An Interesting Poster to look at from the Tri Alpha Energy Team in California
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                                                                                                                      Design of a Custom Insertable Probe Platform for Measurements of C-2W Inner Divertor Plasma Parameters
A. M. DuBois, V. Sokolov, K. Knapp, M. C. Thompson, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
             ABSTRACT
A custom motor controlled probe system has been designed to make spatially resolved measurements of temperature, density, flow, and plasma potential in the C-2W inner divertors. Measurements in the inner divertors, which have a radius of 1.7 m and are located on either end of the confinement vessel, are critical in order to gauge exactly how local settings affect the plasma conditions, confinement, and stability in the FRC core. The inner Divertor Insertable Probe Platform (iDIPP) system consists of a custom motor controlled linear rack and pinion transporter that has a 1.9 m travel length in order to reach the center of the divertor. Mounted to the end of the transporter is a 1 m long segmented probe shaft made of individually floating stainless steel rings to prevent shorting out the electrode plates, which are biased up to 5 kV/m. A variety of interchangeable probe tips, including a triple Langmuir probe, a baffled probe, and a Gundestrup probe, can be easily plugged into the end of the probe shaft. Custom UHV coiled cabling comprised of 9 shielded conductors expands/retracts with the motion of the transporter in/out of the divertor. Details of the design of the iDIPP system will be discussed.
MOTIVATION
 FRC core located in CV within separatrix & edge of plasma characterized by region of open field lines called the scrape-off-layer (SOL)
 SOL open field lines terminate on electrode plates in inner divertors
 FRC core greatly affected by local settings in inner divertor
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iDIPP MOTION SYSTEM
Custom heavy duty linear rack & pinion transporter from Kurt J. Lesker Company with magnetically coupled drive manufactured by UHV Design Ltd.
Magnetically coupled UHV rotary drive & stepper motor move 5 cm OD internal transporter shuttle 1.9 m in/out of divertor in 2 minutes or less
MD40 “MagiDrive” rotary feedthrough enables transfer of motion into vacuum environment w/out bellows or dynamic seals
CUSTOM UHV COILED CABLING
 Custom coiled cables for UHV applications from Axon Cable
 9 conductors, individually shielded, 50 Ohm, 30 AWG
 1.9 m resting length
 Tension held to avoid
drooping
 4.5 m extended length
 Not pulled completely straight
 Scoop attached to end of shuttle to scoop & prevent coiled cable from being caught during motion of the shuttle
INTERCHANGABLE TRIPLELANGMUIRPROBE
SEGMENTED PROBE SHAFT
− Passes 50.5 cm in front of biased electrodes
− d = 1.6 cm
− l =1 m
 Probe shaft made of “fish scale” segments so radial electric field (5 kV/m) from biased electrodes not shorted
 Individual floating stainless steel rings isolated with ceramic disks
 Ceramic tube insulates cables from plasma facing components  Internal stainless steel tube improves strength of probe shaft &
provides additional shielding
 Two tees allow access for maintenance & replacing probe tips without removing entire system
 Variety of probe tips plugged into probe shaft
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Contains 2 rotating rings (air-side, vacuum-side) with alternating strip magnets that magnetically couple through SST vacuum sheath
Stepper motor rotates air-side ring, causes vacuum-side ring rotation Vacuum-side ring fixed to pinion gear - drives rack
1.9 meter travel length
2.4 meter length (retracted) 4.3 meter length (extended)
GUNDUSTRUP PROBE
BAFFLED PROBE
       PROBE TIP ASSEMBLIES
Plasma parameters & fluctuation measurements
4.8 cm long, 1.9 cm OD ceramic
Plug directly into segmented probe shaft
D-TACQ with 80 MS/s sampling rate
Future probe types: ■ Emissiveprobe
Measures: V , T , n , V Measures: Flow velocity, V Measures: V , T feep f pi
  Mirror coils between CV & inner divertor flare SOL field lines to reduce particle loss [1,2] & maintain SOL high T - main purpose of divertors
 SOL thickness controlled by varying mirror coil current & funnel limiter defines SOL radial size
 Electrode plates biased to produce inward electric field & E x B velocity shear to suppress rotational instabilities & improve confinement [3]
 Custom motor controlled probe system, iDIPP, designed to make spatially resolved measurements of plasma parameters in inner divertors to gauge effect on FRC
   e
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− 3 Tungsten tips
− d = 0.5 mm
− l = 3 mm
− A = 2.7 mm2
− Spaced 1.6 mm apart
Electrically floating tip measures Vf Other 2 tips floating & biased wrt
each other [4]
𝑇=+ 𝑓
 − − −
9 Tungsten tips d = 0.5 mm
l = 3 mm
− Tungsten tip
− d = 2.4 mm
− Recessed 1.6
mm into shield
Goal: reduce electron flux on probe surface by recessing probe tip into shield [6]
■ 𝝆𝒆<𝝆𝒊
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Mach Probe with 8 faces isolated
from one other measuring I sat
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𝑉−𝑉 𝑒 ln2
Iup/Idown related to Mach number [5] I 𝑀 −𝑀 cot𝜂
𝑰𝒖𝒑 > 𝑰𝒅𝒐𝒘𝒏
  𝑅= 𝑢𝑝 =𝑒𝑥𝑝 ∥ ⊥ I𝑀
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Electrons can’t access probe surface
Ions can reach surface Isat increases so that:
𝐼=𝐼 𝑠𝑎𝑡 𝑒𝑠
𝑉 𝑓𝑙𝑜𝑎𝑡𝑠 𝑉
𝑝𝑟𝑜𝑏𝑒 𝑝𝑙𝑎𝑠𝑚𝑎
    𝐼𝑚 𝑠𝑎𝑡 𝑖
0.61𝑒𝐴 𝑒𝑇 𝑒
4𝑚 𝑖
3𝜋𝑚𝑒
𝑑𝑜𝑤𝑛
𝑐
    iDIPP comprised of heavy duty rack & pinion motion system, custom coiled cabling, segmented probe shaft, interchangable probe tips
[1] M. W. Binderbauer et al., PoP (2015)
[2] Ryutov et al., AIP Conference Proceedings 1721, 06003 (2016) [3] M. Tuszewski et al., PRL 108, 255008 (2012)
[4] S. L. Chen, J. Applied Phys. 36, 2363 (1965)
[5] Patacchini et al., PRE (2009)
[6] Katsumata, Contrib. Plasma Phys. 36, 73-80 (1996)
𝑛 = 𝑒
𝑉 = 𝑉 + 𝑒 ln 𝑝 𝑓 2
V 𝑀= 𝑓𝑙𝑜𝑤
C𝑠
𝑇
Resolve plasma flow speed & direction
V measured by 9th tip f
Radial measurements →change in speed and direction
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Shuttle deflection due to gravity when fully extended with 10 lbs attached < 10 mm
iDIPP requires independent vacuum system for removal from divertor for maintenance & shielding during wall conditioning processes between shots
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Capacitive probe Mirnov probe
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Measurements at multiple positions yields radial profiles
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T measured when 𝑰 > 𝑰
i 𝒔𝒂𝒕 𝒆𝒔
   
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