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

K.M.Likin, A.F.Almagri, D.T.Anderson, F.S.B.Anderson, J.Canik, C.Deng2, C.Domier1, H.J.Lu, J.Radder, S.P.Gerhardt, J.N.Talmadge, K.Zhai

University of Wisconsin-Madison, USA

1UC-Davis, USA; 2UCLA, USA

Electron Cyclotron Heating at B = 0.5 T in HSX

B E k

R

3

1 R B 

grad|B|

• Microwave power at 28 GHz

produces and heats the plasma at the second harmonic of wce

• Wave beam is launched from the low

magnetic field side and is focused on the magnetic axis with a spot size of 4 cm

• Wave beam propagates almost along

grad|B| and grad(ne) that leads to a small ray refraction

• One can expect a sharp absorbed

power profile because modB along the beam axis is inverse to R3

RF Heating in HSX

Pin Toroidal angle, degrees

a.u.

Normalized mod|B| along axis

HSX configurations

• QHS has a helical axis of

symmetry and a very low level of neoclassical transport

• Mirror configurations in HSX are

produced with auxiliary coils in which an additional toroidal mirror term is added to the 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

R, m Z, m R, m Z, m

First Pass Second Pass

Ray Tracing Calculations

• First pass: small refraction

because wave vector is almost parallel to grad|n| and grad|B|

• Second pass: high ray

refraction due to wide beam with 20o divergence

3-D Code is used to estimate absorption in HSX plasma

0.2 0.4 0.6 0.8 1 0.5 1 1.5 2 2.5 3 3.5

Te = 0.4 keV

Te from exp.

Line Average Density, 1018 m-3 Absorption

Single-pass absorption

Absorption, % Effective Plasma Radius

Ne = 2·1018 m-3 Te(0) = 0.4 keV

First pass Two passes Absorbed Power Profile

Absorbed Power Profile (1)

• Single-pass absorbed power profile is

pretty narrow (< 0.2ap)

• Second Pass: Rays are reflected from

the wall and back into the plasma, the absorption is up to 70% while the profile does not broaden

• Absorption versus plasma density is

calculated at constant Te in Maxwellian plasma and based on the TS and ECE data in bi-Maxwellian plasma

• Owing to high non-thermal electron

population at a low plasma density the absorption can be high enough

Absorbed Power Profile (2)

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:

Effective Plasma Radius

High Ne: h = 50 % Low Ne: h = 14 %

Absorption, % Effective Plasma Radius En, keV

High Ne: h = 50 % Low Ne: h = 14 % Pin = 100 kW

) ( ) ( ) ( ) ( r r n r p r En

e e abs

  

#5 Pin #1 #3 #2 #4 #6 Top view

Measurements of RF Power Absorption

• Six absolutely calibrated microwave

detectors are installed around the HSX at 6, 36, 70 and  100 (0.2 m, 0.9 m, 1.6 m and 2.6 m away from RF power launch port, respectively). #3 and #5, #4 and #6 are located symmetrically to the RF launch

Quartz Window mw Detector Amplifier Attenuator

Each antenna is an

• pen ended waveguide

followed by attenuator

Multi-Pass Absorption

• 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

0.2 0.4 0.6 0.8 1 1 2 3 4

MD #1 MD #2 MD #3 MD #4

QHS

Line Average Density, 1018 m-3 Absorption

0.2 0.4 0.6 0.8 1 1 2 3 4

MD #1 MD #2 MD #3 MD #4

Mirror

Line Average Density, 1018 m-3 Absorption

Neutral Gas Breakdown

• Growth rate is determined

from exponential fit to the interferometer central chord signal

• In QHS mode the growth rate

is twice as that in Mirror

• In anti-Mirror mode the gas

breakdown occurs with a very low growth rate Motivation: (1) to study the particle confinement (2) to study the physics of plasma breakdown by X-wave at the second harmonic of wce

Gas Pressure, Torr Growth rate, sec.-1

4000 8000 12000 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03

QHS Mirror anti-Mirror

Growth rate vs. RF electric field

• In QHS mode the growth rate

has been measured at different launched power levels. The growth rate drops with decreasing of RF power and its maximum is shifted towards lower gas pressure

• With ordinary mode the growth

rate is similar to that with X- mode at a low power level

Gas Pressure, Torr Growth rate, sec.-1

4000 8000 12000 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03

X-mode: 40 kW X-mode: 30 kW X-mode: 20 kW O-mode: 40 kW

• High electric field in front of the launching antenna makes the

gas to break down at higher rate

Plasma Density Scan

• In both QHS and Mirror modes the

stored energy is about 20 J at high plasma density ( > 1018 m-3)

• Absorbed power is almost

independent of plasma density

• Radiated power rises with plasma

density

• Energy confinement time is defined

from the experimental data:

• At 1.9·1018 m-3 the energy

confinement time is by a factor of 1.5 higher in QHS as compared to Mirror

0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 2.5 QHS Mirror

Line average density, 1018 m-3 Energy Confinement Time

E , msec.

10 20 30 40 0.0 0.5 1.0 1.5 2.0 2.5

P a bs- QHS P a bs- M ir r or P r a d- QHS P r a d- M ir r or

Line average density, 1018 m-3 P, kW Absorbed and Radiated Power

10 20 30 40 0.0 0.5 1.0 1.5 2.0 2.5

QHS M ir r or

Stored Energy Line average density, 1018 m-3 WE , J

rad abs E E

P P W   

• QHS thermal conductivity is

dominated only by anomalous transport:

• A better model of

anomalous transport in HSX is an Alcator-like dependency (ne in units of 1018 m-3):

ASTRA Code

anom e neo e e

    

s m ne

anom e

/ 35 . 10

2 ,

 

• Te(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).

ASTRA:QHS ASTRA: Mirror Er=0

Stored Energy Increases Linearly with Power

• Fixed density of 1.5·1018 m-3
• Difference in stored energy

between QHS and Mirror reflects 15% difference in volume

• W ~ P in agreement with

 ~ 1/n model

• At lower density, stored energy

is greater than predicted by ASTRA code and TS disagrees with the model

ISS95 scaling ASTRA: QHS ASTRA: Mirror

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

0.5 1 1.5 2 2.5 3 3.5 4 4.5 0.5 1 1.5 2 2.5

r = - 0.2 r = + 0.2 r = + 0.24 r = + 0.5

Line average density, 1018 m-3 Tece, keV

ECE Temperature

Electron Temperature in QHS

• ECE temperature drops with plasma
• density. Tece at r = 0.2 at low and high

plasma density differs from each

• ther by a factor of 8
• Electron temperatures measured by

Thomson Scattering and ECE are in a good agreement only at high plasma density (>1.7·1018 m-3)

Effective plasma radius

0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8

ECE TS

Te, keV

Te profile at 1.9 ·1018 m-3

0.5 1 1.5 2 2.5 3 3.5 4 4.5 0.5 1 1.5 2 2.5

ECE TS

Line average density, 1018 m-3 Te, keV

ECE vs. TS at r = 0.2

1 2 3 4 5 6 7 0.5 1 1.5 2 2.5

QHS Mirror

Line average density, 1018 m-3 Tece, keV

ECE Temperature

QHS vs Mirror

• ECE temperature in QHS and

Mirror configuration are almost the same except at very low plasma density (<0.6·1018 m-3)

• At low plasma density due to a

better confinement of trapped particles the electrons can gain more energy in QHS mode than in Mirror

Bi-Maxwellian plasma

• Model upon bi-Maxwellian

distribution function is used to explain the enhanced stored energy and the high absorption efficiency at low plasma density

• The density and temperature

profiles are taken from TS, ECE and interferometer measurements

• At 0.5·1018 m-3 the plasma stored

energy is 21 J due to super-thermal tail and 5 J due to bulk plasma and the single-pass absorption is about 0.5

• Corresponds to large hard X-ray

emission (poster by Abdou)

2 4 6 0.2 0.4 0.6 0.8 1 Te, keV Effective plasma radius Electron Temperature Profiles Tail: Tt ~ exp(–4r2) Bulk: Tb ~ exp(–4r2) 0.2 0.4 0.6 0.2 0.4 0.6 0.8 1 ne, 1018 m-3 Effective plasma radius Plasma Density Profiles Bulk: nb ~ (1 – r2) Tail: nt ~ exp(–4r2)

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 ·1018 m-3)

• Crashes appear to

be due to an instability on super-thermals

Stored Energy vs. Gas Puffing Location

• At low plasma density the stored energy strongly depends on

gas fueling

Top 180° T Middle 30° T Mini-flange 30° T Middle 180° T

Time, sec.

WE, J

Ne = 0.4·1018 m-3

• When the puffing valve is moved further away from the plasma axis, the

neutral density drops in the plasma centre where the resonant RF-electron interactions take place. Electrons then gain more energy between collisions because they suffer less scattering on neutrals.

Summary

• The microwave multi-pass absorption

efficiency is higher in QHS and Mirror (0.8-0.9) than in anti-Mirror (0.6)

• Density growth rates at breakdown clearly

indicate the difference in particle confinement in different magnetic configurations

• Electron temperature increases linearly with

absorbed power up to at least 600 eV

Summary (cont.)

• ECE and TS data are in a good agreement at

high plasma density

• At low plasma density the ECE radiometer

measures a high non-thermal electron population in QHS and Mirror configurations; higher signal for QHS

• ASTRA modeling shows the need for higher-

power, higher-density to observe differences in central electron temperature between Mirror and QHS