Fast Ferrite ICRF Matching System in Alcator C-Mod*

Transcription

1 Poster QP-00053, 48 th APS-DPP Annua Meeting, Phiadephia, PA, 006 Fast Ferrite ICRF Matching System in Acator C-Mod*. Lin, A. Binus, A. Parisot, S. Wukitch and the Acator C-Mod team MIT, Pasma Science and Fusion Center Cambridge, MA 039, USA *Work supported by US DoE Cooperative agreement DE-FC0-99ER545. Specia thanks to R. Murray, A. Pfeiffer, J. Stierman and P. Koert.

2 Basic Transmission Line Theory : votage refection coeffcient Z :impedance R jx :admittance G jb Z π A short stub ony has reactance : Z 0 j tan( βst, β λ A phase shifter : exp( ± jβ.' ' towards oad, ' ' towards transmitter S : votage standing wave ratio V R P max c net : maximum votage in the ine : equivaent antenna couping resistance : Couped power P forw P ref ps P P ref forw Z V Z max 0 0 S P net S, Z 0 S V Z 0 max 0 Z 50 ohm for C - Mod R c, with V max imited at < 40 kv

3 Present Matching System in C-ModC Stub Phase shifter (ine stretcher Transmitter 50 Ω coaxia ine dc dc Antenna Directiona couper # Directiona couper # Matched side (ow VSWR Un-matched side (high VSWR Goa: dc 0 (zero refection by adjusting the engths of the stub and phase shifter. Stubs and phase shifters are driven by sow step motors. Ony fit for between-discharge adjustment. Lengths are adjusted based on dc and dc from previous discharges. Therefore, the system functions poory in experiments with arge pasma parameter variation discharge by discharge. Reativey high power refection (i.e. dc may appear within a discharge, for exampe, during ELMs and L-H transitions. 3

4 Matching both L and H Modes It is difficut to keep the refection coefficient ow in both L and H modes because of the oad change, which is argey determined by the density pedesta profie in C-Mod. Poor match In this discharge, the match in H-modes was good, but poor in the L- mode period. Good match dc changed from 0.83 to 0.89 in L and H-mode, corresponding to R c from 5 Ohm to 3 Ohm. 4

5 Matching Edge Locaized Modes ELMs aso affect the antenna oading significanty. Good match Poor match In this discharge, we managed to match the ELMs, but rather high refection in other time periods. 5

6 Rea Time ICRF Matching Frequency moduation: Varying RF source frequency to change impedance Fast (miiseconds Requires a ong transmission ine Requires a wide-band response of the transmitters Tested on JET and LHD. Dieectric iquid: Pumping/fiing dieectric iquid in stubs and/or phase shifters to change eectrica ength Sow (seconds, but good for ong puse operation Probematic handing high power Instaed on LHD and HT-7 Ferrite materia: Varying magnetic fied on ferrite materia to change effective eectrica ength Fast (miiseconds Limited range of eectrica ength variation Tested on ASDEX-Upgrade 3. Being deveoped on SST- 4 A. Kaye et a, th RF topica conference, 997 G. Nomura et a, 4th RF topica conference, 00 3 F. Braun et a, Fusion Eng. and Design, 00 4 D. Bora et a, Nuc. Fusion, 006 6

7 Smith Chart From the ocation of the exampe point, we can get the foowing information: 0.j0.5: from horizonta and vertica coordinates 0.5: dot/dashed circes 6% P ref /P forw : % abes on the dotted circes (/( GjB 0.5j0.68: from constant G circe and constant B ines VSWR S ( /( 3. Dashed circe at VSWR~3 is the arc detection threshod. 7

8 Principe of Doube-stub Matching A stub transforms aong G constant circes (soid circes. The 3/8λ section between the two stubs moves aong a constant VSWR (or circe for 70 degrees. For a given with G <, there are two sets of soutions for the ength of the two stubs. No soution exists if is inside the G circe, i.e. G >. Short Stub # Short Stub # Transmitter st st,st 3/8 λ st Load/Antenna 8

9 Doube-Stub Fast Ferrite Tuning System 80 MHz RF In Tuners were made by Advanced Ferrite Technoogy in Germany. They were on ASDEX-upgrade for a short time period. 3/8 λ Tuner # Tuner # Power Suppies Digita Controer Bench test setup Tuner # Tuner # Transmitter dc,st st st,st 3/8 λ st st,dc dc dc Load/Antenna 9

10 Principe of Ferrite Tuner V V Z p p 0 µε and Z : Phase veocity : Line Impedance µ : Magnetic permeabiity ε : Dieectric constant 0 µ ε A combination of permanent magnets and magnetic fied cois is used for the magnetization of the ferrites. Their permeabiity can be varied around a set point by changing the magnetic fied created by the cois. The eectrica characteristic of the tuner to be varied dynamicay by changing the current in the cois 0

11 Ferrite Tuner Specifications Specification: Center frequency: 60 MHz (used at 80 MHz Input power: MW Max. circuating power: 0 MW Votage standoff: 70 kv peak (3 bar SF 6 Return oss at match: 0 db Matching range: max 0.95, a phases Effective eectrica ength can vary ~35 cm at 80 MHz for current range of ± 50 A.

12 Digita Contro Schematic Diagram Antenna Loading Variation Computer DC P forw, P ref, Φ dc Present L st and L st Init/trigger DC P forw, P ref, Φ Matching Change Ferrite Tuners dc Measured PS I and I L st and L st to get dc 0 Requested PS I and I System Parameters Data Storage MDS-Pus System Power Suppies Current demands Tuned PID Feedback PLC Remote Operation Computation normay takes <5 µs Set 00 µs per cyce to ensure numerica stabiity. Power suppy can sweep coi currents from -50 A to 50 A in 4 ms.

13 Cacuation from DC ony: dc DefineG and b / tan( β Stub Lengths to Match exp( jβ L Re(, st, st st, dc B L ±, Im( G transformation toward the transmitter L ( G, max G max G L / tan ( β st, st We have the soutions when G Stub #: Stub # : st st GL tan( β arctan β b tan( βst, arctan β B L L b < G max st : st, st G No knowedge of stub engths is needed. Good for a coarse ook-up tabe system. However, a feedback system has to act on dc. L 3

14 From dc DC to DC and vice versa exp( dc dc exp( to exp( dc j tan( β : jβ jβ j tan( β st st, st jβ dc, st st st, dc From dc dc dc to exp( exp( j j exp( tan( β dc tan( β : jβ jβ st st jβ st, dc st, st dc, st Because of some non-inearity in the ferrite response, we use the measured DC and DC to cacuate the stub engths. Then use these engths and DC to feedback contro the stub engths. 4

15 5 Stub Lengths from Stub Lengths from DC DC and and DC DC 0 0, and Write ( ( ( (, 8 3 and, tan(, tan( Write exp( exp(,,, W Q V P U PQ W V Q P U PQ jb a jb a j Q j Q j j j P Q P j j st st st st dc st dc st dc dc λ β β β β

16 6 Stub Lengths Cacuation Continued Stub Lengths Cacuation Continued arctan tan( arctan 4 tan(,, Write ( ( ( ( / / ( / ( and ( ( ( ( / / ( / ( Where P U Q W V Q P Q A AC B B Q W U U W C V U V U W W B V V A b a a b b b a a D D W D b a V D b a U b a a b b b a a D D W D b a V D b a U st st st st β β β β ±

17 Stub Lengths for a Pasma Discharge Additiona ines are needed to set the tuners at proper engths. The stub engths depend on the instaation ocation. The figure shows the stub engths required vs. the reative distance of tuner # to the present DC ocation. Soid ines indicates the average ength for the pasma. Dashed ine indicated the range of min and max engths. In the bench test, we used the foowing initia engths L st.55 m L st. m Seected because of dynamic range and simpe configuration. 7

18 Stub engths in Time Stub engths vs. time to achieve a perfect match for a pasma with ELMs 8

19 Time Varying Load Puse generator Stub Phase Shifter TTL controed 4- way RF switch 9

20 Initia Period (Time Traces Power refection coef. on the transmitter side Stub engths to get perfect match FFT current demands Power refection coef. on the antenna side Measured FFT coi currents Measured FFT coi votages Perfect matching was achieved in ess than.5 ms 0

21 Initia Period (Smith Chart Green: oad side Red: source side The system parameters were tuned to eiminate osciation.

22 L-H H Transition Simuation (Time Traces Power refection coef. on the transmitter side Stub engths to get perfect match FFT current demands Power refection coef. on the antenna side Measured FFT coi currents Measured FFT coi votages Perfect matching was re-gained in about ms after the transition.

23 L-H H Transition Simuation (Smith Chart Green: oad side Red: source side The power refection during the excursion caused by the transition was < 5%. 3

24 ELM Simuation (Time Traces Power refection coef. on the transmitter side Stub engths to get perfect match FFT current demands Power refection coef. on the antenna side Measured FFT coi currents Measured FFT coi votages Perfect matching was re-gained in about ms after the end of ELM. 4

25 ELM Simuation (Smith Chart Green: oad side Red: source side The power refection during the excursion caused by the ELM was < 5%. 5

26 Fu Range Test (Time Traces Power refection coef. on the transmitter side Stub engths to get perfect match FFT current demands Power refection coef. on the antenna side Measured FFT coi currents Measured FFT coi votages 6

27 Fu Range Test (Smith Chart Green: oad side Red: source side Expected dc range in pasma discharges The oad at times moved out of the designed parameter space, but the power refection was ow (<0% in a cases. 7

28 Summary A fast ferrite tuning system has been designed and tested successfuy on various scenarios at ow power. The FFT system is being instaed on the transmission ine of the E-port ICRF antenna. It wi run at high power in pasma discharges in the coming F 007 experimenta campaign. It wi significanty enhance the RF matching capabiity and work more robusty for L-H transitions, ELMs, and in experiments that have a arge range of pasma parameter scans. 8

29 Sign-up Name Emai 9