# Presentation: 111282

## Measuring electron gyro-scale fluctuations in NSTX plasmas using a collective scattering system

- David R. Smith*Princeton Plasma Physics Lab*
- In collaboration with:
- E. Mazzucato and H.K. Park*Princeton Plasma Physics Lab*
- W. Lee*Pohang University*
- C.W. Domier and N.C. Luhmann, Jr.*University of California at Davis*
*Plasma Science & Technology Dept. Seminar*
*Princeton Plasma Physics Lab*
*December 7, 2007*

## Motivation
Does electron temperature gradient (ETG) turbulenceproduce meaningful electron thermal transport?

Principles of scattering measurements

The NSTX high-k scattering system
Core fluctuation diagnostic with high sensitivity,spatial localization, k-space selectivity for up tofive distinct wavenumbers, and steerable optics.

Initial measurements and research topics

- Motivation
- Does electron temperature gradient (ETG) turbulenceproduce meaningful electron thermal transport?
- Principles of scattering measurements
- The NSTX high-k scattering system
- Core fluctuation diagnostic with high sensitivity,spatial localization, k-space selectivity for up tofive distinct wavenumbers, and steerable optics.
- Initial measurements and research topics

## ETG turbulence andelectron thermal transport

## Transport sets the plasma size neededto attain fusion-relevant conditions

- Transport sets the plasma size neededto attain fusion-relevant conditions

- The perpendicular particle flux (Γ) andperpendicular energy flux (Q) are key transport quantities

- (species subscripts suppressed above)

- 0th moment:
- 1st moment (SS):
- 2nd moment:
- 3rd moment (SS):

- terms with substantialfluctuation contributions

## Fluctuation-induced transporttypically dominates other contributions

- Fluctuation-induced transporttypically dominates other contributions

- The phase between fluctuations is critical.
- When       leads       (non-adiabatic response),there is a net motion of particles down the density gradient.
- The story is the same for energy and     .

## }

- What drives electron thermal transport?

- ← long disregardeduntil recently

- ← low confidence

- [Doyle et al., NF 47, S18 (2007)]

- good confidence in our understanding

- well studied exp. & comp.

- note that fusion particles will preferentially heat electrons

- NSTX high-k

- ion gyro-scale
- turbulence

- electron gyro-scale
- turbulence

## Electron thermal transport is typicallythe dominant loss channel in NSTX plasmas

- Electron thermal transport is typicallythe dominant loss channel in NSTX plasmas

- Ion thermal transport is at or near neoclassical.
- It is widely held that ExB flow shear stabilizes ITG/TEM turbulence in NSTX.
- [see papers by Kaye (07, 06), Levinton (07), Stutman (06), LeBlanc (04), Synakowski (03), Bourdelle (03), Rewolt (96)]

- Stutman, APS 04

- Common thermal transport model:

- thermal conductivity

- Relation between
- energy flux Qs and heat flux qs:

## Drift-interchange modes are analogousto the Rayleigh-Taylor instability

- Generic
- drift-interchange
- growth rate:

- [G. Hammett, APS 2007 Review Talk]

- Drift-interchange modes are analogousto the Rayleigh-Taylor instability

- “Gravity” is ExB drift
- arising from charge
- separation due to
- B/curvature drifts

## Plasma turbulence is anisotropic with k  k

- Plasma turbulence is anisotropic with k  k

- [Candy, Waltz, and
- the GYRO team]

- Remember this for later...

## ETG instability exhibits a critical gradient

- ETG critical gradient from linear gyrokinetic simulations using GS2[Jenko, PoP 8, 4096 (2001)]

- ETG critical gradient extendsto nonlinear saturation and electron thermal transport

- Horton, PoP 112600 (2004)

## ETG turbulence can saturate at higher amplitudethan ITG/TEM turbulence due to zonal flow dynamics

- drift-wave turbulence
- (ITG/TEM/ETG)

- zonal flows

- turbulent Reynolds stress

- ExB flow shear

- ETG turbulence generates zonal flows less efficiently thanITG/TEM turbulence because the adiabatic-ions in ETG turbulence
- have large gyro-orbits compared to ETG-driven zonal flows.
- Consequently, ETG turbulence saturates at higher amplitude
- and produces more normalized transport than ITG/TEM turbulence.
- [see Dorland, PRL (2000) and Nevins, PoP (2006)]

## Scattering measurementsof plasma turbulence

## The intersection of the probe beam and receiving beamdefines the scattering volume
The scattering angle provides k-space selectivity through the Bragg relation:  k = 2ki sin(/2)

- The intersection of the probe beam and receiving beamdefines the scattering volume
- The scattering angle provides k-space selectivity through the Bragg relation:  k = 2ki sin(/2)

## k = 2ki sin(/2)

- ki

- →

- ks

- →

- k

- →

- Energy conservation:
- ωs = ωi ± ω

- Bragg relation follows fromhigh frequency incident wave:
- ωi  ω ki ≈ ks and
- k = 2ki sin(/2)

- An incident EM wave (ki, ωi) and a plasma fluctuation (k, ω)couple to produce a scattered EM wave (ks, ωs).

- 

- + gives anti-Stokes interaction- gives Stokes interaction

- Conservation relations describe three-wave couplingamong EM waves and a plasma fluctuation

## Fluctuation spectrum can be reconstructedusing multiple detection channels

- The **plasma** performs the **Fourier transform**from real space to k-space

## Anisotropic turbulence and magnetic field variationcombine to improve spatial localization

- Anisotropic turbulence and magnetic field variationcombine to improve spatial localization

- see Mazzucato, PPCF 48, 1749 (2006)

- k’s and θ’s must satisfy:

- For scattering on the midplane, k’s are purely radial.Some k’s within the overlap volume produce scattered light (ks)that does not intersect the receiver, so the scattering volume is constricted.

## Density fluctuations encoded intoscattered power spectrum

- Density fluctuations encoded intoscattered power spectrum

- →

- scatteringvolume

- r

- observation
- point

- →

- ki

- derivedquantities:

- independentquantities:

- Scattered power due to a single, coherent fluctuation:

## Thomson scattering → EM wave scatteringdue to unbound electrons

- Rayleigh scattering → EM wave scattering due to bound electrons

- Scattering cross-section, σ, derives from the dipole radiation of an electron,
- with natural frequency ω0, driven by an EM wave with frequency ω:

- Classical electron radius:

## Incoherent Thomson scattering
Scattering wavelength → λ  λD    (λ=2π/k)
Random electron motion → incoherent radiation
Dipole powers additive → Ps  ne
Near-IR sources → λ ~ 1 m
Scattering angle → 90°
Useful for ne and Te measurements
Collective Thomson scattering
Scattering wavelength → λ  λD
Collective electron motion → coherent radiation
Dipole powers multiplicative → Ps  ne2
Far-IR to mm-wave sources → λ ~ 10 m to 4 mm
Scattering angles → 1° to 40°
Useful for density fluctuation measurements

- Two varieties of Thomson scattering:collective and incoherent

- Incoherent Thomson scattering
- Scattering wavelength → λ  λD    (λ=2π/k)
- Random electron motion → incoherent radiation
- Dipole powers additive → Ps  ne
- Near-IR sources → λ ~ 1 m
- Scattering angle → 90°
- Useful for ne and Te measurements
- Collective Thomson scattering
- Scattering wavelength → λ  λD
- Collective electron motion → coherent radiation
- Dipole powers multiplicative → Ps  ne2
- Far-IR to mm-wave sources → λ ~ 10 m to 4 mm
- Scattering angles → 1° to 40°
- Useful for density fluctuation measurements

## The NSTX high-k scattering system

## 280 GHz (=1 mm) scattering system
Instrumental minimum detectable fluctuation is ñe/ne~10-5
Five detection channels
k┴ spectrum at five discrete k┴
ω spectrum from time domain sampling
Tangential scattering
Probe and receiving beams nearly on equatorial midplane
System sensitive to radial fluctuations
Steerable optics
Scattering volume can be positioned throughout the outer half-plasma
First data during FY06 run campaign

- 280 GHz (=1 mm) scattering system
- Instrumental minimum detectable fluctuation is ñe/ne~10-5
- Five detection channels
- k┴ spectrum at five discrete k┴
- ω spectrum from time domain sampling
- Tangential scattering
- Probe and receiving beams nearly on equatorial midplane
- System sensitive to radial fluctuations
- Steerable optics
- Scattering volume can be positioned throughout the outer half-plasma
- First data during FY06 run campaign

- NSTX “high-k” scattering systemmeasures fluctuations up to k┴e  0.6

## Five detection channels can measurefluctuations at five discrete wavenumbers 

- Five detection channels can measurefluctuations at five discrete wavenumbers

|  | Ch. 1 | Ch. 2 | Ch. 3 | Ch. 4 | Ch. 5 |
| --- | --- | --- | --- | --- | --- |
| k (cm-1) | 21.4 | 17.1 | 12.8 | 9.0 | 5.3 |
| kρe | 0.31 | 0.25 | 0.19 | 0.13 | 0.08 |
| kρs | 18.9 | 15.2 | 11.6 | 8.2 | 4.8 |
| |kθ/kr| | 0.19 | 0.18 | 0.18 | 0.17 | 0.17 |

- Measurements at
- R = 142 cm  and  r/a = 0.79

- higher k,
- smaller λ

- lower k,
- larger λ

## Steerable optics enable good radial coverage

- Inboard  ρ = 0.05
- k┴ρe up to 0.7

- Outboard  ρ = 0.75
- k┴ρe up to 0.2

- Intermediate  ρ = 0.4
- k┴ρe up to 0.3

**Notes:**
Steerable launch and collection optics enable measurements throughout much of the minor radius, from rho=0.1 to 0.8.  The range in k┴ρe, however, changes due to different scattering angles and local plasma conditions.  On the left is the inboard launch configuration with tangencies near ρ=0.05.  In this configuration, the system can measure up to k┴ρe=0.7.  In the intermediate configuration, the probe beam is aimed below the collection mirror.  Here, the system measures fluctuations is the region where the ETG growth rates peak.  On the right is the outboard launch configuration with tangencies near ρ=0.8.  In this configuration, the system can measure up to k┴ρe=0.2.  

## Scattering system layout

- outside test cell
- ~100 mW at 280 GHz
- Overmoded, corrugated waveguide
- low-loss transmission
- Steerable optics
- quasi-optical design
- Heterodyne receiver
- five channels
- reference signal extracted from main beam

## Scattering system pictures

- heterodynereceiver

- collection
- mirror

- exit
- windows

- collection
- optics

- waveguide and launch optics

## 3 cm beam waist (1/e2 intensity radius)at the scattering location
k-space resolution  Δk ≈ 0.7 cm-1

- Quasi-optical design positions beam waist at scattering location and couples scattered light into receiver

- 3 cm beam waist (1/e2 intensity radius)at the scattering location
- k-space resolution  Δk ≈ 0.7 cm-1

## Quadrature heterodyne receiver samples at 7.5 MS/s 

## Ray tracing calculations are neededto design configurations prior to experiments 

- Ray tracing calculations are neededto design configurations prior to experiments

- Launch mirror (probe beam)Horizontal actuator: 1.117 in.Horizontal angle wrt X-axis:  83.0°Vertical actuator:  1.217 in.Vertical angle wrt midplane:  -6.0°
- Collection mirrorHorizontal actuator:  2.144 in.Horizontal angle wrt X-axis:  232.5°Vertical angle wrt midplane:  5.4°(Vertical angle not adjustable)

| Exit window angles |  |  |  |  |
| --- | --- | --- | --- | --- |
|  | Ch. 2 | Ch. 3 | Ch. 4 | Ch. 5 |
| Hor. Ang. | -6.4° | -3.9° | -0.7° | 2.5° |
| Vert. Ang. | -0.2° | -0.7° | -1.1° | -1.5° |

## Ray tracing calculations are neededto ensure k-space alignment 

- Ray tracing calculations are neededto ensure k-space alignment

|  | Ch. 2 | Ch. 3 | Ch. 4 | Ch. 5 |
| --- | --- | --- | --- | --- |
| r/a | 0.27 | 0.28 | 0.29 | 0.30 |
| dmin (cm) | 0.1 | 0.1 | 0.1 | 0.1 |
| k (cm-1) | 0.1 | 0.0 | 0.2 | 0.0 |
| kr (cm-1) | 6.9 | 11.0 | 14.6 | 17.8 |
| kθ (cm-1) | -1.6 | -3.4 | -4.4 | -5.5 |
| k (cm-1) | 7.1 | 11.5 | 15.2 | 18.6 |
| |kθ/kr| | 0.23 | 0.30 | 0.30 | 0.31 |
| kρe | 0.23 | 0.38 | 0.51 | 0.62 |
| kρs | 14 | 22 | 30 | 37 |
| kT (cm-1) | -0.4 | -0.7 | -1.2 | -1.3 |
| fD (MHz) | -0.95 | -1.78 | -2.98 | -3.26 |
| dtan (cm) | -14.5 | -18.4 | -23.5 | -25.5 |

- Data is not junk

- Electron diamag.direction

- Doppler shiftin ion direction

- Offset from
- PB tangency

## Ray tracing calculations are neededto determine scattering volume size 

- Ray tracing calculations are neededto determine scattering volume size

- beamoverlap

- instrument
- selectivitydue to k  k

## Ray tracing calculations are neededto account for refraction 

- Ray tracing calculations are neededto account for refraction

|  | Ch. 1 | Ch. 2 | Ch. 3 | Ch. 4 |
| --- | --- | --- | --- | --- |
| with refraction |  |  |  |  |
| r/a | 0.99 | 1.00 | 0.99 | 0.97 |
| k | 44.1 | 43.8 | 41.2 | 37.4 |
| without refraction |  |  |  |  |
| r/a | 0.81 | 0.82 | 0.82 | 0.84 |
| k | 14.8 | 10.7 | 6.9 | 3.3 |

- with refraction

- without refraction

## Ray tracing calculations are neededto discriminate good data from junk data 

- Ray tracing calculations are neededto discriminate good data from junk data

- Good data

- Mostly good data

- Junk data

## Ray tracing calculations are neededto interpret measurements 

- Ray tracing calculations are neededto interpret measurements

- The time evolution of turbulence parameters,such as kρe and kθ/kr, are needed for comparing data to simulations.

## Initial measurementsand research topics

## Stray light spectral peak and Doppler-shifted fluctuations are common features 

- Stray light spectral peak and Doppler-shifted fluctuations are common features

- High-k measurements at R  135 cm and r/a  0.68 with kρe ~ 0.2-0.3

- Fluctuations initially appear in the **electron direction,**
- then Doppler-shift to the **ion direction** due to VT**.**

## Prominent, persistent fluctuationsobserved in core

- Prominent, persistent fluctuationsobserved in core

- High-k measurements at R  113 cm and r/a  0.25kρe ~ 0.35-0.40 for channel 5

- Features appear in the ion direction due to Doppler shift

## Core fluctuations growwhen R/LTe is near critical value

- Core fluctuations growwhen R/LTe is near critical value

## Prominent, persistent fluctuationsobserved in outer-plasma

- Prominent, persistent fluctuationsobserved in outer-plasma

- High-k measurements at R  135 cm and r/a  0.7kρe ~ 0.1-0.3 for channel 2

- Ion/electron directions are reversed from previous slide.Fluctuations again experience a Doppler-shift to ion direction.

## Using only HHFW heating,unshifted fluctuations observed in core 

- Using only HHFW heating,unshifted fluctuations observed in core

- High-k measurements at R  120 cm and r/a  0.3kρe ~ 0.15-0.35 for channel 4

- Without toroidal rotation from NBI to produce a Doppler-shiftto the ion direction, fluctuations appear in electron direction.

## In some HHFW discharges, peak Te obtainedwhen R/Ln reverses and fluctuations drop 

- In some HHFW discharges, peak Te obtainedwhen R/Ln reverses and fluctuations drop

- High-k measurements at R  120 cm and r/a  0.3

- This is a preliminary observation; counter examples exist.

## Core fluctuations change characterafter RS collapse

- Core fluctuations change characterafter RS collapse

- High-k measurements at R  120 cm and r/a  0.35kρe ~ 0.15-0.25 for channel 4 and 0.25-0.35 for channel 5

- This is a preliminary observation due to stray light concerns.

## The NSTX high-k system is the only core fluctuationdiagnostic capable of addressing these questions:
k-space isotropy
Are fluctuations isotropic in the k-kr plane?
Steerable optics provide the capability to address this.
Mode coupling
What is the phase coherence among three turbulent fluctuations that satisfy frequency and wave vector matching conditions?
Multiple detection channels provide the capabilityto address this.

- The NSTX high-k system is the only core fluctuationdiagnostic capable of addressing these questions:
- k-space isotropy
- Are fluctuations isotropic in the k-kr plane?
- Steerable optics provide the capability to address this.
- Mode coupling
- What is the phase coherence among three turbulent fluctuations that satisfy frequency and wave vector matching conditions?
- Multiple detection channels provide the capabilityto address this.

## Transport is important
ETG turbulence may be important
Collective scattering is a powerful technique for measuring fluctuations with spatial localization andk-space selectivity
The NSTX high-k system is versatile due to steerable optics and multiple detection channels
Electron gyro-scale fluctuations have been observed exhibiting a variety of dynamics in a variety of NSTX plasmas

- Transport is important
- ETG turbulence may be important
- Collective scattering is a powerful technique for measuring fluctuations with spatial localization andk-space selectivity
- The NSTX high-k system is versatile due to steerable optics and multiple detection channels
- Electron gyro-scale fluctuations have been observed exhibiting a variety of dynamics in a variety of NSTX plasmas

## Special thanks to Ben LeBlanc, Ron Bell, Mike Bell, Russ Feder, Stan Kaye, Howard Yuh, Fred Levinton, Joel Hosea, Dave Mikkelson, Greg Hammett, and T.S. Hahm.