First Results From the Alcator C-Mod Lower Hybrid Experiment. R. Parker 1, N. Basse 1, W. Beck 1, S. Bernabei 2, R. Childs 1, N. Greenough 2, M. Grimes 1, D. Gwinn 1, J. Hosea 2, J. Irby 1, D. Johnson 1, A. Kanojia 1, B. LaBombard 1, J. Liptac 1, E. Marmar 1, M. Porkolab 1, D. Terry 1, J. Terry 1, G. Wallace 1, J.R. Wilson 2 1 Plasma Science and Fusion Center, MIT, Cambridge, MA, USA 2 Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA
First Results From the Alcator C-Mod Lower Hybrid Experiment LH power is coupled by a grill consisting of 4 poloidal rows of waveguides with each row composed of 24 waveguides in the toroidal direction. The grill is powered by 12 klystrons operating at 4.6 GHz and capable of 250 kw steady-state. Thus, 3 MW is available at the source; with transmission losses about 1.5 MW is available to be injected to the plasma. A second grill and 4 additional klystrons are planned to be added in an upgrade, bringing the total source power to 4 MW. A master oscillator provides inputs to 12 vector modulators whose phase- and amplitude-controlled outputs feed the inputs to the 12 klystrons. The n -spectrum can be varied dynamically during a plasma pulse over the range 1.5 < n < 3 with a response time of ~ 1 ms. An LHCD system has been implemented on Alcator C-Mod with the goal to investigate steady-state regimes with f BS ~ 70%, β n ~ 3 and H H ~ 1-2 for T pulse ~ 5 s > L/R > τ res.
The klystron output is divided 4 ways by conventional components before being applied with an E-plane transformer to sub-height (5.5 mm) waveguides in the launcher. The launcher consists of two sections of 24 stacked plates, each milled on one side to form four waveguides, and couplers that have the vacuum windows and form the plasma facing grill. RF power is divided by 2 in the rear section of the stacked-plate waveguides by means of a 3 db slot coupler. An H-plane transformer in the forward section of stacked-plate waveguides brings the waveguides to their final dimension of 5.5x60 mm 2. In the initial experiments, the couplers were EDM-machined out of solid blocks of titanium. Al 2 O 3 windows were brazed into the couplers and the ends were machined to conform to standard-shaped C-Mod plasmas. Molybdenum limiters were installed on each side of the couplers and the whole launcher could be moved from a position flush with the limiter to several cm s behind it.
In first experiments, relatively low power (100-200 kw) was applied for 10-20 ms and coupling measurements were made as a function of phase and coupler position. Forward and reflected power was monitored by 48 directional couplers located just before the E-plane transformer feeding the launcher. Reflected power from the grill was monitored by 48 probes in the launcher waveguides. During a discharge, the grill was visually monitored by a camera (exposure time of 2-3 ms, 30 frames/s.) Breakdown was observed even at relatively low power ( ~ 1 kw per guide). Plasma density at the grill mouth was monitored by 6 Langmuir probes, which indicated that close-to-optimum densities for coupling can be achieved by small (few mm) adjustments in the coupler position. A 32 channel X-ray camera has been installed for monitoring the position and energy distribution of the gamma rays produced by fast-electron Bremsstrahlung, however no significant emission has been observed in initial experiments.
Four Klystrons are Mounted in Each of Three Carts in the Alcator C-Mod Cell. Circulators (AFT) Protect Klystrons Klystrons 4.6 GHz, 250 kw CW Circulators 250 kw, 5 s 0.2 db Insertion loss
The Power From Each Klystron is Divided Eight Ways and Feeds Two Adjacent Columns of the Coupler 12 Klystrons 3 MW Source Power 1 Oscillator low power phase shifter 9 10 Attenuator KLYSTRON #5 high power phase shifter 3 db splitter 24 Electronic control of the drive to each klystron provides dynamic control of phase and amplitude applied to each pair of columns in coupler array. Conventional microwave circuitry divides klystron power into four waveguides. Remaining 2-way division takes place in the forward waveguide assembly.
Conventional Components Split Power 4 Ways 50 db Coupler Phase & Amplitude Control 2-Way Splitter Fixed High PWR Phase Shifter 2-Way Splitter (T) 50 db Couplers FWD & REV Launcher Power Monitor
The Launcher Consists of Three Main Components: Couplers and Front Waveguide and Rear Waveguide Assemblies Couplers (4) Viton Seal Front Waveguide Assembly H-plane Transformer 4.75 to 6 cm Rear Waveguide Assembly Microwave Window Gaskets 3 db Power Splitter E plane Transformer 22 to 5.5 mm
Final 2 Way Power Split is by Means of Slot Coupler Rear WG has internal 3 db splitters in each plate of each stack Simple one slot coupler was not satisfactory with shorted 4th port (too much precision required) Two slot coupler with added post has greater margin for error Rejected Power Injection to plasma Post added P input E-Plane Transformer From Splitter
The Forward Waveguide Assembly Consists of 25 Milled and Stacked 304 SS Plates Four slots are milled in one side of each plate. When stacked and bolted together, the channels formed by the slots of one plate pressed to the flat side of the adjacent plate form the waveguides. An H-plane transformer is incorporated, changing the width from that of WR 187 waveguide (4.75 cm) to 6 cm width of the coupler waveguides. Vacuum shroud
The Couplers are Fabricated From Blocks of Titanium by Wire Electrical Discharge Machining (EDM) 19 cm RF gasket Cooled vacuum flange Viton seal flange Titanium Waveguides Bolt sleeve Half-wave thick Al 2 O 3 windows are brazed into guides Vacuum barrier provided by viton seal between coupler and vacuum flanges. Stress on bolts maintained with sleeves to compensate for temperature excursions.
Brazing Al 2 O 3 Ceramic Windows into Titanium Waveguides Required Special Procedures Due to Difference in CTE s Stresses induced by cooling after braze reaches solidus temperature on cooldown induced cracks in outer windows Problem was solved by thinning walls at end windows and brazing thin ceramic strip to outside of window
The Couplers Are Protected by Molybdenum Limiters Probes
Vector Modulators and I/Q Detectors are Used to Control the RF Phases and Amplitudes in Each Pair of Waveguide Columns
Varying Phase in Adjacent Pairs of WG s Controls n Spectrum These curves are derived from amplitudes and phases measured at the end of the forward waveguide assembly. 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.
Optimal Densities Achieved by Inserting Grill to Within 1-2 mm of Limiter n Te
n Te
Breakdown was Observed at < 1 kw/ Guide But Improved With Conditioning
Arcs Occuring During Conditioning Produce Density Bursts at Grill n Te
Efficient Coupling Observed Over Wide Range of Phase (n ) Results agree qualitatively with code calculations (see next poster) but detailed comparisons are complicated by variations in density and gradient at grill face.
After 3 months in Alcator C-Mod, the Ti Grills Had Been Substantially Eroded Ti dust formed by interaction with plasma and/or D 2 not RF. Note erosion on sides of couplers.
The Mechanism Leading to Disintegration of the Couplers Has Not Been Identified Analysis shows that the dust is mostly Ti, probably TiD 2. Dust appeared after D2 backfills (p ~ 500 T) for Thomson calibration. Ti reacts with D 2 to form TiD 2, but the reaction is slow at low temperature. Attempt to reproduce phenomena at 60 C D 2 bake failed. Removal of oxide layer via plasma operation a factor? Ti has been used without incident in other machines, notably for divertor plates in ASDEX. ITER plans to use Ti for the flexible attachments used to attach blanket modules to the vessel will it be a problem? We are continuing to investigate mechanism for Ti disintegration in plasmas using glow and RF discharge sources but Stainless Steel will be used for replacement grills.
An Imaging X-Ray Spectrometer Has Been Installed to Detect Fast Electron Bremmstrahlung C-Mod Cross section with pinhole camera B-port a c =5mm a d =5mm Spatial resolution: Estimation: r ~ 2a / #chords=1.4cm Geometry: r = (1 + D/d)a c ~ 1.7cm D=123cm d=40cm
HXR Pinhole Camera for Detection of LH Driven Electron Profiles Shield and Mounting Structure Electronics Electronics Assembly CZT detectors to measure photons from 20-250 kev 32 channels: spatial resolution of ~1.5 cm Compact design at B-Hor midplane Integrated and modular electronics designed for high count rates Fast digitization and software processing techniques ~3.5 cm Pb gamma shielding Adjustable mount fixed to igloo All 32 channels are working and have been aligned and calibrated.
No Significant Fast Electron Bremsstrahlung for RF Power ~ 150 kw Modelling indicates weak tail formation at this power-see Liptac poster
Summary A Lower Hybrid Current Drive Experiment has been implemented on Alcator C-Mod. Up to 3 MW of source power is available at 4.6 GHz, for pulse lengths long compared to the resistive diffusion time. Power is coupled through a grill consisting of 4 poloidal rows of 24 waveguides with dimensions 0.55x6 cm 2. Phase control allows dynamic control of n with response time of 1 ms. Probes mounted in the grill indicate that optimum densities for coupling are easily achieved by adjusting the grill position. In initial experiments, breakdown in the vacuum side of the grill at a level of ~0.5 kw per guide was observed but showed signs of conditioning with successive pulses. However Ti disintegration due to interaction with D2 necessitated grill removal 3+ months after installation in Alcator C-Mod. A spatially imaging X-ray spectrometer has been installed to detect fast electron bremsstrahlung; no indications of fast electrons were observed in first low-power experiments.
Replacement launcher being fabricated from stainless steel. Experiments scheduled to resume in December.