Plasma and Fusion Research: Regular Articles Volume 4, 002 (2009)

Development of Advanced Pellet Injector Systemsfor Plasma Fueling)

Ryuichi SAKAMOTO, Hiroshi YAMADA and LHD experimental groupNational Institute for Fusion Science, Toki, Gifu 509-5292, Japan

(Received 21 August 2008 / Accepted 25 November 2008)

Two types of solid hydrogen pellet injection systems have been developed, and plasma refueling experi-ments have been performed using these pellet injectors. One is an in-situ pipe-gun type pellet injector, which hasthe simplest design of all pellet injectors. This in-situ pipe-gun injector has 10 injection barrels, each of whichcan independently inject cylindrical solid hydrogen pellets (3.4 and 3.8 mm in diameter and length, respectively)at velocities up to 1,200 m/s. The other is a repetitive pellet injector with a screw extruder, which can form a3.0 mm solid hydrogen rod continuously at extrusion rates up to 55 mm/s. This extruder allows consecutivepellet injection up to 11 Hz without time limit. Both of these pellet injectors employ compact cryo-coolers tosolidify hydrogen; therefore, they can be operated using only electrical input instead of a complicated liquid he-lium supply system. In particular, using a combination of the repetitive pellet injector with cryo-coolers providesa steady-state capability with minimum maintenance.

c 2009 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: solid hydrogen pellet, fueling, cryo-cooler, pellet injector, steady state operation

DOI: 10.1585/pfr.4.002

1. IntroductionCore fueling will play a primary role in future fusion

reactors in which magnetically confined burning plasmacan be sustained by its own particle heating. Gas puff-ing, which provides fuel particles by blowing neutral gasonto a plasma surface, has been typically used for fuel-ing high-temperature plasma in toroidal magnetic fusionresearch. However, its capability is limited in large-scalehigh-temperature plasmas, since a thick, hot scrape-offlayer prevents penetration of the neutrals, and the particlescannot be provided efficiently to the core plasma. Pellet in-jection, which injects cryogenic hydrogen pellets into thecore plasma directly at high speeds (typically more than1,000 m/s), is cited as an alternative means of efficient corefueling. Pellet injection has been investigated since thepast thirty years [13], and many studies have indicatedthat pellet injection contributes to not only efficient fuel-ing but also improvements in plasma confinement proper-ties. In addition, the pellet injection has merits that be-come significant in a fusion reactor. High-efficiency corefueling enables minimization of the amount of tritium tobe used, and it may reduce the tritium inventory in a vac-uum vessel. Pellet injection can provide fuel particles tothe core plasma with negligible energy requirements com-pared with other core fueling options, such as neutral beaminjection (NBI). Despite these advantages, the pellet injec-tion is not included in the infrastructure facilities due to itslimited use in present plasma experiments and technolog-

authors e-mail: sakamoto@LHD.nifs.ac.jp) This article is based on the invited talk at the 24th JSPF Annual Meeting(2007, Himeji).

ical difficulties. In order to operate the pellet injector sys-tem as an infrastructure facility for plasma experiments, ahighly reliable and stable system is required. One of themajor difficulties in the current pellet injector system isthe use of liquid helium for hydrogen solidification. Theliquid helium usage causes reduction in the utilization ratedue to the need for its replenishment. Use of a closed-cyclehelium refrigerator is a valid solution [4], but it becomes arelatively large facility requiring considerable maintenanceand reduction in operational flexibility.

From the perspective of fueling for fusion reactors,demonstration of steady-state pellet injection is essential.Several research groups have already developed repetitivepellet injectors that use prepared solid hydrogen in advanceof the pellet injections. Plasma experiments with repetitivepellet injections have been conducted on JET [5], ASDEX-upgrade [6], DIII-D [7], and JT-60U [8, 9]. These injectorsperform adequately for previous plasma experiments, butare inadequate for fueling a fusion reactor due to the lim-ited quantity of solid hydrogen production. We have devel-oped pellet injection as one of the infrastructure facilitiesnot only for existing plasma experiments but also for futurefusion reactors. Highly reliable pellet injectors for stableoperation have been developed, and many successive im-provements have been made. The first pellet injector thatwe have developed is a conventional in-situ pipe-gun typepellet injector [10], and the second is a repetitive pellet in-jector with steady-state, low-maintenance injection capa-bility [11]. Both injectors employ compact cryo-coolers,which enable stable operation in absence of liquid helium.

c 2009 The Japan Society of PlasmaScience and Nuclear Fusion Research

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Plasma and Fusion Research: Regular Articles Volume 4, 002 (2009)

In particular, using a combination of the repetitive pelletinjector with cryo-coolers provides a steady-state capabil-ity with minimum maintenance.

In this paper, two pellet injectors developed at the Na-tional Institute for Fusion Science (NIFS) are described.The repetitive pellet injector is described in detail.

2. in-situ Pipe-Gun Type Pellet Injec-torIn order to produce a solid hydrogen pellet, a very low

temperature below the triple point (13.9 K for H2, 18.7 Kfor D2, 20.6 K for T2 [12]) is required. Liquid heliumcooling has been employed in all previous pellet injectors.Here, we employ Gifford-McMahon cycle compact cryo-coolers(SHI SRDK-415D), which utilizes electrical inputonly instead of liquid helium as new initiative in the world.

First, we developed a conventional in-situ pipe-guntype pellet injector in order to examine the adequacy of pel-let injection for fueling high-temperature plasmas. In thein-situ pipe-gun concept, a solid hydrogen pellet is directlyformed in an injection barrel, and then pneumatically ac-celerated by high-pressure propellant gas [13]. As there areno moving parts in this cryogenic system, stable and reli-able operation is possible. Although the in-situ pipe-gunconcept basically can inject a single pellet, this injector caninject 10 pellets using 10 parallel injection barrels.

Combination of the compact cryo-cooler and in-situpipe-gun concept results in stable and reliable operation.The pellet injector has demonstrated stable and reliableperformance on the Large Helical Device (LHD) duringthe 11 year experimental project. The experimental resultsindicated the efficacy of the pellet injection not only for

Fig. 1 System diagram of the repetitive pellet injector.

efficient fueling but also for plasma confinement improve-ment [14].

3. Development of Repetitive PelletInjectorImprovements of plasma confinement properties by

means of pellet fueling were reported in many previousstudies [1, 15]. However, these improvements were tran-sient and the sustainability of good plasma confinementproperties has to demonstrate in order to extrapolate thescenarios to a fusion reactor. We have developed a reactor-oriented repetitive pellet injector with steady-state opera-tion capability. Its key design features are as follows:

Cooling by compact cryo-coolers: The pellet injectoris operated using electrical input only, which is themost fundamental facility in a laboratory, instead of acomplicated liquid helium supply system. This designenables flexible and reliable operation. Screw extruder for solid hydrogen production: The

Screw-extruder can produce solid hydrogen rod viasimultaneous replenishment, liquefaction, and solidi-fication of hydrogen gas. This design enables steady-state operation in principle. Pneumatic pipe-gun pellet acceleration: This design

enables reliable and reproducible pellet injection witheasy injection-timing control.

A system diagram of the repetitive pellet injector isshown in Fig. 1. The repetitive pellet injector consists of agas supply system, a cryo system and a differential pump-ing system. All control objects (contact I/O, 100 ch;

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Plasma and Fusion Research: Regular Articles Volume 4, 002 (2009)

Fig. 2 (a) Conceptual drawing of the screw extruder and (b) photograph of the extruded solid hydrogen rod.

analog I/O, 30 ch) can be operated remotely, and auto-matically using a programmable logical controller (PLC)placed in the experimental hall. An operator can give di-rections to the PLC via a TCP/IP-based network using agraphical user interface program on a Windows-based PCplaced in the control room.

3.1 Solid hydrogen extrusion with screw ex-truder

In the previous solid hydrogen extruder, such as theORNL extruder [16], solid hydrogen is formed in the cryo-cylinder in advance, and then it is extruded from a nozzleby pressurizing with a piston. The usable amount of solidhydrogen is restricted by the capacity of the cryo-cylinderand the time required in preparing successive batches ofsolid hydrogen. On the other hand, the screw extrusionconcept, which was proposed by Mitsubishi Heavy Indus-try [17] and developed by PELIN Laboratory [18, 19], canproduce a solid hydrogen rod continuously. We adoptedthe screw extrusion concept as a key design of the repet-itive pellet injector. A conceptual drawing of the screwextruder is shown in Fig. 2 (a). Making use of our pre-vious experience on pellet injector development, we em-ployed two Gifford-McMahon cycle 4 K cryo-coolers tocool the cryo-cylinder. The total cooling capacity of thecryo-coolers is 20 W at 8 K. The lower part of a coppercryo-cylinder, whi