Upgrade of the diagnostic neutral beam injector for the TCV tokamak

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1 Fusion Engineering and Design 66/68 (2003) 899/904 Upgrade of the diagnostic neutral beam injector for the TCV tokamak Alexander N. Karpushov a, *, G.F. Abdrashitov b, I.I. Averboukh b, P. Bosshard a, I. Condrea a, B.P. Duval a, A.A. Ivanov b, V.V. Kolmogorov b, J. Mlynar a, A. Perez a, I.V. Shikhovtsev b, A.N. Shukaev b, H. Weisen a a Centre de Recherches en Physique des Plasmas, Association EURATOM */Confédération Suisse, EPFL, 1015 Lausanne, Switzerland b Budker Institute of Nuclear Physics, , Novosibirsk, Russia Abstract A diagnostic neutral beam injector (DNBI) [CRPP report LRP 710/01, CRPP-EPFL, 2001; EPS Conf. Contr. Fusion Plasma Phys., 25A (2001) 365] has been installed on tokamak à configuration variable (TCV) [Plasma Phys. Control Fusion, 36 (1994) B277; Plasma Phys. Control Fusion, 43 (2001) A161; Plasma Phys. Control Fusion, to be published] for the purpose of providing local measurements of plasma ion temperature, velocity and impurity density by Charge exchange recombination spectroscopy (CXRS) [EPS Conf. Contr. Fusion Plasma Phys., 25A (2001) 365]. The system recently underwent a technical upgrade, which allowed to increase the full neutral beam current density by a factor of two (from 0.5 to 1 A at 52 kev injection energy) and to extend the operational range of the diagnostic. This was achieved by means of a new, larger ion source, with an increased extraction area and corresponding enhancements of the power supplies. # 2003 Elsevier B.V. All rights reserved. Keywords: Diagnostic neutral beam injector; TCV tokmak; CXRS 1. Experimental setup of the CXRS and DNBI on the TCV * Corresponding author. Tel.: / ; fax: / address: alexander.karpushov@epfl.ch (A.N. Karpushov). The diagnostic neutral beam injector (DNBI) [1], manufactured by the Budker Institute of Nuclear Physics (BINP) in Novosibirsk, was commissioned at the tokamak à configuration variable (TCV) Tokamak [3 /5] in It provides, together with Charge exchange recombination spectroscopy (CXRS), a diagnostic for a local measurement of the plasma ion temperature and rotation velocity from the impurity (carbon) line Doppler broadening and line shift [2,6]. The experimental setup is shown in Fig. 1. The full (/50 cm) DNB path in the plasma is imaged via a quartz vacuum window and two in-vessel adjustable relay mirrors onto a set of 16 optical fibres connected to a Czerny-Turner spectrometer. The effective radial spatial resolution, 1.5 /3 cm, is determined by the intersection of the viewing chords (only a subset is shown in the figure) with /03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi: /s (03)

2 900 A.N. Karpushov et al. / Fusion Engineering and Design 66/68 (2003) 899/904 Fig. 1. Experimental setup of the CXRS on TCV (horizontal cross-section). 1, TCV vacuum vessel; 2, beam axis; 3, thermocouple array at the inner wall; 4, plasma axis; 5, CXRS view lines for low-field plasma side; 6, CXRS diagnostic head. the flux surfaces at the beam position. The spectra are imaged onto a 2D detector equipped with a front-illuminated CCD from EEV, with a minimal readout time of 10 ms. Carbon being the most abundant impurity ( /3%), the transition of choice is CVI (n /8 0/7) at l/5291 Å. The modest power of the DNBI allows non-perturbative measurements, but implies low active CX signal intensities as compared with devices equipped with heating beams. Initially the low active to passive spectroscopic signal ratio (A/P / 25% for Žn e of 1/2/10 19 m 3 and A/P/5% for 5/10 19 m 3 ), together with poor photon statistics, severely limited the performance of CXRS on TCV. An upgrade of the optical system provided a S/N increase by a factor of /5 by enhancing the system throughput [6] (Fig. 5(A, B)). 2. DNBI characteristics The DNBI (Fig. 2) (similar to [7]) was designed for the operational energy range 20 /55 kev, initially with an equivalent beam current of the full energy component of 0.5 A at 50 kev, within a 10 cm diameter at the beam focus. The beam is injected at a toroidal angle of in the horizontal mid-plane. The beam dimensions and alignment were verified inside the TCV vessel by an array of thermocouples on the central column facing the DNB entrance port. The neutral particle density of beam fractions delivered to the plasma (calculated from the relative intensities of the Doppler-shifted H a lines) with full, 1/2, 1/3 and /1/18 energies was 40:35:20:5%, respectively). The plasma source (Fig. 3) is powered by /5 kw, 4.55 MHz RF generator. The ions are extraction and acceleration system consists of four molybdenum grids with a 4 m curvature for focussing at the plasma. The first (plasma) and the second (extracting) grids are power supplied by a high voltage modulator (max. 55 kv, 3.7 A), the third (accelerating) grid is biased at /450 V and the fourth (suppressing) grid is grounded. The neutralisation efficiency of the full energy fraction is about 50%. A bending magnet in the DNBI tank to separates the residual ions from the neutral beam particles and an insertable, segmented calorimeter is used to measurement of the beam profile. Two liquid He cryopumps, each with a pumping speed of 24 m 3 s 1, limit the pressure at the beam duct B/10 3 Pa during operation. For upgraded performance the diameter of the RF plasma chamber was increased from 10 to 12 cm, the extraction area diameter was increased from 72 to 92 mm and the number of cylindrical apertures in the grids was increased from 163 to 241. The full energy (52 kev) beam fraction was increased from 0.5 to /1 A and the total extracted ion current from 1.65 to 2.7 A. Optimisation of the RF plasma discharge resulted in a full energy fraction delivered to the plasma of /50%. The parameters of the beam before and after upgrade are listed in Table 1. The injector can operate either under the central TCV control system, or independently for tests and commissioning. For the latter, the injector is controlled by a local PC. A Java routine establishes the communication between the DNBI control software and the TCV control system. The DNBI is controlled by CAMAC modules,

3 A.N. Karpushov et al. / Fusion Engineering and Design 66/68 (2003) 899/ Fig. 2. Diagnostic neutral beam injector on TCV 1, ion source; 2, neutraliser; 3, ion source gate valve; 4, cryopumps; 5, magnetic separator with diaphragm; 6, vacuum vessel; 7, movable calorimeter; 8, TCV gate valve. Table 1 DNBI parameters Parameter Before upgrades (1999) After upgrades (2002) Fig. 3. RF ion source. 1, gas puffing; 2, ignition; 3, permanent magnets; 4, water cooling; 5, plasma chamber; 6, RF antenna; 7, plasma grid; 8, extracting grid; 9, accelerating grid; 10, grounded grid; 11, isolator; 12, neutraliser. with the timing and other beam parameters pre-set by the DNBI control software, with values supplied by the TCV central control system. A code was developed to calculate the beam profile and attenuation for each beam fraction in the TCV plasma [8]. The beam attenuation can reach 80% at high density, as verified by thermocouple measurements on the TCV central column. Doppler-shifted H a emission measurement in the DNBI tank are used for monitoring and optimising the beam fractional energy composition (Fig. 4; Table 1). Beam energy (E 0 ) 20/50 kev Up to 53 kev Beam species Hydrogen or Hydrogen deuterium Ion beam current 1.65 A 2.7 A Full energy equivalent /0.5 A ]/1 A current Neutral beam species mix (E 0 :E 0 /2:E 0 /3:E 0 /18) (density) 40:35:20:5% 52:17:26:5%, 56:17:23:4% a Beam divergence 0.6/0.78 /0.78 Maximum overall pulse 2 s 1.8 s duration Beam ON-time range 1 ms/2 s 10/150 ms b Minimal beam OFF-time 2 ms ]/1.5 of ONtime Rise/fall time (10/90%) B/0.2 ms 2.5/0.25 ms The shortest time between 300 s beam pulses Pressure at the exit during operation B/10 5 mbar a For 0.2 ms DNBI pulse. b Optimised for 50 ms. 3. Application to CXRS ion temperature measurements The CXRS T i measurements on TCV are used to study the dependence of the energy confinement on the plasma shape and plasma behaviour in the

4 902 A.N. Karpushov et al. / Fusion Engineering and Design 66/68 (2003) 899/904 Fig. 4. H a emission: fundamental and three Doppler-shifted beam components. presence of additional ECR heating, especially in advanced scenarios with ITB creation by off-axis heating [6]. The DNBI and CXRS system upgrades now allow to measure the ion temperature profile for average plasma density up to 6/10 19 m 3 with an active/passive ratio ]/20%. The CX carbon VI 5291 Å line broadening before and after DNBI and CX detection system upgrades are shown in Fig. 5. The CX line intensity for ohmic discharges with different plasma densities are shown in Fig. 6. The active/passive signal ratio reaches 80% for TCV ohmic discharges with low (/10 19 m 3 ) density (Fig. 6(C)). The availability of the DNBI reached /80% of the TCV shots in 2002, with a reliability /90%. CXRS T i data are available for /250 TCV shots following the final DNBI upgrade in The integration time of typically 100 ms is adequate for stationary plasma conditions, as achieved in TCV advanced tokamak experiments. Fig. 5. Plasma CX measurements of CVI lines 5291 Å: before CXRS and DNBI upgrades, Žn e /3/10 19 m 3 ; after CXRS and before DNBI upgrades, Žn e /6/10 19 m 3 ; after CXRS and DNBI upgrades, Žn e /6/10 19 m 3, passive (circles points), active (stars), sum (squares) and active spectral fits (line).

5 A.N. Karpushov et al. / Fusion Engineering and Design 66/68 (2003) 899/ Fig. 6. CX CVI lines, 5291 Å intensity (solid lines) for TCV ohmic shots: after CXRS and before DNBI upgrades, Žn e /6/10 19 m 3 ; after CXRS and DNBI upgrades, Žn e /6/10 19 m 3 ; after CXRS and DNBI upgrades, Žn e /2/10 19 m 3, high elongated plasma; active signals (circles points) and extrapolation of the passive signals (squares). Since TCV does not use additional ion heating, there is a limit on the maximum injected power beyond which the ion temperature is significantly perturbed by the diagnostic. The DNBI-plasma interaction was studied with a neutral particle analyser (NPA). Its high-energy channels (sensitive to the neutrals with energy 3/8 kev escaping plasma) measure an increase of the CX flux correlated with DNBI operation. This was shown to be due to a fast hydrogen ion population resulting from the slowing-down of beam particles [9]. This investigation of fast ion relaxation has shown that in TCV ohmic TCV discharges, 70/ 90% of the DNB absorbed power is deposited on the electrons and only the remainder on the ions. The ion heating increases the ion temperature (measured using the NPA) by only 2/10%, which is regarded as negligible. 4. Outlook A further increase of the active to passive signal ratio of the CXRS measurements and an extension the operational range up to n e ]/10 20 m 3 requires an increase of the observed full energy fraction line brightness in the observation region without further increasing the injected power. The upgrades considered include the replacement of the RF DNBI plasma source by an arc source. This is expected to provide an increase of the density of the full energy beam fraction from /50 to /80%

6 904 A.N. Karpushov et al. / Fusion Engineering and Design 66/68 (2003) 899/904 at the cost of frequent replacements of the cathode. The observed beam brightness could also be increased by making the beam elliptical, using source grids with slits instead of the current arrangement of holes. Acknowledgements The authors acknowledge the support of the CRPP and BINP staff, who were involved in the upgrades described and the support of the Swiss National Science Foundation. References [1] J. Mlynar, A.N. Shukaev, P. Bosshard, B.P. Duval, A.A. Ivanov, M. Kollegov, V.V. Kolmogorov, V. Llobert, R.A. Pitts, H. Weisen, Diagnostic Neutral Beam injector at the TCV Tokamak, CRPP report LRP 710/01, CRPP-EPFL, Lausanne, [2] P. Bosshard, B.P. Duval, J. Mlynar, H. Weisen, 28th EPS Conference on Contr. Fusion Plasma Phys., Madera, Portugal, ECA Vol. 25A, 2001, pp. 365/368. [3] F. Hofmann, J.B. Lister, W. Anton, S. Barry, R. Behn, S. Bemel, G. Besson, et al., Plasma Phys. Control Fusion 36 (1994) B277/B287. [4] F. Hofmann, R. Behn, S Coda, T.P. Goodman, M. Henderson, P. Lavanchy, Ph. Marmillod, et al., Plasma Phys. Control Fusion 43 (2001) A161/A173. [5] J.-M. Moret, ECH physics and new operational regimes on TCV, Plasma Phys. Control. Fusion, 44 (2002) B85/B97. [6] P. Bosshard, B.P. Duval, A. Karpushov, J. Mlynar, 29th EPS Conference on Plasma Phys. Contr. Fusion, Montreux, ECA Vol. 26B 2002, P [7] A.A. Ivanov, V.I. Davydenko, P.P. Deichuli, A. Kreter, V.V. Mishagin, A.A. Podminagin, I.V. Shikhovtseu, B. Schweer, R. Uhlemann, Rev. Sci. Instrum. 71 (2002) 3728/ [8] J. Mlynar, TCV DNBI Profile and Attenuation Studies with Code Manual, CRPP report LRP 692/01, CRPP-EPFL, Lausanne, [9] A. Karpushov, P. Bosshard, B.P. Duval, J. Mlynar, 29th EPS Conference on Plasma Phys. Contr. Fusion, Montreux, ECA 26B (2002) P