Neutral beam injector for active plasma spectroscopy

REVIEW OF SCIENTIFIC INSTRUMENTS

VOLUME 75, NUMBER 5

MAY 2004

Neutral beam injector for active plasma spectroscopy S. A. Korepanov, G. F. Abdrashitov,a) D. Beals, V. I. Davydenko, P. P. Deichuli,a) R. Granetz, A. A. Ivanov, V. V. Kolmogorov, V. V. Mishagin,b) M. Puiatti,a) B. Rowan, N. V. Stupishin, G. I. Shulzhenko,b) and M. Valisa Budker Institute of Nuclear Physics, 630090, Novosibirsk, Russia

共Presented on 12 September 2003; published 17 May 2004兲 A diagnostic beam system has been developed for the RFX reversed field pinch, Padova, Italy. Currently the system is loaned to Alcator C-mod, MIT, Boston. The system is primarily used for measurement of the ion temperature by charge-exchange recombination spectroscopy and for magnetic field measurements via motional Stark effect. The system comprises an ion source, beam duct equipped with vacuum pumps and various diagnostics of the beam. The ion source provides 50 keV, 5 A hydrogen beam. Ions are extracted from a plasma created by an arc-discharge source and, after accelerating and focusing, are neutralized in a gas target. A plasma emitter, which is formed by collisionless expansion of a plasma jet on to the grids, has low perpendicular ion temperature. These results are in rather low 共0.01 rad兲 angular divergence of the extracted ion beam. The grids of ion optical system are spherically curved providing geometric focusing of the beam at a distance 4 m. Current density at the focal plane reaches 100 mA/cm2. Arc-discharge plasma box provides highly ionized plasma, so that the extracted beam has about 80% of full energy specie. The injector provides 50 ms duration pulse each 5 min. In order to increase the signal to noise ratio the beam can be modulated with a frequency variable up to 250 Hz. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1699513兴

tion ⫽20 mc. The diagnostic injector based on arc-discharge plasma source with longer beam pulse and higher energy was developed recently for MSE diagnostic at RFX device, Padova 共now on loan to Alcator C-mod, MIT, Boston, MA兲. It has pulse duration 50 ms, beam current 5 A, particle energy 50 keV, and angular divergence ⬃0.5°. This paper contains the injector description and results of the measurements of the beam parameters.

I. INTRODUCTION

The hydrogen diagnostic beams are widely exploited in magnetic fusion devices, providing measurements of local parameters of the plasma and magnetic fields. Plasma density and temperature profiles, parameters of plasma turbulence, cross field transport, magnetic field value, etc., can be measured using the neutral beams. Charge exchange recombination spectroscopy 共CXRS兲 and motional Stark effect 共MSE兲 diagnostic in large plasma devices require a hydrogen beam with an energy ⫽50 keV. The equivalent neutral beam current should be about 1 A or higher to provide reasonable signals in the detection system. Besides that, beam species mix is critically important for the diagnostics. Neutral hydrogen beams usually contain atoms with three different energies: H(E), H(E/2), H(E/3), originating from atomic and molecular ions extracted from the ion source with the same kinetic energy E. The penetration depth of the lower energy species into the dense plasma core is significantly smaller. Therefore, higher full energy species fraction in a hydrogen beam enables to provide higher signals in the detection system. In the Budker Institute of Nuclear Physics, Novosibirsk a number of different diagnostic neutral beam injectors based on arc-discharge plasma source have been developed since 1975.1,2 These injectors are distinguished by a high 共up to 90%兲 proton fraction in the beam. The beams had an ion current up to 4 A, particle energy up to 40 keV, pulse dura-

II. INJECTOR BEAMLINE

The developed injector is based on an arc-discharge plasma source. The neutral beam is provided by extraction of ions from the plasma emitter, accelerating to the desired energy and subsequent neutralization by charge exchange in a gas target 共51% efficiency for 50 keV and hydrogen target兲 in the neutralizer tube. Remaining ions are deflected by a bending magnet and aimed on to a residual ion dump. The neutral beam is injected into a plasma under study. A retractable calorimeter for beam profile and position measurements is installed at the exit of the injector tank. In Fig. 1 the general layout of the diagnostic neutral beam injector is

a兲

Also at: MIT Plasma Science & Fusion Center, NW 17-186, 175 Albany Street, Cambridge, MA 02139. b兲 Also at: Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy. 0034-6748/2004/75(5)/1829/3/$20.00

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FIG. 1. Diagnostic neutral beam injector setup at RFX facility. 1829

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FIG. 4. Schematic of the typical wave forms of all main systems. FIG. 2. Arc-discharge ion source: 共1兲 soft metal case; 共2兲 gas valves; 共3兲 arc-discharge plasma generator; 共4兲 permanent magnets; 共5兲 ion optic system; 共6兲 ceramic spacers; 共7兲 grid cooling manifold.

shown as it is installed at the RFX device. The focal point of the diagnostic beam is at 4 m distance from the ion source and close to the plasma center. The vacuum system comprises two liquid–helium cryogenic pumps and one turbomolecular pump with 250 l/s pumping speed, which is used for initial pump down of the injector vessel and during regeneration of the cryogenic pumps. Each cryopump installed on the top of the injector tank has a measured hydrogen pumping speed of 24 000 l/s in the molecular flow regime. III. ION SOURCE

The ion source consists of an arc-discharge plasma source and a four grid electrostatic accelerator. It is shown in Fig. 2. A cold cathode arc-discharge plasma generator produces a highly ionized plasma jet. As a result of its collisionless expansion, the ion current density falls down to that required for optimal beam extraction. At the same time, the transverse ion temperature in the diverging plasma decreases, which results in a small beam divergence. The gas is introduced into the plasma box using two pulsed gas valves. The first gas valve is located near the cathode. It puffs the gas during quite small period of time. To initiate the arc discharge, a high voltage pulse is applied between special trigger electrode and cathode body. During the discharge, the gas is supplied by second gas valve located near anode. The plasma stream expands from the anode orifice into a cylindrical volume. To obtain homogeneous ion current density at

FIG. 3. Calculated ion beam trajectories.

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the plasma grid, the outer surface of it is covered by an array of Nd–Fe–B permanent magnets. The magnetic field strength at the inner wall of the expander is 0.2 T and falls down radially to less than 0.01 T at 2 cm distance from the wall. The plasma emitter for extraction of a 5 A ion beam has been obtained at discharge current of about 500 A. In the accelerator, there is a set of four nested grids with 421 circular apertures diameter 4 mm configured in a hexagonal pattern. The grids are mounted on the water-cooled flanges, making possible full heat removal between the pulses. In order to focus the beam on to the desired point inside the plasma, the grids are formed to be spherical segments with the 5 m curvature radius. A distinctive feature of the ion optic system 共IOS兲 is that all grids have equal hole step. It results in the difference of the beam formation on axis of IOS and at periphery of the grid. The calculations show that IOS with identical grids with radius of curvature R has the focal length F⬃0.8* R. Thus the focal length of the IOS is ⬃4 m. The geometry of the elementary cell was optimized by using 2D computer code AXCEL3 to obtain small angular divergence of the beam. The results of simulations with the AXCEL code are shown in Fig. 3. It is worthwhile to note that the accelerating electrode is combined of two grids. This increases effective thickness of the grid and results in formation of a negative potential barrier for back-streaming electrons at the electrode potential as small as ⫺200 V. IV. EXPERIMENTAL RESULTS

Main subsystems of the injector operate in a temporal sequence shown in Fig. 4. First, the gas valves are turned on starting to introduce the gas into the arc-discharge plasma box. About 20 ms later, a current in ‘‘magnetic insulation’’

FIG. 5. Oscilloscope display of the accelerating voltage and extracted ion current with 10 ms⫻10 ms modulation.

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FIG. 8. Spectrum of the H␣ lines emitted by 50 keV, 4.5 A beam. Species ⫹ density is 77% for H⫹ , 10.7% and 12.3% for H⫹ 2 and H3 , correspondingly.

FIG. 6. Measurements of beam size vs beam current at the 4 m distance from ion source.

coil starts. It creates longitudinal magnetic field along the arc-discharge channel that increases plasma output several times. Then arc current power supply is started. Several milliseconds later, an ignition pulse is applied. The arcdischarge begins at low level 共up to 250 A兲 to prevent high voltage 共HV兲 breakdown at the front of the HV pulse. The arc-discharge current is raised up to normal value 共500 A兲 with ⬃400 ␮s delay from high voltage start. The typical waveforms of the extracted current and accelerating voltage are shown in Fig. 5.

The beam profiles were measured by an array of the secondary emission detectors 共SEDs兲. The V-shaped dependence of the beam radius upon the beam current has been obtained near 4 m focal distance 共Fig. 6兲. The minimum beam radius corresponds to ⬃0.5° angular divergence. The segmented retractable calorimeter located at 2.2 m from ion source 共Fig. 1兲 is used to measure beam current density and beam position. The current density profile measured by the calorimeter is shown in Fig. 7. The ion beam species mix has been measured for 50 keV beam. The beam composition was analyzed by using H␣ Doppler shift spectroscopy. In the measurements we evaluated the beam species mix by taking into account the ratios of the integrated Doppler-shifted light peaks of full, half, and third energy components of the beam.4 The spectrum of the light emitted by the beam is shown in Fig. 8. The proton fraction amounts to ⬇80% in terms of the particle density in the beam. The injector is capable of providing a deuterium beam of correspondingly reduced current compared to the hydrogen beam. Switching the beam to deuterium and back to hydrogen requires only several shots to be done for grids conditioning. The beam can be modulated with frequency up to 250 Hz. Since 2002, the injector has been working at Alcator C-Mod tokamak 共MIT, Boston, MA兲 for MSE plasma spectroscopy and neutron diagnostics as well. 1

FIG. 7. Beam current density profile measured by segmented calorimeter located 2.2 m from the ion source.

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V. I. Davidenko, I. I. Morozov, and G. V. Roslyakov, Proton source for AMBAL facility, PTE, 1986, N6, c. 39– 42 共in Russian兲. 2 G. F. Abdrashitov, V. I. Davydenko, P. P. Deichuli, D. J. Den Hartrog, G. Fiksel, A. A. Ivanov, S. A. Korepanov, S. V. Murakhtin, and G. I. Shulzhenko, Rev. Sci. Instrum. 72, 594 共2001兲. 3 P. Spaedtke, Ing. Buro fur Naturwissenschaft und Programmentwicklung, AXCEL code, Junkernstrasse 99, D-65205 Wiesbaden, Germany. 4 A. A. Ivanov et al., Characterization of ion species mix of the textor diagnostic hydrogen beam injector with an RF and arc-discharge plasma box, preprint Budker INP 2002-41, Novosibirsk, 2002, 15p.