Characterization of Wave Propagating Parallel and Anti...
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Characterization of Wave Propagating Parallel and Anti-Parallel Downstream of a High Power Helicon Source J. Prager*; T. Ziemba; R. Winglee; R. Roberson Department of Earth and Space Sciences University of Washington Seattle, WA 98195-1310 *[email protected] Half Wavelength Helical Antenna - diameter = 7 cm - length = 13 cm - right handed helicity DC Magnetic Field - six magnets - solenoid field in the source - B = 400 G in the source - dipole field downstream Ar Plasma Properties in the Source - neutral flow rate ~1 torr L/s - density ~ 2 x 10 20 m -3 - Te = 5 - 7 eV - Power = 30 kW (into plasma) - Plasma accelerated in downstream direction only - Downstream neutral pressure less than 1 microtorr HPHX Source -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Impacts Side of Chamber Connects to Ambient Earth Magnetic Field Magnetic Field - Top View -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Int. Langmuir Magnetic Field - Side View Plasma Emitter Repeller Discriminator Suppressor Collector -18 V -97 V -97 V 0 - 300 V 10 kW V out Retarding Field Analyzer (RFA) Wave Downstream of HPHX Antiparallel Wave Propagation Conclusions A retarding field analyzer (RFA) is used to measure the ion velocity distribution functions (IVDF). Due to the short shot length, it is difficult to scan discriminator voltage during a single shot. To deal with this we rely on the high shot-to- shot reproducibilty and vary the discriminator voltage on a shot-to-shot basis. Over many shots we sweep through a range of discriminator voltages. The IVDF is proportional to the derivative of the collector current with respect to the discriminator voltage. The RFA was used to collect data with the oriface in three orientations (0 o , 45 o , 90 o ). If the plasma had no bulk velocity then the IVDFs should look identical for all three probe orientations. If there is a thermal plasma with a beam component, then as the probe is rotated, the peak from the thermal plasma should remain while the beam component disappears. At HPHX we see the IVDF decrease as the probe is rotated. This indicates that the ions are flowing with a bulk velocity that is pointed downstream of HPHX, which is radically different from what is ovserved at other helicons. There is still thermal plasma when the RFA is rotated, but the RFA is not sensitive enough to measure it. This was conffirmed by rotating a planar probe. Thus HPHX is producing a flowing plasma. Reference: Prager J, Winglee R, Ziemba T, Roberson B, Quentin G, Plasma Source Sci Technol. 17, 025003, 2008 The three components of the wave magnetic field were measured downstream of HPHX on axis as a function of axial position using a 3-axis B-dot probe. These measurements demonstrate that a right handed, circular polarized wave is propagating downstream of HPHX. Using this data, we measured the wavelength, which changes as a function of axial location due to the wave propagating through a magnetic field and density profile that varies with distance. This measured wavelength was then compared to helicon theory. Since helicons are bounded whistler waves, we made an assumption about the bounding radius. This assumption was based the size of the helicon antenna and a radial plasma density profile. The calculated wavelentgth did not match the measured wavelength. The wavelength for a freely propagating whistler wave was also compared to the measurments. This result demonstrates that a freely propagating (not radially bounded) whistler wave is propagating downstream of HPHX. Based on a radial cut using a 3-axis B-dot near the face of HPHX, the wave appears to show signs of being bounded. Comparing this radial cut with the radial density profile indicates that the plasma boundary is not what is boundig the wave at this location. These results indicate that the wave shows initial signs of being bounded, since it was generated inside a quartz tube; however, by the time it has propagated far downstream, the wave is freely propagaing. The temporal profile of the ion saturation current shows that the current rises faster and reaches a higher peak value in the case of antiparallel propagation. This effect is likely due to increased plasma speeds rather than increased plasma production. The differences can also be seen in the radial profiles. Early on in time, the plasma is peaked on axis (more so for the antiparallel case). However later on in time, the antiparal profile becomes a hollow distribution. The peaked profile is typical of the helicon, while the hollow profile is atypical. The temporal evolution of the IVDF also showed remarkable differences between the parallel and anti-parallel propagation cases. The parallel propagation case had a single peak that started at high energy and then migrated to lower energy towards the end of the shot. The anti-parallel case starts with a single peak. As the distribution evolves a second peak forms at low energy. This second peak indicates a two acceleration mechanisms. Both peaks then increase in speed. The rate at which the speed increases is the same. The axial profile of ΔB z was measured, and it shows that ΔB z is larger for the case of antiparallel propagation. The eight-fold increase cannot be accounted for soley by an increased plasma density, which only increases by a factor of two. The axial profile of the wave magnetic field was measured. In the antiparallel case, the measured wavelength matches well with the freely propagating whistler wavelength. However, the wave looks different. In the parallel case, all three components had a similar magnitude; whereas in the antiparallel case, the B z has a near zero magnitude. It is possible that a surface wave is also being propagated in the antiparallel case. This would help explain the hollow density profile, the second peak in the IVDF, the radical differences in ΔB z and differences in the magnetic wave field measurements. We have demonstrated that the plasma downstream of HPHX is a flowing plasma not a small beam component with a thermal background. We have demonstrated that the wave propagating downstream of HPHX is initially a bounded wave as it leaves the antenna region. As the wave propagates it transitions to a freely propagating whistler wave. We have investigated the plasma flowing downstream of HPHX when the wave propagates parallel and antiparallel to the magnetic field. Radical differences are observed in these two cases. The data indicate that there is potentially a second mechanism involved in accelerating the ions. There is also the possibilty that a surface wave is being propagated. Thanks Thanks to Tina Saloutos, Shawn Campbell, John Carscadden and David Peters.
Transcript of Characterization of Wave Propagating Parallel and Anti...
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Characterization of Wave Propagating Paralleland Anti-Parallel Downstream of a High Power Helicon Source
J. Prager*; T. Ziemba; R. Winglee; R. RobersonDepartment of Earth and Space Sciences
University of WashingtonSeattle, WA 98195-1310
Half Wavelength Helical Antenna - diameter = 7 cm - length = 13 cm - right handed helicityDC Magnetic Field - six magnets - solenoid field in the source - B = 400 G in the source - dipole field downstreamAr Plasma Properties in the Source - neutral flow rate ~1 torr L/s
- density ~ 2 x 1020 m-3
- Te = 5 - 7 eV - Power = 30 kW (into plasma) - Plasma accelerated in downstream direction only - Downstream neutral pressure less than 1 microtorr
HPHX Source
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Impacts Side of ChamberConnects to Ambient Earth Magnetic Field
Magnetic Field - Top View
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Int. Langmuir
Magnetic Field - Side View
Plasma
Emitter
Repeller
Discrim
inator
Suppressor
Collector
-18 V
-97 V-97 V 0 - 300 V
10 kΩ
Vout
Retarding Field Analyzer (RFA)
Wave Downstream of HPHX
Antiparallel Wave Propagation
Conclusions
A retarding field analyzer (RFA) is used to measure the ion velocity distribution functions (IVDF). Due to the short shot length, it is difficult to scan discriminator voltage during a single shot. To deal with this we rely on the high shot-to-shot reproducibilty and vary the discriminator voltage on a shot-to-shot basis. Over many shots we sweep through a range of discriminator voltages. The IVDF is proportional to the derivative of the collector current with respect to the discriminator voltage.
The RFA was used to collect data with the oriface in three orientations (0o, 45o, 90o). If the plasma had no bulk velocity then the IVDFs should look identical for all three probe orientations. If there is a thermal plasma with a beam component, then as the probe is rotated, the peak from the thermal plasma should remain while the beam component disappears. At HPHX we see the IVDF decrease as the probe is rotated. This indicates that the ions are flowing with a bulk velocity that is pointed downstream of HPHX, which is radically different from what is ovserved at other helicons. There is still thermal plasma when the RFA is rotated, but the RFA is not sensitive enough to measure it. This was conffirmed by rotating a planar probe. Thus HPHX is producing a flowing plasma.
Reference:Prager J, Winglee R, Ziemba T, Roberson B, Quentin G, Plasma Source Sci Technol. 17, 025003, 2008
The three components of the wave magnetic field were measured downstream of HPHX on axis as a function of axial position using a 3-axis B-dot probe. These measurements demonstrate that a right handed, circular polarized wave is propagating downstream of HPHX.
Using this data, we measured the wavelength, which changes as a function of axial location due to the wave propagating through a magnetic field and density profile that varies with distance. This measured wavelength was then compared to helicon theory. Since helicons are bounded whistler waves, we made an assumption about the bounding radius. This assumption was based the size of the helicon antenna and a radial plasma density profile. The calculated wavelentgth did not match the measured wavelength. The wavelength for a freely propagating whistler wave was also compared to the measurments. This result demonstrates that a freely propagating (not radially bounded) whistler wave is propagating downstream of HPHX.
Based on a radial cut using a 3-axis B-dot near the face of HPHX, the wave appears to show signs of being bounded. Comparing this radial cut with the radial density profile indicates that the plasma boundary is not what is boundig the wave at this location. These results indicate that the wave shows initial signs of being bounded, since it was generated inside a quartz tube; however, by the time it has propagated far downstream, the wave is freely propagaing.
The temporal profile of the ion saturation current shows that the current rises faster and reaches a higher peak value in the case of antiparallel propagation. This effect is likely due to increased plasma speeds rather than increased plasma production.
The differences can also be seen in the radial profiles. Early on in time, the plasma is peaked on axis (more so for the antiparallel case). However later on in time, the antiparal profile becomes a hollow distribution. The peaked profile is typical of the helicon, while the hollow profile is atypical.
The temporal evolution of the IVDF also showed remarkable differences between the parallel and anti-parallel propagation cases. The parallel propagation case had a single peak that started at high energy and then migrated to lower energy towards the end of the shot. The anti-parallel case starts with a single peak. As the distribution evolves a second peak forms at low energy. This second peak indicates a two acceleration mechanisms. Both peaks then increase in speed. The rate at which the speed increases is the same.
The axial profile of ∆Bz was measured, and it shows that ∆Bz is larger for the case of antiparallel propagation. The eight-fold increase cannot be accounted for soley by an increased plasma density, which only increases by a factor of two.
The axial profile of the wave magnetic field was measured. In the antiparallel case, the measured wavelength matches well with the freely propagating whistler wavelength. However, the wave looks different. In the parallel case, all three components had a similar magnitude; whereas in the antiparallel case, the Bz has a near zero magnitude.
It is possible that a surface wave is also being propagated in the antiparallel case. This would help explain the hollow density profile, the second peak in the IVDF, the radical differences in ∆Bz and differences in the magnetic wave field measurements.
We have demonstrated that the plasma downstream of HPHX is a flowing plasma not a small beam component with a thermal background.
We have demonstrated that the wave propagating downstream of HPHX is initially a bounded wave as it leaves the antenna region. As the wave propagates it transitions to a freely propagating whistler wave.
We have investigated the plasma flowing downstream of HPHX when the wave propagates parallel and antiparallel to the magnetic field. Radical differences are observed in these two cases. The data indicate that there is potentially a second mechanism involved in accelerating the ions. There is also the possibilty that a surface wave is being propagated.
ThanksThanks to Tina Saloutos, Shawn Campbell, John Carscadden and David Peters.
