NIO2BEAM. Sono più strettamente di competenza NIO2BEAM:

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1 NIO2BEAM Si propone di simulare e misurare le caratteristiche di una matrice di fascetti di ioni negativi H -, contribuendo anche allo sviluppo di una sorgente a radiofrequenza di media potenza [NIO1 (Negative Ion Optimization 1st step), 100 ma, 60 kv, 9 beamlets di 12 ma] presso RFX (Padova). Tale sorgente è fondamentale per acquisire competenze nella costruzione di iniettori di fasci per reattori termonucleari a fusione (ITER e successivi). Sono più strettamente di competenza NIO2BEAM: 1) lo sviluppo di codici PIC-MC per la simulazione dell estrazione di ioni negativi dalla sorgente; 2) la costruzione di un misuratore di emittanza veloce (tipo Allison scanner) per verificare la sorgente e validare i risultati dei codici ; 3) l'adattamento del sistema RF a carichi variabili. Durata temporale esperimento: 3 anni [ ] Responsabile Nazionale: M. Cavenago (1R, LNL) Sezioni coinvolte: LNL, MI

2 Idea principale: comprensione dei meccanismi di base della formazione ed estrazione di ioni negativi (N-NBI: mezzo per riscaldare un plasma in futuri reattori a fusione termonucleare controllata). Central Solenoid Blanket Module Outer Intercoil Structure Vacuum Vessel Toroidal Field Coil Cryostat Port Plug (EC Heating) Poloidal Field Coil Divertor Machine Gravity Supports Torus Cryopump

3 Negative ion sources for neutral beam systems magnetically confined fusion plasmas are heated by neutral beam injection (>> 150 kev D + ); cannot accelerate neutral atoms; accelerate H + then neutralize by charge exchange inefficient at >100 kev; however, with H -, the neutralization efficiency remains high out to 500 kev; now use negative-ion based neutral beam systems capable of producing multiampere beams of H and D negative ions. Accelerator Ion Neutraliser source Residual Ion Dump (RID) Plasma

4 Negative ion sources for neutral beam systems HV Bushing (-1 MV inside) 16.7 MW D 0 delivered to the ITER plasma Electrostatic RID (deflection of the residual charged beams) Neutralization to D 0 in the gas neutralizer D - ions accelerated with 1 MV Extraction of 40 A D - 5 m Negative ions D - are generated in the ion source (required 200 A/m2D-)

5 ITER NBI requirements Neutral beam injection is required since the beginning of ITER operation The NBI system consists of 2 (+1) beams for Auxiliary Heating and Current Drive Beam parameters: P=16.5MW, I=40A, V=1MV (to heat the core plasma), t pulse=3600s 2+1 NBI DNB tangential injection Plan view Vertical cross section view On/off axis injection by tilting the beam axis vertically

6 R&D issues In the development of large volume H ion sources, e.g., for N-NBI negative-neutral beam injection N-NBI into fusion plasmas, the key R&D issues are: 1) improvement of the production efficiency of H ions under low pressure condition, 2) development of large volume H ion sources with spatial uniformity, 3) optimization of the H extraction from the source, and 4) high energy acceleration and good beam optics. Details of the source of H- (blue): 1) neutralization with H+ (red); 2) volume production; 3) surface production. The plasma grid (PG) can have up to 1280 holes.

7 NIO1 design: 3 x 3 beamlet matrix, water cooling of intermediate electrodes, modularity of the ion source. Optical diagnostic ports are also provided.

8 NIO2BEAM: sviluppo di codici di simulazione (indagini preliminari) electrons (blue) and H- (red).

9 Emittance scanners The beam emittance is the six-dimensional distribution of all position coordinates along the three configuration space directions and the associated velocity coordinates. The emittance is typically projected into the two-dimensional subsets {x x }, {y y }, and {z z }. Transverse projections are experimentally accomplished with an entrance slit scanning along the x direction while sampling the beamlets of all y coordinates and vice versa. For each slit position the corresponding trajectory angle distribution, x or y, respectively, is determined from the downstream particle position distribution measured with a second slit or a set of equidistantly spaced probes. Each measured signal contains only a tiny fraction of the entire beam, making the signals small and sensitive to bias and noise.

10 Allison scanner Allison developed a scanner that measures the trajectory angle distribution with an electric sweep while a mechanical scan probes the particle position distribution. The figure shows the entrance slit passing a beamlet that is electrically swept across the exit slit to measure the trajectory angle distribution. Charged particles that pass through both slits are collected in a Faraday cup, which features secondary electron suppression. A grounded shield surrounds the entire assembly to intercept any charged particles not passing through the entrance slit.

11 NIO2BEAM: meters Maximumbeampowerishigh(7kW); beam divergence under 3-5 mrad is desired While the codes provide phase space plot in the (x,x') plane (also depending on plasma temperature), up to now measurement was performed only in (x,y) plane Phase space information will allow real benchmarking of codes Allison scanner concept is compact. With a fast electronics, cooling problems can be improved. Tentative parameters: distance 1 m separate (x,x') and (y,y') scanners +/- 40 mm x window +/- 22 (or 30) mrad x' window 3D Allison scanner design (E. Fagotti)

12 -V +V Deflection plates must avoid beam reflection and still give a uniform deflection: note the staircase and/or new lamellar design as shown above Adequate pumping must be provided, so that no secondary plasma is formed between the deflection plates.

13 NIO2BEAM: rf coupling A natural question arises in radiofrequency source design: what is the best frequency f? Usually: f=1 MHz Inductively Coupled Plasma (ICP) sources of IPP and ITER design; f=13.56 MHz (commercial Capacitatively Coupled sources for semicon industry). Systematic frequency scan are rare. fixed f 1)Typical rf circuit with fixed frequency generator: note the unconvenience of mechanical moving parts 3a) A relatively small source like NIO1 (ICP) offers the possibility of a systematic f scan (albeit at low power and with cap switching). 3b) After chosing f approximately, the rf circuit can be finalized: plasma is a strongly variable load, so that standard circuit (1) needs variable cap (expensive ad delicate). Our circuit (2) (developed for induction rf oven for ECRIS) can accomodate fair load variation. Furthermore, we are investigating an auxiliary load scheme (3) to compensate plasma. 2) rf circuit used with radiofrequency oven: note the use of fixed caps and the frequency tunability f +/- 10% aux load 3) Concept of a new system with an auxiliary load to compensate plasma load variation

14 PIC simulations The extractor used in the experiments consists of three electrodes, the plasma grid (PG), the extraction grid (EG) and the acceleration grid (AG). The electrodes have an extraction aperture. A pair of magnets that create a weak magnetic field near the PG is located in the EG.

15 PIC simulations Region close to the extraction opening modeled with a 2D slab geometry. The x axis is taken to be parallel to the direction of the H beam extracted from the aperture, while the y axis is parallel to the PG.

16 PIC simulations The motion of the charged particles H ions, H+ ions, and electrons in their selfconsistent electric field is calculated by the PIC method. The trajectories of the particles are calculated by numerically solving the equations of motion m j dv qj( ) dt = E + v B where m j,v, q j, E and B are the charged particle mass, velocity, charge, electric field, and magnetic field for the particles of species j, respectively. To obtain the charge density on a grid point, a particle charge is assigned to each grid point by linear interpolation. The particle densities n j are also calculated from the charge densities on the grid points. Poisson s equation for the electrostatic potential 2 φ = H e en ( n n )/ ε + H 0 is solved by the finite difference method, where ε 0 is the permittivity of free space.

17 PIC simulations Parametri geometrici (dimensioni lineari sorgente, PG, EG, AG); Potenziali applicati (PG, EG, AG); Condizioni al contorno; Caricamento particelle nella zona della sorgente; Distribuzione iniziale di velocità delle particelle; Temperatura delle diverse popolazioni; Frazione relativa delle diverse popolazioni (constraint di neutralità globale); Presenza o meno di un campo magnetico trsversale (particelle magnetizzate o non magnetizzate in presenza di campo non nullo); Re-iniezione o assorbimento di particelle alle diverse pareti; Effetti delle collisioni tra le diverse specie (Coulomb, charge-exchange); Effetti dei processi di produzione e distruzione di ioni H - attraverso metodi MC: mutual neutralization (H - +H + ->H+H), associative dissociation (H - +H->e+2H oppure e+h 2 ), electron detachment (H - +e->h+2e). Meccanismo e tassi di produzione degli ioni: volume production vs Cs-seeded surface production; Et cetera

18 Anagrafica NIO2BEAM 2009 LNL Ricercatore Qualifica % CAVENAGO Marco [RN] I Ric. INFN 70 FAGOTTI Enrico Ass. INFN 40 POGGI Marco I Tec. INFN 30 COMUNIAN Michele Ric. INFN 20 CANELLA Stefania I Tec. INFN 20 KULEVOY T. Ass. INFN 50 PETRENKO S. Ass. INFN 100 MI Ricercatore Qualifica % POZZOLI Roberto Univ. PO 20 DE LUCA Franca Univ. PA 10 ROME Massimiliano [RL] Univ. RU 20 BETTEGA Giovanni Ass. Unimi 20 Numero totale dei ricercatori 4 Ricercatori Full Time Equivalent 0.7 Numero totale dei ricercatori 7 Ricercatori Full Time Equivalent 3.3

19 Richieste Finanziarie NIO2BEAM 2009 (preliminary) LNL MI Missioni interne 2 1 Missioni estero Consumo Inventariabile 28 2 Apparati 5 Totale 50 9

20 Richieste Finanziarie NIO2BEAM (even more preliminary) MI ME CONSUMO INVENTARI ABILE APPARATI TOTALE 2009 LNL MI LNL MI LNL MI