenna surrounds a pyrex cylinder providing the necessary insulating boundary condition, and the array is mounted at one end of the vacuum chamber. Power to the antennae will be provided by a single RF source, operating at 1 - 34 MHz with a 20 kW output.
Multiple helicon sources at the end of Alpha.
The alpha experiment uses seven helicon sources to make a large area plasma. As mentioned above, the larger the diameter of the helicon source, the lower the achievable density. For Alfvén wave studies we require a fairly high density, while in order to minimize effects from the plasma boundary we would like a large cross sectional area. Thus, rather than us a single helicon source, we use seven smaller sources packed in a hexagonal array.
Originally
we had planned to use a single RF power supply and matching/distribution network
to each of the seven source. This had been shown effective in other studies
of multiple sources. However, we found that for our plasma conditions (likely
due to the large magnetic field and high density) this would not work. Studies
with two tubes and single RF supply and matching network show that no more
than
one antenna at a time would achieve helicon mode. With separate matching networks
for each antenna we were able to get multiple helicon operation. Thus, for alpha
we have chosen to use seven separate power supplies and matching networks.
In
addition to making for a more manageable RF power system, this allows
us to separately adjust each source, and taylor the plasma for particular
experiments.
The figure at the left shows an end view of the helicon array on alpha
with two helicon sources in operation.
Matching circuits tried. The inductor is the helicon antenna.
Multiple helicon operation with all seven sources was first successful in the fall of 2002. Below are photos of the 7 tube operation in argon. At this point, each of the sources acts separately, rather than creating a uniform plasma (as hoped). Careful adjustment of the magnetic field and RF power allows us to make a uniform plasma. However, to do so requires very specialized conditions, and a weak magnetic field. In addition, the plasma is not very stable: the sources couple to each other, and eventually one or more sources "goes out". Details of multi-source operation can be found in a paper. Because of the difficulty in obtaining multi-source operation, research in this direction has been suspended for now.
Multiple helicon operation. In the right photo are visible
the 7 blue cores of the individual sources.
Density profile across two of the helicon sources during
multi-source operation. Here the individual source can be identified by
the density peak.
Research on multiple helicon sources has been suspended for now, and research efforts now focus on the HelCat device. Our results indicated that indeed, multiple source operation is possible. However, a uniform density and temperature profile is only obtainable under very limited plasma conditions (density, temperature and magnetic field). Further research might identify additional regimes at some future time.
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