# Presentation: 471493

## presented 
by
Gary Taylor
In collaboration with
Cynthia Phillips, Phil Ryan,
Randy Wilson 
and 
 The NSTX RF Research Team

*presented *
*by*
- Gary Taylor
*In collaboration with*
- Cynthia Phillips, Phil Ryan,
- Randy Wilson
*and*
-  The NSTX RF Research Team

**NSTX Wave-Particle Physics, Heating and Current Drive ****5-Year Research Plan**

*NSTX 5-Year Research Plan Review Meeting*
*December 12-13, 2002*

## High Harmonic Fast Waves (HHFW)
5-Year Research Goals
Research Status:
		- HHFW System
		- Experimental Results
		- Theory & Modeling
5-Year Research Plan

Electron Bernstein Waves (EBW)
5-Year Research Goals
Research Status:
		- Mode Conversion Theory
		- Mode Conversion & Coupling Experiments
		- Technology Issues
5-Year Research Plan

- 5-Year Research Goals
- Research Status:
- 		- *HHFW System*
- 		- *Experimental Results*
- 		- *Theory & Modeling*
- 5-Year Research Plan
**Electron Bernstein Waves (EBW)**
- 5-Year Research Goals
- Research Status:
- 		- *Mode Conversion* *Theory*
- 		- *Mode Conversion & Coupling Experiments*
- 		- *Technology Issues*
- 5-Year Research Plan

**Outline**

## HHFW Research

## ST’s need auxiliary current drive (CD)

High  plasma makes Lower Hybrid and conventional electron cyclotron CD (ECCD) impossible

HHFW in high  plasmas has strong single pass absorption on electrons

			- can allow off-axis deposition

- High  plasma makes Lower Hybrid and conventional electron cyclotron CD (ECCD) impossible
- HHFW in high  plasmas has strong single pass absorption on electrons
- 			- *can allow off-axis deposition*

**HHFW Provide a Tool for Electron Heating & Current Drive in High ******** ST Plasmas**

## HHFW 5-Year Research Goals 

- HHFW 5-year research objective to provide heating & CD tools to supplement OH
- Enable preliminary assessment of ST performance
- 		- *HHFW-assisted startup*
*	**	**- Pressure profile modification *
- 		- *HHFW CD-assisted discharge sustainment*
- 10-Year goal to use HHFW, with other tools, for
-      pulse > skin operation

## Status of HHFW Research

## Utilizes TFTR ICRF transmitters & transmission line
30 MHz frequency corresponds to D = 9-13
6 MW total power from 6 transmitters for up to 5 s
12 Element antenna with active phase control allows wide range of wave spectra
			- kT = ± (3-14) m-1
			- can be varied during shot

- 30 MHz frequency corresponds to D = 9-13
- 6 MW total power from 6 transmitters for up to 5 s
- 12 Element antenna with active phase control allows wide range of wave spectra
- 			- *k**T** = ± (3-14) m**-1*
- 			- *can be varied during shot*

**Flexible System for High Power HHFW has been Installed on NSTX**

## HHFW 12 Element Antenna Array Provides Good Spectral Selectivity

**HHFW 12 Element Antenna Array Provides Good Spectral Selectivity**

-  Digital phase feedback system sets phase between straps
-  Antenna utilizes BN insulators to minimize RF sheaths

## HHFW Primarily Heats Electrons in NSTX, as Expected from Theory

- For non-NBI NSTX plasmas HHFW deposits all its power into electrons
- 		- *No experimental evidence for direct thermal ion heating*
- 		- *HHFW does heat NBI ions*
- Energy confinement on NSTX follows conventional scaling predictions when heat is applied via the electron channel
- Improved electron energy confinement has been observed

## HHFW Provides Strong Electron Heating

- Te(0)
- (keV)

- Ip
- (MA)

- 0

- 0.5

- 1.4

- 0

- TIME (s)

- PNBI
- (MW)

- PRF
- (MW)

-  Confinement consistent with predictions of standard scaling
-   *[L-Mode: ITER97L, H-Mode ITER98Pby2]*

- B = 0.44 T
**k**T = 14 m-1
- ne0 = 4.0x1019 m-3

- 0

- 4

## Some HHFW-Heated Discharges Display Behavior of Internal Transport Barrier

**Some HHFW-Heated Discharges Display Behavior of Internal Transport Barrier**

-  Te increases strongly
-   inside half radius
-  Density profile doesn’t
-   show change
-  Ti (0) rises with Te(0)
***e* progressively
-   decreases in the
-   central region

- Prf = 2.5 MW
- Ip = 800 kA

- Te
- (keV)

- 0

- 5

- 0.4

- 1.4

- R (m)

## Differences in Loop Voltage with Directed Spectra Consistent with HHFW CD

- Experiment performed at low electron  and current to maximize effect of HHFW CD on loop voltage
- Compare discharges with wave phased ± (3-7) m-1
- Adjust power levels and fueling to match density and temperature profiles
- Loop voltage differences seen when no central MHD (sawteeth, m=1)
- Driven current inferred from analysis of magnetic signals comparable to theoretical predictions

## Less Loop Voltage to Maintain IP With Co Phasing; Magnetic Signal Analysis Estimates Icd = 110 kA (0.05 A/W) 

- HHFW On

- Loop
- Voltage
- (V)

- 0

- 1.0

- TIME (s)

- 0.6

- 0.2

- kT = ±7 m-1

- V = 0.23 V

- Counter-CD

- Co-CD

-  TORIC Icd = 95 kA (0.05 A/W)
-  CURRAY Icd = 162 kA (0.08 A/W)

## CD Efficiency Consistent with DIII-D & TFTR CD Experiments

-  Operation at increased
-   Te required to meet
-   NSTX goals:
-       - *More RF power *
*        **and improved *
*        **confinement regime*
*        **should allow this*

- Trapping significantly reduces HHFW-driven current:
- 	- *Diamagnetic effects at high **** may*
*             **reduce trapping*

*C. Petty et al., Plasma Physics and Controlled Fusion ****43**** (2001) 1747*

## High  Poloidal H-Mode Plasmas Provide Excellent Candidate for Long Pulse Sustainment

-  Vloop ~ 0
-  ~ 40% bootstrap
-   fraction

## Evidence of HHFW Interaction with Fast Ions, as Predicted

- Damping on beam ions may reduce CD efficiency
- At high harmonic numbers (N ≥ 9) ion damping can be important due to large ki
- 		- *On NSTX k******i** ~ 10 for 80 keV beam ion*
- 		- *Damping maximum at ~ 35 keV for N = 9 *
- Neutral particle analyzer shows fast ion tail build-up during NBI + HHFW and decay after NBI turn-off
- 		- *D**+** tail extends to 130 keV*
*      **- Tail saturates in time during HHFW*

## Tail Reduced at Lower B, Higher 

- B0=4.5 kG
- B0=4.0 kG
- B0=3.5 kG

- beam injection
- energy

- B0=4.5 kG
- B0=4.0 kG
- B0=3.5 kG

**NBI**

**HHFW**

- NPA

**Tail Reduced at Lower B, Higher ******

- Larger e promotes greater off-axis electron absorption reducing power available to centralized fast ion population

## Further Code Development Needed to Model Interaction Between HHFW and Fast Neutral Beam Ions

- 1-D METS code generalized to model wave propagation and absorption with significant non-thermal population
- Initial results indicate that beam distribution can be approximated by Maxwellian with same average energy
- Effect of beam anisotropy to be evaluated in 2003
- 2-D non-Maxwellian effects could also be important, may need to generalize 2-D codes to include non-Maxwellian species in dielectric tensor operator

## Modeling HHFW in NSTX Shows Electron Absorption Dominates; IBW Conversion Not Significant

- METS 1-D and AORSA 2-D all-order codes show no excitation for short wavelength modes in present NSTX
-     plasmas
- Further numerical studies needed to determine if IBW conversion is important at higher B fields and/or higher ion 
- WKB ray tracing codes may be applicable due to:
- 		-  absence of significant IBW conversion
- 		-  wavelength < equilibrium gradient scalelength

## Excellent Agreement Between Ray Tracing Codesfor HHFW Current Drive Experiments

**Excellent Agreement Between Ray Tracing Codes****for HHFW Current Drive Experiments**

- HPRT

- 1/2

- 

- Co

- 2.1 MW

- 1.1 MW

- CURRAY

- 2

- HPRT

- CURRAY

- 

- 

- 0.8

- 1/2

- 

- 0.8

- Counter

- 1.1 MW

- Counter

- Pe
- (MW/m3)

- Pe
- (MW/m3)

- 2

- Co

- 2.1 MW

-  AORSA & TORIC   predict similar   deposition to ray   tracing codes

## HHFW 5-Year Research Plan

## HHFW 5-Year Research Plan Focused On Evaluating the Effectiveness of HHFW as an ST Research Tool

-  HHFW research plan has five major components:
*	**- Study dependence of HHFW coupling on plasma *
*             **configuration & density*
*	**- Explore HHFW coupling with neutral beam heating*
*            *
*	**- Investigate CD and wave-particle interactions**	**    **	*
*	**- Develop solenoid-free plasma startup*
*	**- Improve technical performance of the HHFW system *

## Study Dependence of HHFW Coupling on Plasma Configuration & Density

- _2003-4_:
* **Explore coupling into double null discharge and vary inner  *
*  **and outer gaps; previously studied limiter and single null*
* **Study effect of increasing density on heating efficiency*
* **Density control will be explored over a wider range due to *
*  **improved fueling & wall conditioning*

## Explore HHFW Coupling with Neutral Beam Heating

- _2003-4_:
* **Modify internal inductance with early heating; reduce *
*  **volt-sec consumption and increase q(0)*
* **HHFW heating efficiency in presence of strong neutral*
*  **beam injection; dependence on target **** and density *
* **Study HHFW H-mode access*
- _2005-6_:
* **Initial feedback control of HHFW heating to maintain J(R) &*
*  **P(R); study off-axis deposition at high **** to broaden electron*
*  **pressure profile*

## Investigate CD and Wave-Particle Interactions - I

- _2003_:
* **Operate HHFW reliably at higher power levels, with*
*  **improved high voltage antenna feed*
- _2004_:
* **Measure J(R) with motional stark effect (MSE) diagnostic *
- _2005-6_:
* **Investigate dependence of CD efficiency on RF power,  *
*  **density, temperature and antenna phasing*
* **Explore reduction in off-axis CD efficiency due to trapping *
*  **and possible increase in CD efficiency due to diamagnetic*
*  **effect at high ***
*  *

## Investigate CD and Wave-Particle Interactions - II

- _2006_:
* **Feedback antenna phasing on MSE J(R) & rtEFIT*
- _2007-8_:
* **HHFW with full feedback control of antenna phase*
*  **using MSE LIF system to obtain real time J(R) & P(R)*
*  *

## Develop Solenoid-Free Plasma Startup

- _2004-5_:
* **Couple HHFW into Coaxial Helicity Injection (CHI) startup*
* **HHFW heating with CHI to develop bootstrap current*
*  *
* **HHFW CD phasing with CHI for direct current drive*
* **HHFW handoff to NBI*
- _2006-8_:
* **HHFW-assisted ramp to high ****pol*
* **Use HHFW to optimize flux consumption in high performance *
*  **plasmas*

## Improve Technical Performance of HHFW System

- _2003-4_:
* **Dedicated experiments to elucidate HHFW antenna power *
*  **limits & reliability issues*
- _2005_:
* **Possibly modify HHFW antenna to be double-end fed;*
*  **reduces voltage for same power & removes hard ground*
- _2006_:
* **If asymmetry in launch spectrum remains a problem for CD *
*  **may tilt antenna straps*

## EBW Research

## EBWs May Enable Local Heating, Current Drive and Te(R,t) Measurements on ST Plasmas

- Electron cyclotron heating, CD and radiometry not viable
-     for spherical torus (ST) plasmas, where pe >> ce
- EBWs propagate when pe >> ce and strongly absorb at EC resonances, allowing EBW heating, CD and radiometry in STs
- Local EBW heating and CD are potentially important for non-inductive startup and MHD suppression in an ST
- EBWs can couple to electromagnetic waves near the upper hybrid resonance (UHR) that surrounds ST plasmas

**Notes:**
Electron Bernstein waves may enable local heating, current drive and electron temperature profile measurements in high beta, spherical torus plasmas where we can’t use conventional electron cyclotron heating, current drive and radiometry because the electron plasma frequency is much bigger than the electron cyclotron frequency.

Electron Bernstein waves can propagate when the electron plasma frequency is much greater than the electron cyclotron frequency and they are strongly absorbed near electron cylotron resonances, so EBW radiometer  

The study of EBW emission can yield important information about the viability of EBW heating and current.

## EBW 5-Year Research Goals 

- 5-year research program has four goals:
- 	- *Demonstrate efficient coupling of X-mode or O- *
*      **mode waves to EBWs*
- 	- *Control spatial location where EBWs damp and *
*      **heat electrons*
-     - *Test EBW-assisted non-inductive current startup,*
*      **alone or in combination with HHFW and/or CHI*
*    **- Test suppression of neoclassical tearing modes*
*       **with EBW heating and/or current drive*
- Plan to install ~ 1 MW by 2006, ~ 3 MW by 2007

## Status of EBW Research

## EBW Experiments on CDX-U and NSTX Have Focused on Maximizing EBW Conversion to X-Mode (B-X)

- If *L**n** *is short at the UHR, EBW can tunnel to the fast X-mode: *EBW to X-mode conversion efficiency (C**BX**) very sensitive to L**n*

- Measurement of B-X emission evaluates the efficiency of the X-B process for heating and CD
- Mode conversion to the O-mode (B-X-O) also possible; studied on W-7AS and MAST

*EBW Emission *
*Frequency = 6 GHz*

- 0            1.0           2.0           3.0

- 0

- 100

*C**BX*
*(%)*

*L**n** (cm)*

**Notes:**
One important characteristic of the mode-converted EBW emission is the degree of localization of the emission source with respect to the major radius.

In order to determine the EBW source localization, we perturbed the CDX-U plasma temperature profile with a sequence short gas puffs. The modulation in the edge density and temperature was measured by a Langmuir probe (shown in green in the schematic cross section). The evolution of the edge electron temperature is shown here by the upper green plot. During a sequence of similar discharges the EBW radiometer receive frequency was scanned and the time of peak EBW radiation temperature,Trad, was measured. An example of the EBW Trad evolution is shown in red in the lower plot.

## On CDX-U, Limiter Shortened Ln  to 0.7cm, Increasing CBX to > 95%, in Good Agreement with Theory

## Need CBX > 80% for Viable EBW Heating and CD System on NSTX

- Measured *C**BX* < 5% for NSTX L-Mode plasmas,
-     10-15% during H-Modes
- Reproduce CDX-U experiments with local limiter on NSTX next year, for both B-X and B-X-O conversion
- Results from experiment on NSTX using HHFW antenna tiles to shorten *L**n* this year were very encouraging:
-               *- achieved C**BX** ≤ 50%*

## CBX Increased from 10% to 50% as Ln Shortened from 2 to 0.7 cm, Agreeing with Theory

*EBW Emission Frequency = 11.6 GHz*

-   Will attempt similar experiment with
-    O-Mode antenna next year

**Notes:**
Plotting the evolution of the EBW Trad at one emission frequency you can see that the emission level increases by about a factor of three at the L-H transition. Above I show the evolution of the Demission, you can see the sudden drop in recycling at the L to H transition. At the time shown by the vertical red line we acquired the density and temperature profile with a Thomson scattering system. Trad/Te increased to 15% at the L-H transition and gradually decreased to 10% as the edge recycling increased. At the time the Thomson scattering data was taken the density scale length at the upper hybrid resonance was about one centimeter implying a mode conversion efficiency of 11%, in good agreement with the Trad/Te value.

## EBW Heating and CD May Optimize Equilibrium for High  Plasmas by Suppressing MHD

- Trapped particle effects make high field side (HFS) EBW power deposition more attractive
- Greatest access to HFS for fundamental EBW frequencies
- EBW heating and current drive modeling with GENRAY ray tracing and CQL3D bounce-averaged Fokker-Planck codes

**Notes:**
With Oak Ridge National Laboratory we are planning EBW heating and current drive experiments on NSTX. We have started EBW coupling experiments with an existing 15kW, 18 GHz system.

We plan to upgrade the existing ORNL antenna with a local limiter in the near future.

In the long term we would like to install four 28 GHz gyrotrons which already exist at ORNL, these would be capable of delivering over 1 MW of pulsed power. Power would be launched from a translatable mirror launcher on the midplane.

## In ~ 20% NSTX Plasma, EBWCD Efficiency Comparable to ECCD and Very Localized

*1 MW 14.5 GHz RF at 5**o** above mid-plane, -0.1 < n**//  **< 0.1 *
*CD efficiency = 0.065 A/W, n**eo** = 3x10**19** m**-3**, T**eo** = 1keV*

- CD localization supports requirements
-     for NTM suppression

**Notes:**
For viable mode-converted EBW heating and current drive or for an EBW Te diagnostic it is essential to optimize and define Ln at the upper hybrid resonance. On CDX-U, we plan to use a multi-tipped Langmuir probe to continuously measure Ln at the antenna location. On NSTX, we will use microwave reflectometry to measure Ln. We are also going to use local limiters to define Ln at the EBW antenna on CDX-U.

## Status of EBW RF Source Technology

- Focus NSTX program is Bo ~ 0.4-0.5 T plasma operation requiring fundamental EBW RF source at ~ 15 GHz
- No long pulse, high power ~ 15 GHz sources
- Four 28 GHz, 350 kW, CW gyrotrons at ORNL might be retuned to operate at 15.3 GHz (~ 200kW/tube)
- 		- Retuning needs to be tested
- 		- Provides only ~ 800 kW with four tubes
- Prefer to develop new megawatt level ~ 15 GHz tube;
- 		- MIT proposes 800 kW tube with ~ 50% efficiency
- 		- MIT estimates 18-24 month development
- 		- Need to issue request for cost & schedule quote in    	  early 2003

**Notes:**
For viable mode-converted EBW heating and current drive or for an EBW Te diagnostic it is essential to optimize and define Ln at the upper hybrid resonance. On CDX-U, we plan to use a multi-tipped Langmuir probe to continuously measure Ln at the antenna location. On NSTX, we will use microwave reflectometry to measure Ln. We are also going to use local limiters to define Ln at the EBW antenna on CDX-U.

## Design Requirements for EBW RF Launcher

- EBW launcher design presently undefined
- Need well defined n// spectrum, good focusing and
-     some beam steering
- Use either focusing mirrors or phased 4-8 element array
- Polarization control by external waveguide or grooved mirrors
- Use local limiter to steepen *L**n* at the mode conversion layer for both X-B and O-X-B launch

**Notes:**
For viable mode-converted EBW heating and current drive or for an EBW Te diagnostic it is essential to optimize and define Ln at the upper hybrid resonance. On CDX-U, we plan to use a multi-tipped Langmuir probe to continuously measure Ln at the antenna location. On NSTX, we will use microwave reflectometry to measure Ln. We are also going to use local limiters to define Ln at the EBW antenna on CDX-U.

## EBW 5-Year Research Plan

## EBW Research in 2003

-  Complete GENRAY/CQL3D scoping study for NSTX
-  GENRAY/CQL3D modeling of EBW startup
-  Determine importance of relativistic effects in EBW
-   propagation & damping, and edge parametric instabilities
-  Complete conceptual design for EBW antenna
-  Request quote for ~ 1 MW, 15 GHz tube
-  MAST to test O-X-B heating

## EBW Research in 2004-5

-  Obtain ≥ 80% B-X and/or B-X-O conversion on NSTX
-  Complete design of 1-3 MW, 15 GHz EBW heating and
-   current drive system
-  Include radial transport effects in CQL3D modeling of EBW
-   current drive
-  Begin install of 1 MW, 15 GHz heating system

## EBW Research in 2006

-  Complete installation of 1MW, 15 GHz EBW heating and
-   CD system
-  Demonstrate coupling to EBW's with ~ 1 MW, 15 GHz
-  Study spatial control of electron heating by EBWs

## EBW Research in 2007-8

-  Begin experiments with 1-3 MW, 15 GHz
-  Demonstrate plasma current generation & control
-  Study plasma EBW startup
-  Investigate NTM suppression by EBW heating and/or CD

## HHFW and EBW Heating and CD Can Provide Tools to Enable Solenoid-Free ST Operation at High 

-  Strong HHFW electron heating seen, initial evidence for
-   HHFW CD and interaction between HHFW and NBI ions
-   observed
-  5-year goal to demonstrate HHFW-assisted startup, pressure
-   profile modification and HHFW CD-assisted sustainment
-  > 95 % B-X conversion attained on CDX-U, ~ 50% so far on
-   NSTX; plan to obtain > 80% conversion on NSTX
-  Modeling indicates efficient localized, off-axis, EBW CD is
-   possible on NSTX
-  Install ~ 1 MW EBW system by 2006, ~ 3 MW in 2007;
-   5-Year goal to test EBW startup and NTM suppression

## HHFW/EBW Tools

***IPPA: 5 year***

***IPPA: 10 yr***

- MSE CIF

*MSE CIF*

*MSE LIF (J, E**r**, P);polarimetry*

*Feedback with MSE, heating, CD, rtEFIT*

*CHERS 18 ch*

*MPTS 20 ch, 60 Hz*

*MPTS Upgrades*

*Edge Reflectometry*

*Fluctuations & wave deposition*

*HHFW Phase Control*

*7 MW NBI,*
*6 MW HHFW*

*Upgrade HHFW Feed*

*HHFW Strap*
*Reorientation*

*1 MW EBW                         3 MW EBW*

*7 MW NBI,*
*3 MW HHFW*

*CHERS 51 ch*

***HHFW/EBW Tools***

***HHFW/EBW Physics***

*HHFW heating : density, configuration dependence  6 MW HHFW, with NBI*

*HHFW CD V**loop*

*HHFW CD measure ∆J*

*HHFW CD, long pulse, ∆J feedback*

*HHFW:*****
*feedback with heating, CD*

*HHFW wave/particle interactions*

*EBW emissions & coupling*

*EBW  CD, NTM Control, Startup*

*CHI & HHFW    CHI, HHFW, NBI Ramp to high ****p*

*Optimize*
*Startup   *

*HHFW and P(r) modification*