Tracer-encapsulated pellet injector for plasma diagnosticsS. Sudo, I. Viniar, A. Lukin, P. Reznichenko, and A. Umov Citation: Review of Scientific Instruments 76, 053507 (2005); doi: 10.1063/1.1899323 View online: http://dx.doi.org/10.1063/1.1899323 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/76/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spectroscopic diagnostics for ablation cloud of tracer-encapsulated solid pellet in LHDa) Rev. Sci. Instrum. 79, 10F541 (2008); 10.1063/1.2957928 Iron K α measurement of LHD plasmas using a wide band and compact x-ray crystal spectrometer Rev. Sci. Instrum. 77, 10F328 (2006); 10.1063/1.2347622 High throughput ultrasoft x-ray polychromator for embedded impurity pellet injection studies Rev. Sci. Instrum. 76, 013508 (2005); 10.1063/1.1832191 Hydrogen-encapsulated impurity pellet injector for plasma diagnostics Rev. Sci. Instrum. 72, 2575 (2001); 10.1063/1.1368858 Tracer-encapsulated cryogenic pellet production for particle transport diagnostics Rev. Sci. Instrum. 68, 2717 (1997); 10.1063/1.1148185

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Tracer-encapsulated pellet injector for plasma diagnosticsS. Sudoa!

National Institute for Fusion Science, Oroshi-cho, 322-6, Toki, Gifu 509-5292, Japan

I. Viniar, A. Lukin, P. Reznichenko, and A. UmovPELIN Laboratory, Ltd., 2, Admiral Makarov Str., Moscow, 125212, Russia

sReceived 21 December 2004; accepted 27 February 2005; published online 22 April 2005d

An injector for making solid hydrogen pellets around impurity cores has been developed for plasmatransport study in large helical device. A technique has been employed for automatic loading carbonor polystyrene cores of 0.2 mm diameter from a gun magazine to a light-gas gun barrel. The injectoris equipped with a cryocooler and is able to form a 3.2 mm long and 3 mm diameter cylindrical solidhydrogen pellet at 7–8 K with an impurity core in its center within 6 min and to inject it in thelight-gas gun up to 1 km/s.© 2005 American Institute of Physics.fDOI: 10.1063/1.1899323g

I. INTRODUCTION

Knowledge of plasma transport is a key for achievinghigh performance plasmas confinement in fusion devices.Experimental data on laser ablation or injection of shell-freeimpurity pellets show an extended profile for plasma pertur-bation, making transport coefficients difficult to measure, es-pecially near the plasma column axis.1,2 Injection of impuritycores inserted into polystyrene capsules gives a local plasmaperturbation, however, some extra impurities appear due tocapsule evaporation and ionization.3 Professor S. Sudo cameup with a proposal for using pellets made from ice of bulkplasmas components, i.e., hydrogen isotopes, as capsules.4

When such pellet enters the plasma, the outer solid hydrogenlayer is ablated first, keeping the impurity core from ablationup to the plasma column axis. This results in an intensiveablation of the core providing the small localization of thedeposited tracer ions. Experimentally observed evolution ofthe impurity ions gives data for analytical calculation of dif-fusion coefficient of the bulk plasmas.

Two attempts were undertaken in order to introduce anapproach and tracer-encapsulated pellet injectors in plasmainvestigations. The reliability of a first injector was notenough due to problems with cores loading from a magazineinto hydrogen pellets. The reliability and technology of pelletformation and acceleration in a second injector was sufficientand efficient, however, proof-of-principle operation wasdemonstrated using steel balls. When metal cores werechanged to nonmetal carbon and polystyrene, some addi-tional problems appeared. More light cores were blown awayand were lost more easily in a loading unit. Besides, theinjector cooling system was based on liquid helium heat ex-changes, however, it requires a liquid helium flask to beplaced near the injector. It is not acceptable for experimentson large helical devicesLHDd, so an injector with a coolingsystem based on a cryocooler and technology for makingpellets with nonmetal cores has been developed for LHDplasma transport study.

II. TRACER-ENCAPSULATED PELLET FORMATIONTECHNIQUE

In the above-mentioned first injector a core was inputinto two halves of a hydrogen pellet, which was transportedto a light-gas gun barrel for injection. The basic concept ofanother technique of tracer-encapsulated pellet formationwas a decision to generate a solid hydrogen pellet just insidethe barrelsin situd, to whose axis before mounts an impuritycore. In other words, to condense hydrogen around the corerather than input the core into solid hydrogen. The core canbe moved to the barrel axis via vacuum pickup technique,where the core will be held at the top end of the injectionneedle, at the expense of no condensable gas continuouslysucked up through the needle. The same gas may be used topush the core out of the gun magazine. Finally, showingthrough the inside duct of the barrel, one can observe thepellet formation process in real time and obtain reliable in-formation about the presence of the impurity core inside thepellet. This drastically increases the reliability of injection,since an operator can see the pellet generated with an impu-rity core inside of it just before injection. In the event that thecore is not mounted on the barrel axis before formation ofthe solid hydrogen pellet around it, the operator can proceedwith the process of loading the next core from the magazinewithout hydrogen pellet formation. Such technology wasproposed, patented, and employed in the second injector.6,7

However, carbon and polystyrene cores are nonmetal mate-rials and they are easily taken off by suction gas to the barrel.So, a technology has been developed for such cores to beloaded.

The stages of tracer-encapsulated pellet formation arerepresented in Fig. 1. An injection needle is preset at the topof a gun magazine, where every impurity core is placed intoa separate cell. A small amount of hydrogen gas is slowlyadmitted into the barrel and condensed on the walls of apellet former creating a solid hydrogen shell, which is cooledto 8–9 K fFig. 1sadg. The needle comes down and is presetunder the cell with the next core to be loadedfFig. 1sbdg. Thegas flow blows through the magazine cell and pushes thecore out of it. Because the whole gas flow is sucked offadElectronic mail: sudo@nifs.ac.jp

REVIEW OF SCIENTIFIC INSTRUMENTS76, 053507s2005d

0034-6748/2005/76~5!/053507/7/$22.50 © 2005 American Institute of Physics76, 053507-1

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through the needle and the formed shell closed a channelthrough which the needle can enter the barrel, the core fallenout of the magazine cell hits the top of the needle, trying toget inside of it together with the gas flow. However, theneedle diameter is so chosen, that the core could not getthrough it, but closes the hole in the needle like a gag. Thetemperature of the pellet former is increased to 30 K and thehydrogen shell is evaporated being opened the channel intothe barrel. A weak gas flow leaks through an untight sealingof the needle hole to hold the core on the needle top duringits upward movement. The needle sets the core to the barrelaxis as shown in Fig. 1scd. The pellet former is cooled to 9 Konce more; at the time of hydrogen inflow, it condenses onthe pellet former walls, gradually filling up the whole of itscross section and even getting inside the needlefFig. 1sddg.The needle is pulled out of the pellet back to the relativelywarm magazine, leaving the core and an empty channel in-side fFig. 1sedg, and hydrogen, condensed in the top of theneedle, sublimates from the needle and recondenses again inthe channel formed by the extracted needle, filling it andcompleting the pellet formation as shown in Fig. 1sfd. Thus,the needle is used as a vacuum tweezers to capture and holdup a core and as a source of hydrogen to complete the pelletformation. The pellet, formed around the impurity core, islocated exactly in the injector barrel. It can be observedthrough the barrel duct subject to suitable illumination andavailability of an optical system with long-focus lens. Aftervisual confirmation of the presence of the impurity core in-side the pellet, it is possible to inject it into plasmas.

III. INJECTOR DESIGN

A cold head of a cryocooler, named cooler, is connectedto the pellet former and used to cool it down together with athermal screen as shown in a schematic diagram in Fig. 2.Connected to the pellet former are an injector barrel with aheater wound on its walls, a loading unit, incorporating aneedle movable by a step motor and a magazine including 50cells with impurity cores, two heaters for rough and finetemperature regulation, a tube to supply a propellant gasfrom a fast valve, with an optical window located on thebarrel axis to observe its bore and inner surface of the pelletformer using a video camera, named barrel camera. Con-

nected to the barrel end is an illumination set, incorporatinga mirror movable by an electromagnet and an infrared lightemission diode. A gate valve, to which a diagnostic chamberwith four optical windows is linked up, is attached to theillumination set. A laser with an expander and an optical fiberwith two slits located in a distance of 5.5 mm connected to aphotomultiplier, which is used as a photodetector, are placedon the axis of two windows to create a light barrier to theflying-out pellet. Mounted on the axis of the other two win-dows are another video camera, named pellet camera, and apulse flash to photograph the pellet in flight. A microphone isattached to the diagnostic chamber flange, to capture the pel-lets after acceleration. The injector pumping system consistsof two rotary vacuum pumps and one turbomolecular one.Cylinders of hydrogen for pellet formation and helium forpellet acceleration are connected to a gas supply system suit-able for automatic remote control. The gas supply systemconsists of six valves with pneumatic actuators controlled byelectromagnetic valves, two manual needle valves for accu-rate gas flow adjustment, and three pressure sensors.

IV. CONTROL SYSTEM

A control system provides remote operation of the injec-tor and includes two main parts:sid a programmable logiccontrollersPLCd with an analog inputs board and digital out-puts boards installed in a PLC rack for low-level control andsii d a personal computersPCd for high-level control and man-machine interface. The PLC and PC are connected by a serialinterface RS232 using a serial port COM1. A control soft-ware operates under OS Windows 2000. A video capture cardis used for visual feedback control.

A control program provides automatic operation of theloading unit, a temperature controller, and the gas supplysystem. A main program window is shown in Fig. 3. Thereare four virtual button and a core numerical identification onthe right side of the window: “load core,” “form pellet,”“injection,” and “standby.” When an operator presses the“load core” button, the loading unit automatically loads the

FIG. 1. Sequence of operations for pellet formation around an impuritycore.

FIG. 2. Schematic diagram of the injector.

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core whose number is preset in the frame of the core identi-fication. To form a pellet around the core loaded into thebarrel in automatic mode it is necessary to press the button“form pellet.” If one presses the button “injection” the injec-tor is waiting for an external pulse for immediate shooting.One can make injection manually using a button with a smallarrow in the fast valve voltage pictogram on the left side ofthe main window. The standby mode is used for keepinginjector in cold state for a long period.

Besides main four buttons for the injector automatic re-mote control there are some buttons and frames for manualcontrol and adjustment which are useful for checking theinjector performance. In particular, every valve can beopened manually by clicking on its pictogram. The illumina-tion set, the step motor of the loading unit, the barrel heater,etc., can be controlled manually also.

The control program uses a video signal to estimate theneedle position on the computer monitor and to control itsmovement. This visual feedback is employed for more intel-lectual step motor operation. On the computer monitor with apicture of the barrel bore, shown by the barrel camera, arectangular frame is set in such a way that its center justcoincides with the center of the barrel cross sectionfFig.4sadg. The needle is installed by the step motor at the maxi-mum possible top positionfFig. 4sbdg. The step motor pullsthe needle down, however, due to a mechanical backlash, theneedle starts to go down after some steps of the motor op-eration, which are counted and recorded by the program.When the needle starts to go down a command from PLCgives a task to go down for 1.5 mm and simultaneouslycounts the number of motor steps. The top needle position onthe monitor is calculated in pixels as the place of maximumgradient of intensity along the center vertical line, passedthrough the needle axis. The visual area should be lightenedwithout significant imperfections for accurate calculationsand correct needle positioning.

Counting the number of pixels, when the needle top goesdown for 1.5 mm, and takes into account the linear travel ofevery motor steps0.005 mmd, one can estimate the scale ofpixels per one motor step. After that, the needle position inrelation to the center of the rectangular frame as well as toany other position out of the visible area in the frame can becalculated. Further, the distance between the maximum topposition of the needle and the magazine is equal to 39 mm,so the distance between the center of the frame and themagazine with cores are also calculated. The sizes of cells

s0.25–0.27 mmd and its distribution in the magazine areknown and written in the program data file for every maga-zine.

To load a core the needle goes above the center andcomes exactly to the center from the upper side to escape themechanical backlash. Knowing the distance from the framecenter to the definite core to be loaded, the necessary numberof motor steps is calculated including steps to overcome thebacklash during the needle upward movement. The needlegoes down, captures the core, comes back, and stops forshort periods1 sd a little bit below the frame center. Theprogram determines the exact needle position on the visualarea and sends a new command to put the core just in thecenter of the frame. So, when the needle is visible on themonitor a picture analysis is employed with feedback to con-trol the needle movement. Step calculations without feed-back control are used for the needle movement outside thevisible frame.

To protect the needle from damages there are internalPLC limits for the step motor operation. The needle will bemoved down, if the temperature of the pellet former is above9 K, and moved up, if the temperature is above 16 K.

The control program allows using two different maga-zines with cores. To switch the current magazine one has toclick on the button “magazine number 1sor 2d.” To save thecomputer monitor image, including automatic savings everypredetermined time intervals1,2,3, and so on, in secondsd,one can use a special video panel, which appears on demand.

FIG. 3. Program window of the injector control system.

FIG. 4. Photos illustrating the needle positioning and its movement controlin the depicted rectangular frame inserted into the barrel cross-section.

053507-3 Tracer-encapsulated pellet injector Rev. Sci. Instrum. 76, 053507 ~2005!

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V. INJECTOR OPERATION

Before operation the gun magazine was loaded by thecarbon or polystyrene cores, as shown in Fig. 5, and mountedto the injector. The needle outer and inner diameters were0.60 and 0.15 mm, respectively, so that cores ranging from0.17 to 0.23 mm in size could not penetrate it. The coolerwith 1 W cooling power at 4.2 K was able to reduce thepellet former temperature from 293 to 6 K within 45 min.The temperature was measured with a Lake Shore sensor andregulated with a temperature controller and two heaters overthe range from 5 to 30 K due to the power release variation.The inside of the vacuum chamber containing the cryocoolerand the gun barrel is evacuated by a turbomolecular pump,while the inside of the loading unit and the pellet former isevacuated only by the rotary pump with adjusting the pres-sure as shown in Fig. 6.

The technology for the formation of solid hydrogen pel-lets around impurity cores involved the sequence of opera-tions shown in Fig. 7 with snapshots of capsule and pellettaken by charge-coupled device cameras.

The full cycle of tracer-encapsulated pellet formationand injection takes about 5–7 min. The typical temperatureand fuel gas pressure dependences versus time during forma-tion process and injection are shown in Fig. 6. Temperaturevariations are depicted by dash lines, pressure ones are givenby solid curves during the pellet formation and injection. Inthe beginning of the cycle there is a temperature jumps1d inorder to evaporate possible condensate from the barrel andloading unit. During this time the needle position is calcu-lated and it is moved to the beginning of the magazine. A

pressure jump to 30 mbarsad appears due to gas admissionand condensation into the pellet former during the tempera-ture stabilizations2d at 11 K. A solid hydrogen shell isformed and the channel between the barrel and magazine isclosed allowing the needle to move down and to capture acore from the magazine cell. Just after that, a temperaturejump s3d is initiated by heaters to evaporate the shell. Apressure jump to 0.62 barsbd is appeared due to shell evapo-ration and vapor and suction gas pumping through theneedle. The core is installed in the center of the barrel crosssection during this period. The temperature is reduced andmaintained near the 13 Ks4d for some seconds to sublimatea thin film formed from suction gas. This film is formedwhen the temperature is reduced from 15 to 13 K. At thisperiod hydrogen gas at 50 mbar is admitted into the barrelinstead of suction gasscd to keep the core on the needle. Thetemperature is reduced to 9 Ks5d; pumping of the hydrogengas is stopped and it is condensed inside the pellet former.When the pressure is increased to 30 mbarsdd the hydrogenadmission into the barrel is terminated. After condensationthe temperature is increased to 10–11 Ks6d in order to pullaway the needle from the pellet smoothly. The temperature isreduced to 7–8 K and a pressure jumpsed due to pellet shoot-ing is appeared. The cycle can be started again.

In case of malfunction in the automatic motor-aided corepositioning, the operator looking at the computer monitor isable to adjust its position by programming extra step motorrevolutions for the needle to move up or down. The time forone core to be loaded ranged from 70 to 130 s, depending onthe distance between the cell and the axis of the barrel bore.The capture and positioning of the core in the channel werereliable to 90%. To make the pellet length smaller, a heaterwound around the barrel and the propellant gas delivery tubeon both sides of the pellet former was used. The heatercaused the hydrogen frozen up on its walls to sublimate.

Prior to injection, the shell temperature was decreased to7–8 K to enhance the ice strength. The gate valve wasopened, and the fast valve admitted helium under a pressureof 2 MPa into the barrel. The gas caused the pellet with thecore to accelerate up to 1.0 km/s. Flying into the diagnosticchamber, the pellet intersected a light barrier. Two signalssent out from the photomultiplier were recorded in a digitaloscilloscope and one of them triggered a pulse flash to makea snapshot of the pellet in flight. Photographs of the pelletscontaining impurity cores and without any core are shown inFig. 8. The velocity of a pellet was estimated from the time ittook to travel a distance of 5.5 mm from one slit to anotherone of the optical fiber connected to the photomultiplier.Representative velocities ranged from 0.7 to 1.0 km/s.

The technology for the production of visually control-lable tracer-encapsulated pellets makes it possible to guaran-tee almost 100% presence of the impurity core at the pelletcenter prior to injection, since in its absence the operator willturn on the mode of loading a new core.

VI. DISCUSSION

The technology for the tracer-encapsulated pellet forma-tion as well as a suitable injector for introducing these pellets

FIG. 5. Carbon cores outsidesad and insidesbd the light-gas gun magazine.

FIG. 6. Temperature and pressure monitoring during pellet formation andinjection cycle: dash lines-temperature, solid curves-pressure.

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into a plasma have been developed and successfully testedbefore installation on LHD. Hydrogen pellets allow a non-metal impurity core to be introduced and provides experi-mental plasma transport characteristics in the LHD to a depthof about half the plasma small radius. A differential pumpingsystem with a guide tube for pellet transportation to the

plasma and cutting off a propellant gas has to be designedand installed between the injector and LHD. In continuationof this development a technology of tritium-encapsulateddeuterium pellets is under investigation. Such technology al-lows reduced tritium exhaust from the plasma column andtritium contamination of the reactor components. This

FIG. 7. Sequence of the tracer-encapsulated pellet operations with snapshots.

053507-5 Tracer-encapsulated pellet injector Rev. Sci. Instrum. 76, 053507 ~2005!

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FIG. 7. sContinuedd.

053507-6 Sudo et al. Rev. Sci. Instrum. 76, 053507 ~2005!

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prolongs a fusion device exploitation and reduces require-ments to the wall reactor materials.

ACKNOWLEDGMENTS

This work was done partly in the framework of theLIME program sGrants-in-Aid for Scientific Research No.

11210101 by the Research Promotion Bureau, Ministryof Education, Culture, Sports, Science and Technology,Japand.

1H. Kaneko, K. Kondo, O. Motojimaet al., Nucl. Fusion 27s7d, 1075s1987d.

2P. B. Parks, J. S. Leffler, and R. K. Fisher, Nucl. Fusion28s3d, 477s1988d.

3K. Khlopenkov and S. Sudo., Rev. Sci. Instrum.69, 3194s1998d.4S. Sudo, J. Plasma Fusion Res.69s11d, 1349s1993d.5S. Sudo, H. Itoh, and K. Khlopenkov, Rev. Sci. Instrum.68, 2717s1997d.6I. Viniar, P. Reznichenko, A. Lukinet al., Rev. Sci. Instrum.72, 2575s2001d.

7I. Viniar and S. Sudo, Patent RF No. 2,230,377s2004d in Russia.

FIG. 8. Snapshots of solid hydrogen pellets in flight withsad, sbd, scd andwithout sdd impurity cores inside.

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