25-27 January 2006 Alcator C-Mod PAC Meeting 1

Lower Hybrid

Ron ParkerAlcator C-Mod PAC Meeting

25-27 January 2006

25-27 January 2006 Alcator C-Mod PAC Meeting 2

Goal of Lower Hybrid Current Drive Experiments

• Use Lower Hybrid Current Drive to supplement the bootstrap current in order to develop attractive steady-state regimes with:

- high bootstrap fraction (≥ 70%)- high βn (≤ 3)- good confinement (HITER89P ~ 2.5)

• Demonstrate such regimes can be sustained for several resistive diffusion times.

If successful, the results should provide a basis for advanced tokamak operation of ITER and for attractive tokamak reactors.

25-27 January 2006 Alcator C-Mod PAC Meeting 3

The Main Tools Are 8 MW ICRF and 3→4 MW LHRF RF Source Power

ICRF Heating LH Heating and Current Drive

Phase I Present Capability

Phase II

D- and E- Antennas

J-Antenna J- and E- Antennas

Frequency 80 MHz 50-80 MHz J: 50-80 MHz E: 80 MHz

Power 2 MW each 4 MW 4 MW each

Antenna 2 Straps each

4 Straps 4 Straps each

Phase I Present Capability

Phase II

Frequency 4.6 GHz 4.6 GHz

Power 3 MW 4 MW

Antenna 4X22 WaveguideGrill (1)

4X24 WaveguideGrills (2)

N⎢⎢ (Variable)

1.5-3 1.5-3

• LH Phase II is an upgrade that is planned to begin with design in 2006.

• Second LH antenna would be fabricated in FY 07 and installed in FY 08. Power upgrade requires incremental funding for FY 08 commissioning.

25-27 January 2006 Alcator C-Mod PAC Meeting 4

Early Modeling Using Accome Code1 Showed Potential of LHCD to Access AT Regimes in C-Mod

1P. T. Bonoli, R. R. Parker, M. Porkolab, et. al., Nucl. Fusion 40 (2000) 1251

Ideal MHD stable reversed-shear regimes could be found with fBS =0.74, fLH=0.24and fFW=0.01

25-27 January 2006 Alcator C-Mod PAC Meeting 5

Penetration and Absorption of LH Waves is Sensitive to n||.....

n||0=2.33

∆φ = 90°

°=∆ 90φfaghadj

n||0=3.09

∆φ = 120°

25-27 January 2006 Alcator C-Mod PAC Meeting 6

…..as is the Deposition of the Driven Current

202 kA

120 kA

N|| = 2.33N|| = 3.09

Pgrill = 2.0 MW

N|| = 2.33N|| = 3.09

Self-consistent modeling using GENRAY and CQL3D – Liptac, Bonoli, Harvey, APS 05

25-27 January 2006 Alcator C-Mod PAC Meeting 7

Amplitude and Phase of Each Klystron Drive Controls Phase Progression Along Grill and Therefore n||

KLYSTRON

low power phase shifter

24

1

9

3 dB splitter

10

#5

high power phase shifter

Common RF Oscillator

~

Antenna structure or “grill”

25-27 January 2006 Alcator C-Mod PAC Meeting 8

The Control System Sets Amplitude and Phase in Each Pair of Waveguide Columns with 1 ms Time Response

Each coupler and probe has 2 diodes and data channels: ~ 500 signals!

25-27 January 2006 Alcator C-Mod PAC Meeting 9

Power From 12 Klystrons (250 kW @ 4.6 GHz) in C-Mod Cell is Transported via Waveguides to the LH Launcher

25-27 January 2006 Alcator C-Mod PAC Meeting 10

The Launcher Divides RF Power From the 12 Klystrons into 96 Waveguides

Rear Waveguide Assembly

E planeTransformers22 to 5.5 mm

Front WaveguideAssembly

Microwave Windows

RF Gaskets

Couplers (4)

3 dB PowerSplitters

O-ringSeal

25-27 January 2006 Alcator C-Mod PAC Meeting 11

The LH Grill is Composed of Four Rows of Waveguides

Probes

Molylimiters

The LH antenna is composed offour rows of waveguides. The variation of the RF amplitude and phase along each row is the same.

Probes mounted in the face of the grillrecord the density at the grill mouth. These data are used to compare couplingefficiency with code calculations.

25-27 January 2006 Alcator C-Mod PAC Meeting 12

The Spectra in Each Waveguide Row are Nearly Identical

Fourier transform of E-field measured at mouth of rear waveguide assembly These curves are derived from

amplitudes and phases measured at the end of the Rear Waveguide Assembly relative to the input to the splitter network.

The measurements have recentlybeen extended to include theForward Waveguide Assembly and give nearly identical results.

The high power phase shifter was assumed to be fixed at 90°.

The spectra indicate the range of n|| achievable with ~ 1 ms time response.

25-27 January 2006 Alcator C-Mod PAC Meeting 13

Progress Since the Last PAC Meeting

• LH System installation completed and commissioned at the level of ~ 200 kW.

• Two dedicated run days to measure coupling efficiency vs. phase and density at grills.

• Although waveguides showed signs of conditioning to higher power, observation of excessive dust in grill interrupted experiments.

• After removal of launcher, substantial erosion of Ti coupler was found. dust was identified to be TiD2 or TiH2.

• R&D program initiated to fabricate grills from 304L SS. Launcherreinstalled with SS grills in mid-January.

• Extensive calibrations of all diode detectors (vs. power)

25-27 January 2006 Alcator C-Mod PAC Meeting 14

Efficient Coupling Observed Over Wide Range of Phase (n||)

0 1 2 3 4 50

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5Reflection Coefficient vs Density

Refle

ctio

n C

oef

ficie

nt

Density (1012 cm-3)

60o

90o

120o

135o

150o

180o

60 80 100 120 140 160 1800

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Phase (degrees)

Refle

ctio

n C

oef

ficie

nt

Ideal and Approximate PhasingIdealApprox.

Above: Measured reflection coeficientvs. phase and density.

n=3x1012 cm-3

(~ 1mm gap tolimiter)

Right: Calculated reflection coefficients for dn/dx~6x1012 cm-4 and no vacuum gap as functions of density and phase.

25-27 January 2006 Alcator C-Mod PAC Meeting 15

After 3 months in Alcator C-Mod, the Ti Grills Had Been Substantially Eroded

TiD2 dust formed by interaction with plasma and/or D2 – not RF.

Note erosion on sides of couplers.

25-27 January 2006 Alcator C-Mod PAC Meeting 16

Objectives for Upcoming Run Campaign

• Careful comparison of reflection coefficient with code predictions vs. density and phase at low to moderate power.

• Increase absorbed power to levels of at least 0.5 MW and hopefully higher, testing schemes for coupler protection.

• Produce and detect fast electrons, compare with theory regarding energy and spatial distribution – HXRs, ECE.

• Look for indications of driven current – e.g., from loop voltage changes, fast electrons, MSE, Alfvén cascades, sawtooth period, etc.

25-27 January 2006 Alcator C-Mod PAC Meeting 17

Distribution Function Tail and HXR Spectra areSensitive to n||

The low initial value of n|| for 90o means that the wave is resonant with higher velocity electrons, increasing the brems-strahlung emission in the higher energy channels relative to 120o. J. Liptac, APS 05

N|| = 2.33N|| = 3.09

25-27 January 2006 Alcator C-Mod PAC Meeting 18

An Imaging X-Ray Spectrometer Has Been Installed toDetect Fast Electron Bremsstrahlung (~ 30-200 keV)

Spatial resolution:Estimation: ∆r ~ 2a / #chords=1.4cm

Geometry: ∆r = (1 + D/d)ac ~ 1.7cm

ac=5mmad=5mm

C-Mod Cross section with pinhole camera

B-port

J. Liptac

D=123cm d=40cm

25-27 January 2006 Alcator C-Mod PAC Meeting 19

Phase II of the LHCD System Will Add 1 MW Source Power and an Additional Coupler

• With two couplers, and 4 MW total source power, the grill power densitywould be reduced to levels achieved for long pulses in JET.

• Additional flexibility provided by a second grill permits exploitation of theadvantages provided by “compound spectra.”

• Based on Phase I experience, simplified approaches to the launcher design will be investigated to facilitate window fabrication and reduce launcherlosses – an opportunity for developing and testing an ITER-relevant design.

• Phase II design will begin this year. Fabrication of antenna would be completed in FY 07 and installed in FY 08. Power upgrade requires incremental funding to come on line in FY 08.

25-27 January 2006 Alcator C-Mod PAC Meeting 20

Phase II Power Flux in the Grill Will be Reduced to Levels Achieved in Long Pulses at JET

0

2

4

6

8

10

0 5 10 15 20 25

power flux in the waveguides

f2b (GHz 2 cm)

kW/cm 2

breakdown weak conditioning

waveguide dimensions

6.0x0.55 cm 2

*

2.3 MW net power (12 Klystrons - 24 kW/guide)

1.5 MW net power (8 Klystrons - 15.5 kW/guide)

* JET 2 secJET, TS long pulse

PBX-M 0.5 sec

25-27 January 2006 Alcator C-Mod PAC Meeting 21

Compound Spectra Permits Further Control of the Driven Current Profile

287 kA

159 kA

290 kA

252 kA 189 kA

Pgrills = 2.7 MWPgrill = 2.7 MW

J. Liptac

25-27 January 2006 Alcator C-Mod PAC Meeting 22

Summary

• Lower Hybrid System has been commissioned and operated at ~ 200 kW level. High coupling efficiency was achieved.

• Experiments were interrupted due to strong interaction of Ti grills with H2/D2 and/or plasma. (Attempts to replicate interaction off-line so far unsuccessful.)

• Ti Grills have been replaced by SS grills and experiments will resume in the upcoming campaign.

• Near term goal is to raise power to at least 0.5 MW where non-thermal electron tails predicted to become detectable

• An upgrade is planned for a second antenna and an increase in source power to 4 MW – important in lowering the grill power density and exploiting benefits of compound spectra in enhancing efficiency and control of Jφ(r).