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Interpretation and implementation of an ion sensitive probe as a plasma potential diagnostic on Alcator C-Mod
Journal of Nuclear Materials 415 (2011) S1143–S1146
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Interpretation and implementation of an ion sensitive probe as a plasma potential diagnostic on Alcator C-Mod R. Ochoukov ⇑,1, D.G. Whyte, B. Lipschultz, B. LaBombard, S. Wukitch PSFC MIT, 77 Mass. Avenue, Cambridge, MA 02139, USA
a r t i c l e
i n f o
Article history: Available online 2 November 2010
a b s t r a c t An ion sensitive probe (ISP) is developed as a robust diagnostic for measuring plasma potentials (UP) in magnetized plasmas of Alcator C-Mod. The ISP relies on the large difference between the ion and electron gyro-radii (qi/qe 60) to reduce electron collection at a collector recessed at a distance qi behind a separately biased wall. We develop a new ISP method based on voltage sweeping to measure UP that is independent of the precise position and shape of the collector: UP is found to be the wall voltage when net current to the collector vanishes during an increasing voltage sweep. We have designed and installed an ISP to study UP and radio frequency (RF) sheath rectification on Alcator C-Mod. Preliminary results show that the ISP is a viable plasma potential diagnostic in Alcator C-Mod plasmas and UP measurements are in good agreement with independent results obtained with an emissive probe. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction One of the most fundamental parameters in tokamak plasmas is the plasma potential (UP). Several probes are capable of measuring UP in tokamaks and the Langmuir probe is the most common of them [1]. However, one must infer UP from other plasma parameters, such as the electron temperature (Te) and the electron and ion saturation currents (Ie SAT and ISAT, respectively), when using Langmuir probes [1]. UP can be measured directly with a hot emissive probe [2] but these probes rely on electron emitting filaments that are fragile. Due to electron emission limits, emissive probes are restricted to plasmas with densities below 1018 m 3, limiting emissive probe usage to the far scrape-off layer (SOL) of tokamak plasmas. It has been proposed [3,4] to use the large difference between the electron and ion gyro-radii (qe and qi, respectively) in magnetized plasmas to measure ion properties and UP directly. Standard theory demands that the collector be precisely positioned at a distance h qi behind a wall to precisely reduce electron collection to make the collector float at the plasma potential. The ion motion is assumed to be demagnetized and space-charge effects are ignored. Different variations of the original Katsumata design [3,4] are the ion sensitive probe (ISP) [5–7], the plug probe [8], the tunnel probe [9], the ball-pen probe [10], and the baffle probe [11]. We will refer to these probes as the ISP throughout the paper.
⇑ Corresponding author. Address: NW17, 175 Albany St., Cambridge, MA 02139, USA. E-mail address: [email protected] (R. Ochoukov). 1 Presenting author. 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.10.053
Our goal is to develop a robust long-lived alternative to the emissive probe that can access high density (>1020 m 3) regions of tokamak SOL plasmas. Specifically, we explore the ISP as a plasma potential diagnostic and develop a technique to measure UP that is insensitive to the exact positioning of the collector by examining the current collection as a function of a sweeping wall potential.
2. Experimental apparatus 2.1. ISP probe in DIONISOS The ISP was initially studied in a linear magnetized RF plasma, DIONISOS [12]. The magnetic field was B = 0.04 T and the working gas was argon at neutral gas pressure PAr = 0.26 Pa. The plasma density range was 1016–1018 m 3, Te range was 7–15 eV, and qe was 0.2 mm. The main components of the ISP (Fig. 1) were (1) a circular stainless steel ‘‘collector’’, diameter dCOLL = 4.69 mm, (2) a cylindrical stainless steel ‘‘wall’’, inner diameter IDWALL = 6 mm and outer diameter ODWALL = 6.8 mm, and (3) cylindrical alumina shield, IDSHIELD = 6.8 mm and ODSHIELD = 11.3 mm. The electrically isolated collector and wall were biased using a bipolar power supply. The wall (VWALL) and the collector (VCOLL) potentials (with respect to the vacuum chamber) were swept together at the same frequency (f = 1–10 Hz, triangular waveform, VSWEEP = ±80 V) with a constant relative bias between them (VBIAS VWALL VCOLL). The probe entrance was positioned at the center of the plasma column with the collector surface aligned parallel to the magnetic field. The collector recess distance, h, was varied between 0 and 10 mm. A
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R. Ochoukov et al. / Journal of Nuclear Materials 415 (2011) S1143–S1146
Fig. 1. A schematic of DIONISOS ISP. h = recess height (h is defined negative when recessed behind wall); solid – conductor, hashed – insulator; cylindrical symmetry.
The S3 is located 10° toroidally clockwise (when viewed from the top) from the nearest ‘‘J-port’’ ICRF antenna limiter. For typical magnetic topology, the two probes are magnetically connected to the J antenna limiter tiles provided that the S3 is in the shadow of the RF limiter (RPROBE > RRF LIMITER, RRF LIMITER = 0.915 m at the mid-plane). The probes are never magnetically connected to the other ICRF antennae at D- and E-port. The ISP collector recess distance is fixed at 50 lm, the probe cross-section is rectangular 3 9.7 mm2, the wall and the collector are separated by a layer of mica 0.5 mm thick, and the wall thickness is 0.8 mm. The surface of the collector is aligned to the local magnetic flux surfaces in the SOL of a typical C-Mod plasma shot. Both the wall and the collector are molybdenum and the outer insulator is boron nitrite. The collector and wall voltages (with respected to the vessel wall) are swept at 1 kHz, triangular waveform, and peak-to-peak voltage of 200 V. The bias voltage was either 0 or 20 V.
detailed description of DIONISOS and its ISP studies can be found elsewhere [13].
3. Experimental results and discussion
2.2. ISP probe on Alcator C-Mod
The plasma potential can be determined by examining a current–voltage (I–V) characteristic of the probe collector as a function of the wall (not the collector) voltage. In the simplest picture of an ISP no ion is electrostatically allowed to enter the probe volume once the wall voltage, and hence vacuum potential of the probe volume, is raised above the plasma potential. Therefore, UP should be found at the wall voltage at which the measured net current to the collector vanishes. This technique is similar to determining UP from a Langmuir probe I–V (Fig. 1a in [1]), however, the UP location is no longer obscured by Ie SAT fluctuations commonly observed in Langmuir probe measurements. Fig. 3 shows two typical I–V curves where the wall and the collector were both swept simultaneously with a constant VBIAS. The plasma
A modified version of the ISP has been installed on Alcator C-Mod [14] on a reciprocating (along the major radius R) probe station, the Surface Science Station or S3 (Fig. 2), located on the horizontal K-port, 22 cm below the mid-plane. The S3 also contains an emissive probe, aligned to the same flux surface of a typical plasma shot (toroidal field BT = 5.4 T, plasma current IP = 1 mA, poloidal field BP 0.18 BT) as the ISP but located 2 cm above the ISP (see Fig. 2), for independent UP measurements. The two probes are enclosed inside a molybdenum tiled cover, 5.1 mm thick, and plasma enters the cover through two apertures 3.8 7.6 mm2.
3.1. DIONISOS results
Fig. 2. A schematic of the S3 diagnostic head in Alcator C-Mod. ISP and emissive probe orientations are shown. The diameter of the diagnostic head is 57 mm.
R. Ochoukov et al. / Journal of Nuclear Materials 415 (2011) S1143–S1146
Fig. 3. ISP collector current in DIONISOS as a function of the wall potential for two cases: VWALL > VCOLLECTOR (triangles), and VWALL < VCOLLECTOR (squares). PRF = 1600 W, h = 5 mm. Ion collection is negative current.
potential, determined independently by an emissive probe, was 15 V in this particular case. The collector current remains in the ion collection regime at all times when the bias voltage is positive (VWALL > VCOLLECTOR). The ion current decreases to zero once the wall potential is raised above the plasma potential. If the bias voltage is negative (VWALL < VCOLLECTOR) one measures substantial net electron current for VWALL < UP, but which also goes to zero when VWALL > UP. Note that previous interpretations of the ISP (e.g. [10]) have sought to find UP simply through the floating potential of the collector while applying a positive VBIAS to avoid secondary electron collection [15]. However, it is clear from Fig. 3 that such a method cannot identify UP since the collector current is zero at all VWALL > UP. In addition, the experimental results eliminate secondary electrons as a cause of the electron collection since the electron collection at the wall is unchanged by VBIAS or VWALL sweeps even as electron collection vanishes at the collector. Using this technique, based on sweeping, we verified UP measurements against emissive probe results in DIONISOS over a wide range of densities and plasma regimes [13]. The main source of uncertainty in the ISP measurements is defining the ‘‘knee’’ location in the collected current. It is likely that the ion temperature determines the roundedness/sharpness of the knee without affecting the location of the knee, which is set by the plasma potential. Importantly, we have found that the UP measurement is completely unaffected by h as long as h > qi. Rather the key to successful UP measurement is control of relative bias between wall and collector voltages during sweeps (Fig. 3): this makes the ISP very attractive for tokamak applications since these are readily controlled externally, while h is difficult to change. The dynamic range of the ISP is large since one may choose to collect electrons or ions based on VBIAS.
S1145
Fig. 4. ISP collector current in Alcator C-Mod as a function of the wall potential for two cases: RF heated (J antenna, t = 1.42 s, circles) and ohmic (squares, t = 1.54 s) plasmas. VBIAS = 20 V. Ion collection is negative current. RPROBE = 0.912 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane. Upper null (UN), deuterium, L-mode plasma.
with the emissive probe show fluctuating values of 100–150 V. The discrepancy in the two UP measurements is likely due to the difference in the magnetic mapping between the two probes – the two probes are 2 cm apart poloidally. The modified ISP is currently being used to study RF rectification of the plasma sheath in Alcator C-Mod. It has been speculated that RF sheath rectification may have a threshold-like behavior as a function of the local plasma density [16]. To test this hypothesis, as well, as to further test ISP measurements against the emissive probe, we performed a central density scan of an RF heated discharge (Fig. 5). Both the emissive probe and the ISP show the same threshold behavior of the plasma potential at t 1.0 s during the RF phase of the discharge. When the UP values are at their peak, as measured by the emissive probe, the ISP collector current remained in the ion collection regime (negative) at all wall voltages, implying that UP was >VWALL MAX = 120 V.
3.2. Preliminary Alcator C-Mod results The modified version of the ISP also works as a plasma potential diagnostic in Alcator C-Mod. Fig. 4 shows two I–V traces: RF heated and ohmic. Both curves show the same feature that we identified in DIONISOS plasmas – the collector current vanishes as VWALL is increased over a certain potential that we identify as the plasma potential. UP for the ohmic case is 20 V, which is in agreement with expectations that UP 3 Te, where Te 5–10 eV from independent Langmuir probe measurements. The plasma potential increases to 70 V in the RF heated case, while UP measurements
Fig. 5. Comparison of plasma potential measurements between ISP and emissive probe in Alcator C-Mod during RF heated discharge. RPROBE = 0.915 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane. UN, deuterium, L-mode plasma. Central line-averaged plasma density is also shown. VBIAS = 20 V. RF on time is shown as dashed area.
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R. Ochoukov et al. / Journal of Nuclear Materials 415 (2011) S1143–S1146
limiter, while it is <1018 m 3 in the shadow of the limiter. Overall, our initial results show that the ISP can function as a plasma potential diagnostic in high density tokamak plasmas, such as the SOL in Alcator C-Mod, and is also suitable to study RF sheath rectification. 4. Conclusion
Fig. 6. Comparison of plasma potential measurements between ISP and emissive probe in Alcator C-Mod during RF heated discharge. RPROBE = 0.909 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane. UN, deuterium, L-mode plasma. Central line-averaged plasma density is also shown. VBIAS = 20 V. RF on time is shown as dashed area.
Fig. 5 is an example of UP dependence on the plasma density during RF heated discharges on field lines directly (magnetically) connected to the active RF antenna (RPROBE = 0.915 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the midplane) with the other end of the field line terminated inside the S3 diagnostic head. The threshold behavior of UP is no longer present on field lines that are not intercepted by the active antenna (RPROBE = 0.909 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane) (Fig. 6). The field line now passes in front of the active RF antenna and is bounded between the horizontal plate of the lower outer divertor and the S3 diagnostic head. This behavior is consistent with the hypothesis that RF sheath rectification is due to a slow wave propagating from active RF antennae along field lines [16]. The amplitude of the slow wave quickly decays to zero across field lines once outside the antenna. Note, that the low UP values (<20 V), measured by the emissive probe are likely due to the emissive probe’s inability to supply enough electrons to float to the plasma potential – the plasma density is 1019–1020 m 3 in the region between the separatrix and the
The ISP appears to be a viable, robust, and long-lived alternative to the emissive probe as a plasma potential diagnostic in magnetized high density plasmas. We have developed a new method to measure the plasma potential with the ISP that is independent of the precise position and shape of the collector – the plasma potential is equal to the wall potential when charge current on the probe collector vanishes. The ISP measurements of UP show good agreement with emissive probe results over a wide range of densities and regimes in DIONISOS. Our preliminary results also show that the ISP can function as a plasma potential diagnostic in the high density SOL plasma of Alcator C-Mod. The ISP is currently being used on Alcator C-Mod to study sheath rectification in RF heated plasmas. Acknowledgements This work was supported by US Department of Energy award DE-FC02-99ER54512. We also thank Peter W. Stahle for his assistance in the installation of DIONISOS experiment at MIT. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
B. Lipschultz et al., J. Vac. Sci. Technol., A 4 (3) (1986) 1810–1816. R.F. Kemp, J.M. Sellen Jr., Rev. Sci. Instrum. 37 (1966) 455. I. Katsumata, M. Okazaki, Jpn. J. Appl. Phys. 6 (1967) 123–124. I. Katsumata, Contrib. Plasma Phys. 36 (S) (1996) 73–80. K. Uehara et al., J. Phys. Soc. Jpn. 66 (4) (1997) 921–924. N. Ezumi et al., J. Nucl. Mater. 313–316 (2003) 696–700. N. Ezumi et al., J. Nucl. Mater. 337–339 (2005) 1106–1110. V.I. Demidov, S.V. Ratynskaia, K. Rypdal, Rev. Sci. Instrum. 70 (1999) 4266. M. Kocan et al., J. Nucl. Mater. 363–365 (2007) 1436. J. Adamek et al., Czech. J. Phys. 55 (3) (2005) 235. V.I. Demidov et al., Rev. Sci. Instrum. 74 (2003) 4558. G.M. Wright et al., J. Nucl. Mater. 363–365 (2007) 977–983. R. Ochoukov et al., Rev. Sci. Instrum. 81 (2010) 10E111. I.H. Hutchinson, R. Boivin, F. Bombarda, et al., Phys. Plasmas 1 (1994) 1511. M.J. McCarrick et al., Rev. Sci. Instrum. 56 (1985) 1463. J.R. Myra, D.A. D’Ippolito, Phys. Rev. Lett. 101 (2008) 195004.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Interpretation and implementation of an ion sensitive probe as a plasma potential diagnostic on Alcator C-Mod R. Ochoukov ⇑,1, D.G. Whyte, B. Lipschultz, B. LaBombard, S. Wukitch PSFC MIT, 77 Mass. Avenue, Cambridge, MA 02139, USA
a r t i c l e
i n f o
Article history: Available online 2 November 2010
a b s t r a c t An ion sensitive probe (ISP) is developed as a robust diagnostic for measuring plasma potentials (UP) in magnetized plasmas of Alcator C-Mod. The ISP relies on the large difference between the ion and electron gyro-radii (qi/qe 60) to reduce electron collection at a collector recessed at a distance qi behind a separately biased wall. We develop a new ISP method based on voltage sweeping to measure UP that is independent of the precise position and shape of the collector: UP is found to be the wall voltage when net current to the collector vanishes during an increasing voltage sweep. We have designed and installed an ISP to study UP and radio frequency (RF) sheath rectification on Alcator C-Mod. Preliminary results show that the ISP is a viable plasma potential diagnostic in Alcator C-Mod plasmas and UP measurements are in good agreement with independent results obtained with an emissive probe. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction One of the most fundamental parameters in tokamak plasmas is the plasma potential (UP). Several probes are capable of measuring UP in tokamaks and the Langmuir probe is the most common of them [1]. However, one must infer UP from other plasma parameters, such as the electron temperature (Te) and the electron and ion saturation currents (Ie SAT and ISAT, respectively), when using Langmuir probes [1]. UP can be measured directly with a hot emissive probe [2] but these probes rely on electron emitting filaments that are fragile. Due to electron emission limits, emissive probes are restricted to plasmas with densities below 1018 m 3, limiting emissive probe usage to the far scrape-off layer (SOL) of tokamak plasmas. It has been proposed [3,4] to use the large difference between the electron and ion gyro-radii (qe and qi, respectively) in magnetized plasmas to measure ion properties and UP directly. Standard theory demands that the collector be precisely positioned at a distance h qi behind a wall to precisely reduce electron collection to make the collector float at the plasma potential. The ion motion is assumed to be demagnetized and space-charge effects are ignored. Different variations of the original Katsumata design [3,4] are the ion sensitive probe (ISP) [5–7], the plug probe [8], the tunnel probe [9], the ball-pen probe [10], and the baffle probe [11]. We will refer to these probes as the ISP throughout the paper.
⇑ Corresponding author. Address: NW17, 175 Albany St., Cambridge, MA 02139, USA. E-mail address: [email protected] (R. Ochoukov). 1 Presenting author. 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.10.053
Our goal is to develop a robust long-lived alternative to the emissive probe that can access high density (>1020 m 3) regions of tokamak SOL plasmas. Specifically, we explore the ISP as a plasma potential diagnostic and develop a technique to measure UP that is insensitive to the exact positioning of the collector by examining the current collection as a function of a sweeping wall potential.
2. Experimental apparatus 2.1. ISP probe in DIONISOS The ISP was initially studied in a linear magnetized RF plasma, DIONISOS [12]. The magnetic field was B = 0.04 T and the working gas was argon at neutral gas pressure PAr = 0.26 Pa. The plasma density range was 1016–1018 m 3, Te range was 7–15 eV, and qe was 0.2 mm. The main components of the ISP (Fig. 1) were (1) a circular stainless steel ‘‘collector’’, diameter dCOLL = 4.69 mm, (2) a cylindrical stainless steel ‘‘wall’’, inner diameter IDWALL = 6 mm and outer diameter ODWALL = 6.8 mm, and (3) cylindrical alumina shield, IDSHIELD = 6.8 mm and ODSHIELD = 11.3 mm. The electrically isolated collector and wall were biased using a bipolar power supply. The wall (VWALL) and the collector (VCOLL) potentials (with respect to the vacuum chamber) were swept together at the same frequency (f = 1–10 Hz, triangular waveform, VSWEEP = ±80 V) with a constant relative bias between them (VBIAS VWALL VCOLL). The probe entrance was positioned at the center of the plasma column with the collector surface aligned parallel to the magnetic field. The collector recess distance, h, was varied between 0 and 10 mm. A
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Fig. 1. A schematic of DIONISOS ISP. h = recess height (h is defined negative when recessed behind wall); solid – conductor, hashed – insulator; cylindrical symmetry.
The S3 is located 10° toroidally clockwise (when viewed from the top) from the nearest ‘‘J-port’’ ICRF antenna limiter. For typical magnetic topology, the two probes are magnetically connected to the J antenna limiter tiles provided that the S3 is in the shadow of the RF limiter (RPROBE > RRF LIMITER, RRF LIMITER = 0.915 m at the mid-plane). The probes are never magnetically connected to the other ICRF antennae at D- and E-port. The ISP collector recess distance is fixed at 50 lm, the probe cross-section is rectangular 3 9.7 mm2, the wall and the collector are separated by a layer of mica 0.5 mm thick, and the wall thickness is 0.8 mm. The surface of the collector is aligned to the local magnetic flux surfaces in the SOL of a typical C-Mod plasma shot. Both the wall and the collector are molybdenum and the outer insulator is boron nitrite. The collector and wall voltages (with respected to the vessel wall) are swept at 1 kHz, triangular waveform, and peak-to-peak voltage of 200 V. The bias voltage was either 0 or 20 V.
detailed description of DIONISOS and its ISP studies can be found elsewhere [13].
3. Experimental results and discussion
2.2. ISP probe on Alcator C-Mod
The plasma potential can be determined by examining a current–voltage (I–V) characteristic of the probe collector as a function of the wall (not the collector) voltage. In the simplest picture of an ISP no ion is electrostatically allowed to enter the probe volume once the wall voltage, and hence vacuum potential of the probe volume, is raised above the plasma potential. Therefore, UP should be found at the wall voltage at which the measured net current to the collector vanishes. This technique is similar to determining UP from a Langmuir probe I–V (Fig. 1a in [1]), however, the UP location is no longer obscured by Ie SAT fluctuations commonly observed in Langmuir probe measurements. Fig. 3 shows two typical I–V curves where the wall and the collector were both swept simultaneously with a constant VBIAS. The plasma
A modified version of the ISP has been installed on Alcator C-Mod [14] on a reciprocating (along the major radius R) probe station, the Surface Science Station or S3 (Fig. 2), located on the horizontal K-port, 22 cm below the mid-plane. The S3 also contains an emissive probe, aligned to the same flux surface of a typical plasma shot (toroidal field BT = 5.4 T, plasma current IP = 1 mA, poloidal field BP 0.18 BT) as the ISP but located 2 cm above the ISP (see Fig. 2), for independent UP measurements. The two probes are enclosed inside a molybdenum tiled cover, 5.1 mm thick, and plasma enters the cover through two apertures 3.8 7.6 mm2.
3.1. DIONISOS results
Fig. 2. A schematic of the S3 diagnostic head in Alcator C-Mod. ISP and emissive probe orientations are shown. The diameter of the diagnostic head is 57 mm.
R. Ochoukov et al. / Journal of Nuclear Materials 415 (2011) S1143–S1146
Fig. 3. ISP collector current in DIONISOS as a function of the wall potential for two cases: VWALL > VCOLLECTOR (triangles), and VWALL < VCOLLECTOR (squares). PRF = 1600 W, h = 5 mm. Ion collection is negative current.
potential, determined independently by an emissive probe, was 15 V in this particular case. The collector current remains in the ion collection regime at all times when the bias voltage is positive (VWALL > VCOLLECTOR). The ion current decreases to zero once the wall potential is raised above the plasma potential. If the bias voltage is negative (VWALL < VCOLLECTOR) one measures substantial net electron current for VWALL < UP, but which also goes to zero when VWALL > UP. Note that previous interpretations of the ISP (e.g. [10]) have sought to find UP simply through the floating potential of the collector while applying a positive VBIAS to avoid secondary electron collection [15]. However, it is clear from Fig. 3 that such a method cannot identify UP since the collector current is zero at all VWALL > UP. In addition, the experimental results eliminate secondary electrons as a cause of the electron collection since the electron collection at the wall is unchanged by VBIAS or VWALL sweeps even as electron collection vanishes at the collector. Using this technique, based on sweeping, we verified UP measurements against emissive probe results in DIONISOS over a wide range of densities and plasma regimes [13]. The main source of uncertainty in the ISP measurements is defining the ‘‘knee’’ location in the collected current. It is likely that the ion temperature determines the roundedness/sharpness of the knee without affecting the location of the knee, which is set by the plasma potential. Importantly, we have found that the UP measurement is completely unaffected by h as long as h > qi. Rather the key to successful UP measurement is control of relative bias between wall and collector voltages during sweeps (Fig. 3): this makes the ISP very attractive for tokamak applications since these are readily controlled externally, while h is difficult to change. The dynamic range of the ISP is large since one may choose to collect electrons or ions based on VBIAS.
S1145
Fig. 4. ISP collector current in Alcator C-Mod as a function of the wall potential for two cases: RF heated (J antenna, t = 1.42 s, circles) and ohmic (squares, t = 1.54 s) plasmas. VBIAS = 20 V. Ion collection is negative current. RPROBE = 0.912 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane. Upper null (UN), deuterium, L-mode plasma.
with the emissive probe show fluctuating values of 100–150 V. The discrepancy in the two UP measurements is likely due to the difference in the magnetic mapping between the two probes – the two probes are 2 cm apart poloidally. The modified ISP is currently being used to study RF rectification of the plasma sheath in Alcator C-Mod. It has been speculated that RF sheath rectification may have a threshold-like behavior as a function of the local plasma density [16]. To test this hypothesis, as well, as to further test ISP measurements against the emissive probe, we performed a central density scan of an RF heated discharge (Fig. 5). Both the emissive probe and the ISP show the same threshold behavior of the plasma potential at t 1.0 s during the RF phase of the discharge. When the UP values are at their peak, as measured by the emissive probe, the ISP collector current remained in the ion collection regime (negative) at all wall voltages, implying that UP was >VWALL MAX = 120 V.
3.2. Preliminary Alcator C-Mod results The modified version of the ISP also works as a plasma potential diagnostic in Alcator C-Mod. Fig. 4 shows two I–V traces: RF heated and ohmic. Both curves show the same feature that we identified in DIONISOS plasmas – the collector current vanishes as VWALL is increased over a certain potential that we identify as the plasma potential. UP for the ohmic case is 20 V, which is in agreement with expectations that UP 3 Te, where Te 5–10 eV from independent Langmuir probe measurements. The plasma potential increases to 70 V in the RF heated case, while UP measurements
Fig. 5. Comparison of plasma potential measurements between ISP and emissive probe in Alcator C-Mod during RF heated discharge. RPROBE = 0.915 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane. UN, deuterium, L-mode plasma. Central line-averaged plasma density is also shown. VBIAS = 20 V. RF on time is shown as dashed area.
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R. Ochoukov et al. / Journal of Nuclear Materials 415 (2011) S1143–S1146
limiter, while it is <1018 m 3 in the shadow of the limiter. Overall, our initial results show that the ISP can function as a plasma potential diagnostic in high density tokamak plasmas, such as the SOL in Alcator C-Mod, and is also suitable to study RF sheath rectification. 4. Conclusion
Fig. 6. Comparison of plasma potential measurements between ISP and emissive probe in Alcator C-Mod during RF heated discharge. RPROBE = 0.909 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane. UN, deuterium, L-mode plasma. Central line-averaged plasma density is also shown. VBIAS = 20 V. RF on time is shown as dashed area.
Fig. 5 is an example of UP dependence on the plasma density during RF heated discharges on field lines directly (magnetically) connected to the active RF antenna (RPROBE = 0.915 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the midplane) with the other end of the field line terminated inside the S3 diagnostic head. The threshold behavior of UP is no longer present on field lines that are not intercepted by the active antenna (RPROBE = 0.909 m, RRF LIMITER = 0.915 m, RLIMITER = 0.910 m, RSEPARATRIX = 0.890 m at the mid-plane) (Fig. 6). The field line now passes in front of the active RF antenna and is bounded between the horizontal plate of the lower outer divertor and the S3 diagnostic head. This behavior is consistent with the hypothesis that RF sheath rectification is due to a slow wave propagating from active RF antennae along field lines [16]. The amplitude of the slow wave quickly decays to zero across field lines once outside the antenna. Note, that the low UP values (<20 V), measured by the emissive probe are likely due to the emissive probe’s inability to supply enough electrons to float to the plasma potential – the plasma density is 1019–1020 m 3 in the region between the separatrix and the
The ISP appears to be a viable, robust, and long-lived alternative to the emissive probe as a plasma potential diagnostic in magnetized high density plasmas. We have developed a new method to measure the plasma potential with the ISP that is independent of the precise position and shape of the collector – the plasma potential is equal to the wall potential when charge current on the probe collector vanishes. The ISP measurements of UP show good agreement with emissive probe results over a wide range of densities and regimes in DIONISOS. Our preliminary results also show that the ISP can function as a plasma potential diagnostic in the high density SOL plasma of Alcator C-Mod. The ISP is currently being used on Alcator C-Mod to study sheath rectification in RF heated plasmas. Acknowledgements This work was supported by US Department of Energy award DE-FC02-99ER54512. We also thank Peter W. Stahle for his assistance in the installation of DIONISOS experiment at MIT. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
B. Lipschultz et al., J. Vac. Sci. Technol., A 4 (3) (1986) 1810–1816. R.F. Kemp, J.M. Sellen Jr., Rev. Sci. Instrum. 37 (1966) 455. I. Katsumata, M. Okazaki, Jpn. J. Appl. Phys. 6 (1967) 123–124. I. Katsumata, Contrib. Plasma Phys. 36 (S) (1996) 73–80. K. Uehara et al., J. Phys. Soc. Jpn. 66 (4) (1997) 921–924. N. Ezumi et al., J. Nucl. Mater. 313–316 (2003) 696–700. N. Ezumi et al., J. Nucl. Mater. 337–339 (2005) 1106–1110. V.I. Demidov, S.V. Ratynskaia, K. Rypdal, Rev. Sci. Instrum. 70 (1999) 4266. M. Kocan et al., J. Nucl. Mater. 363–365 (2007) 1436. J. Adamek et al., Czech. J. Phys. 55 (3) (2005) 235. V.I. Demidov et al., Rev. Sci. Instrum. 74 (2003) 4558. G.M. Wright et al., J. Nucl. Mater. 363–365 (2007) 977–983. R. Ochoukov et al., Rev. Sci. Instrum. 81 (2010) 10E111. I.H. Hutchinson, R. Boivin, F. Bombarda, et al., Phys. Plasmas 1 (1994) 1511. M.J. McCarrick et al., Rev. Sci. Instrum. 56 (1985) 1463. J.R. Myra, D.A. D’Ippolito, Phys. Rev. Lett. 101 (2008) 195004.