Cyclotron Heating at B = 0.5 T in HSX K.M.Likin, A.F.Almagri, - PowerPoint PPT Presentation

Electron Cyclotron Heating at B = 0.5 T in HSX K.M.Likin, A.F.Almagri, D.T.Anderson, F.S.B.Anderson, J.Canik, C.Deng 2 , C.Domier 1 , H.J.Lu, J.Radder, S.P.Gerhardt, J.N.Talmadge, K.Zhai University of Wisconsin-Madison, USA 1 UC-Davis, USA; 2 UCLA, USA

RF Heating in HSX  Microwave power at 28 GHz 1 B  produces and heats the plasma at the 3 R second harmonic of w ce  Wave beam is launched from the low magnetic field side and is focused on E the magnetic axis with a spot size of k 4 cm B  Wave beam propagates almost along grad|B| and grad(n e ) that leads to a small ray refraction grad|B|  One can expect a sharp absorbed power profile because modB along R the beam axis is inverse to R 3

HSX configurations Normalized mod|B| along axis  QHS has a helical axis of symmetry and a very low level of neoclassical transport a.u.  Mirror configurations in HSX are produced with auxiliary coils in which an additional toroidal P in mirror term is added to the Toroidal angle, degrees magnetic field spectrum  In Mirror mode the term is added to the main field at the location of launching antenna and In anti-Mirror it is opposite to the main field  Predicted global neoclassical confinement is poor in both Mirror configurations

Ray Tracing Calculations 3-D Code is used to estimate absorption in HSX plasma First Pass Second Pass  First pass: small refraction because wave vector is almost parallel to grad|n| and grad|B| Z, m  Second pass: high ray Z, m refraction due to wide beam with 20 o divergence R, m R, m

Absorbed Power Profile (1)  Single-pass absorbed power profile is Absorbed Power Profile pretty narrow (< 0.2a p ) N e = 2·10 18 m -3 Absorption, %  Second Pass: Rays are reflected from T e (0) = 0.4 keV the wall and back into the plasma, the First pass Two passes absorption is up to 70% while the profile does not broaden Effective Plasma Radius  Absorption versus plasma density is Single-pass absorption calculated at constant T e in 1 Maxwellian plasma and based on the Absorption 0.8 T e from exp. TS and ECE data in bi-Maxwellian 0.6 0.4 plasma T e = 0.4 keV 0.2  Owing to high non-thermal electron 0 population at a low plasma density the 0 0.5 1 1.5 2 2.5 3 3.5 Line Average Density, 10 18 m -3 absorption can be high enough

Absorbed Power Profile (2) P in = 100 kW High N e : h = 50 % Absorption, % High N e : h = 50 % En, keV Low N e : h = 14 % Low N e : h = 14 % Effective Plasma Radius Effective Plasma Radius At low plasma density the energy that electrons can gain between collisions is higher than at high plasma density because high power per particle and longer collision time: p ( r )    abs En ( r ) ( r ) e n ( r ) e

Measurements of RF Power Absorption  Six absolutely calibrated microwave Top view detectors are installed around the HSX #3 #2 at 6  , 36  ,  70  and  100  (0.2 m, 0.9 #4 #1 m, 1.6 m and 2.6 m away from RF P in power launch port, respectively). #3 and #5, #4 and #6 are located symmetrically to the RF launch #6 #5 Attenuator Amplifier Each antenna is an open ended waveguide m w Detector Quartz followed by attenuator Window

Multi-Pass Absorption QHS Mirror 1 1 MD #1 MD #1 Absorption Absorption 0.8 0.8 MD #2 MD #2 MD #3 MD #3 0.6 0.6 MD #4 MD #4 0.4 0.4 0.2 0.2 0 0 0 1 2 3 4 0 1 2 3 4 Line Average Density, 10 18 m -3 Line Average Density, 10 18 m -3  RF Power is absorbed with high efficiency in a few passes through the plasma column in the wide range of plasma density  At low plasma density the efficiency remains high due to the absorption on super-thermal electrons, in QHS their population is higher than in Mirror

Neutral Gas Breakdown Motivation: (1) to study the particle confinement (2) to study the physics of plasma breakdown by X-wave at the second harmonic of w ce  Growth rate is determined 12000 QHS from exponential fit to the Growth rate, sec. -1 Mirror interferometer central chord anti-Mirror 8000 signal  In QHS mode the growth rate 4000 is twice as that in Mirror  In anti-Mirror mode the gas 0 breakdown occurs with a 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 Gas Pressure, Torr very low growth rate

Growth rate vs. RF electric field  In QHS mode the growth rate 12000 X-mode: 40 kW has been measured at different Growth rate, sec. -1 X-mode: 30 kW X-mode: 20 kW launched power levels. The 8000 O-mode: 40 kW growth rate drops with decreasing of RF power and its 4000 maximum is shifted towards lower gas pressure  With ordinary mode the growth 0 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 rate is similar to that with X- Gas Pressure, Torr mode at a low power level  High electric field in front of the launching antenna makes the gas to break down at higher rate

Plasma Density Scan Stored Energy  In both QHS and Mirror modes the 40 QHS 30 M ir r or stored energy is about 20 J at high W E , J 20 plasma density ( > 10 18 m -3 ) 10  Absorbed power is almost 0 0.0 0.5 1.0 1.5 2.0 2.5 Line average density, 10 18 m -3 independent of plasma density Absorbed and Radiated Power  Radiated power rises with plasma 40 density 30 P, kW P a bs- QHS P a bs- M ir r or 20  Energy confinement time is defined P r a d- QHS 10 P r a d- M ir r or from the experimental data: 0 0.0 0.5 1.0 1.5 2.0 2.5 W Line average density, 10 18 m -3   E Energy Confinement Time  2.0 E P P QHS abs rad  E , msec. 1.5 Mirror  At 1.9·10 18 m -3 the energy 1.0 confinement time is by a factor of 0.5 0.0 1.5 higher in QHS as compared to 0.0 0.5 1.0 1.5 2.0 2.5 Line average density, 10 18 m -3 Mirror

ASTRA Code  QHS thermal conductivity is dominated only by anomalous      neo anom transport: ASTRA:QHS e e e  A better model of anomalous transport in HSX is an Alcator-like dependency E r =0 (n e in units of 10 18 m -3 ): ASTRA: Mirror 10 . 35   2 / m s , e anom n e  T e (0) from Thomson scattering is roughly independent of density. Consistent with  ~ 1/n model.  Stored energy should have linear dependence on density but data clearly does not show this (see the previous slide).

Stored Energy Increases Linearly with Power  Fixed density of 1.5·10 18 m -3  Difference in stored energy between QHS and Mirror ISS95 scaling reflects 15% difference in volume  W ~ P in agreement with ASTRA:  ~ 1/n model Mirror ASTRA: QHS  At lower density, stored energy is greater than predicted by ASTRA code and TS disagrees with the model

ECE diagnostic on HSX 4-channel ECE radiometer is used to measure the electron temperature in HSX plasma: one channel is put on the high field side and 3 others on the low field side. At B=0.5 T (on-axis heating) the effective plasma radii in QHS mode are as follows: -0.2, 0.2, 0.24 and 0.5, respectively  All channels have been calibrated on a bench. In experiment, the ECE data have been benchmarked with respect to the Thomson Scattering

Electron Temperature in QHS  ECE temperature drops with plasma ECE Temperature 4.5 density. T ece at r = 0.2 at low and high 4 r = - 0.2 3.5 r = + 0.2 plasma density differs from each T ece , keV 3 r = + 0.24 other by a factor of 8 2.5 r = + 0.5 2  Electron temperatures measured by 1.5 1 Thomson Scattering and ECE are in a 0.5 0 good agreement only at high plasma 0 0.5 1 1.5 2 2.5 density (>1.7·10 18 m -3 ) Line average density, 10 18 m -3 T e profile at 1.9 ·10 18 m -3 ECE vs. TS at r = 0.2 4.5 1 4 ECE 0.8 ECE 3.5 TS 3 T e , keV T e , keV TS 0.6 2.5 2 0.4 1.5 1 0.2 0.5 0 0 0 0.5 1 1.5 2 2.5 0 0.2 0.4 0.6 0.8 Line average density, 10 18 m -3 Effective plasma radius

QHS vs Mirror  ECE temperature in QHS and ECE Temperature 7 Mirror configuration are almost 6 QHS the same except at very low plasma 5 Mirror T ece , keV density (<0.6·10 18 m -3 ) 4 3  At low plasma density due to a 2 better confinement of trapped 1 0 particles the electrons can gain 0 0.5 1 1.5 2 2.5 more energy in QHS mode than in Line average density, 10 18 m -3 Mirror

Bi-Maxwellian plasma Plasma Density Profiles  Model upon bi-Maxwellian 0.6 Bulk: n b ~ (1 – r 2 ) distribution function is used to n e , 10 18 m -3 explain the enhanced stored 0.4 Tail: n t ~ exp( – 4r 2 ) energy and the high absorption 0.2 efficiency at low plasma density 0  The density and temperature 0 0.2 0.4 0.6 0.8 1 profiles are taken from TS, ECE Effective plasma radius and interferometer measurements Electron Temperature Profiles  At 0.5·10 18 m -3 the plasma stored 6 Tail: T t ~ exp( – 4r 2 ) energy is 21 J due to super-thermal T e , keV 4 tail and 5 J due to bulk plasma and 2 the single-pass absorption is about Bulk: T b ~ exp( – 4r 2 ) 0.5 0  Corresponds to large hard X-ray 0 0.2 0.4 0.6 0.8 1 Effective plasma radius emission (poster by Abdou)

Stored Energy and ECE at Low Plasma Density  Diamagnetic loop shows the plasma energy crashes at low plasma density  ECE signals are in phase with the energy crashes  Also observed on soft X-ray emission (see poster by Sakaguchi)

Stored Energy and ECE at High Plasma Density  No stored energy crashes observed at high plasma density (>1.5 ·10 18 m -3 )  Crashes appear to be due to an instability on super-thermals