Exhaust behavior of tritium from the large helicaldevice in the first deuterium plasma experiment
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Exhaust behavior of tritium from the Large Helical Device in the first
deuterium plasma experiment
Masahiro Tanakaa, b*, Naoyuki Suzukia, Hiromi KatoaaNational Institute for Fusion Science, National Institutes of Natural Sciences, 322-6

Oroshi, Toki, Gifu 509-5292, Japanb

The Graduate University for Advanced Studies, SOKENDAI, 322-6 Oroshi, Toki, Gifu

509-5292, Japan*corresponding author: Masahiro Tanaka, e-mail: tanaka.masahiro@nifs.ac.jp, National
Institute for Fusion Science, National Institutes of Natural Sciences, 322-6 Oroshi,
Toki, Gifu 509-5292, Japan

Exhaust behavior of tritium from the Large Helical Device in the first
deuterium plasma experiment
The tritium exhaust behavior from the Large Helical Device (LHD) was observedin the first deuterium plasma experimental campaign. Tritium in the exhaust gas
was monitored at the conducted by use of the ionization chamber and water
bubbler system with the discrimination of chemical forms. The observation
results indicated that (i) tritium on the surface of the first wall and divertor tiles
as plasma facing components was released by the hydrogen isotope exchange
reaction of the glow discharge cleaning operation and the diffusion-limited
process was suggested in the tritium release behavior from the bulk, (ii) theamount of tritium release from the LHD vacuum vessel was about one-third of
the produced tritium and the mostly produced tritium was still retained at the endof plasma experimental campaign, (iii) the ratio of exhausted tritium from the
LHD vacuum vessel was larger than that in the case of JT-60U based on the
carbon materials as the plasma-facing components. It indicated that the tritium
inventory would be reduced and controlled by the kind of plasma-facing
materials.
Keywords: large fusion test device; wall conditioning operation; tritium release;plasma exhaust gas; tritium chemical forms; tritium balance
Subject classification codes: Article

1. Introduction
The management and control of tritium in the fusion facilities are one of the key issues
for public acceptance and the realization of a nuclear fusion reactor. Therefore, thebehavior of tritium should be understood to control the tritium in a fusion test device.
The tritium exhaust behavior and tritium inventory on the deuterium plasma experiment
have been reported by JT-60U, JET, and DIII-D [1-11]. In the deuterium plasma
experiment by a large fusion test device, a small amount of tritium, which has the
maximum energy of 1.01 MeV, is produced by d(d, p)t reaction in core plasma. A partof produced tritium is exhausted from the vacuum vessel via the vacuum pumping
system and the rest of the tritium would be retained in the vacuum vessel. These sourcesof tritium can be utilized as a tracer in a fusion test device. One of the applications is toquantitatively determine the tritium balance in the vacuum vessel and release pathwaysof tritium from a fusion test device. Other applications are to observe tritium exhaust
behavior and determine the chemical forms of tritium in the exhaust gas. The
observation of the tritium behavior in the exhaust gas also indicates the tritium releaseproperties from the plasma-facing components. As a result, the tritium inventory andbehavior in the fusion test device would be revealed.
At National Institute for Fusion Science (NIFS), the first deuterium plasma
experiment using Large Helical Device (LHD), which is the largest helical type fusion
test device equipped with the superconducting magnet system, has been started from 7th
March 2017 [12, 13]. After starting the deuterium plasma experiment, exhausted tritium
was observed during the plasma experiment. Exhausted tritium from the vacuum vessel
has to be recovered by the tritium removal system such as an exhaust detritiation system(EDS) from the viewpoints of public acceptance. Thus, EDS has been installed at thedownstream of vacuum pumping gas lines [14]. All exhaust gas lines from the LHD and
Neutral Beam Injection (NBI) systems and so on had to be connected to the EDS for
tritium recovery. Therefore, the inlet of EDS was suitable for the observation of tritium
behavior in the exhaust gas.
In this study, we observe the tritium in the LHD exhaust gas by the tritium
monitoring system and discuss the tritium release behavior and mass balance of tritiumin the first deuterium plasma experiment.

2. Plasma exhaust system and LHD operations
2.1. The vacuum pump system and wall conditioning system in LHD

The layout of the LHD and the auxiliary system is shown in Fig. 1. The plasma-facing
components in LHD mainly consist of stainless steel for the first wall and carbon fordivertor tiles. These materials combination of plasma-facing components are different
from other large fusion test devices such as JT-60U using the carbon-based first wall [2]and JET using the carbon or beryllium wall in the deuterium plasma phase prior to thepreliminary tritium experiment [8].
The LHD is equipped with various components of vacuum pumping systems for
the LHD vacuum pump system, NBI systems for plasma heating, in-vessel cryosorption
pumps for closed helical divertor (CHD), glow discharge cleaning system and wallbaking system at 368 K, number of plasma diagnostic devices and so on. These
components are installed with many independent vacuum pumping systems that areoperated continuously. However, the produced tritium is mainly exhausted via thevacuum pump systems for the LHD vacuum vessel and NBI, and the cryosorption
pumps for CHD. The vacuum pump systems for LHD vacuum vessel consists of two
systems: one system has 10 cryosorption pumps and 6 turbomolecular pumps, anothersystem has 8 cryosorption pumps and 2 turbomolecular pumps. Two vacuum pumpsystems are installed side by side in a part of the vacuum vessel as shown in Fig. 1. The
total practical pumping speed of hydrogen gas is estimated to be a few hundred m3/s.
The vacuum pump system for an NBI mainly consists of a large cryosorption pumpdeveloped by NIFS and 2 turbomolecular pumps. The NBIs are installed almost equallyin the rest of the vacuum vessel as shown in Fig. 1. The total practical vacuum pumping
speed by 5 NBI vacuum pump systems of hydrogen gas is estimated to be a few
hundred m3/s. It corresponds to the performance of the vacuum pump system for the
LHD vacuum vessel. The five CHD pumping system by means of the cryosorption
system was installed in the LHD divertor region to control the neutral gas pressure. The

practical vacuum pumping speed of hydrogen gas per one system is 9 m3/s [15]. Thesecryosorption pumps were regenerated after plasma experiment in a day or on aweekend. The detailed operation procedures are described in section 2.3.
As for the wall conditioning in the vacuum vessel, the glow discharge cleaning,boronization, and wall baking were conducted. The glow discharge cleaning operation
was carried out after the plasma experiment in a day or weekend as required by the
researcher. One electrode for glow discharge cleaning was installed in the LHD vacuum
vessel in the first deuterium plasma experiment campaign. Helium, hydrogen (H2), anddeuterium (D2) were utilized as the discharge gas. The boronization of the vacuum
vessel during the deuterium plasma experiment was limited to one operation. The
discharge cleaning condition is summarized in Table 1. The wall baking operation at
368 K was also carried out as required. The rate of temperature increase was 5 K/h. The
baking temperature was kept for a few ten hours. These wall conditioning operations
were carried out after the regeneration of the CHD cryosorption pump. The gate values
for the cryosorption pumps were closed during the wall conditioning operation and only
turbomolecular pumps were used for the exhaust of operation gas and outgas.

2.2. Exhaust detritiation system and tritium monitoring system
A schematic diagram of the LHD vacuum pumping system and the exhaust
detritiation system (EDS) is shown in Fig. 2. The exhaust gas lines from all vacuum
pumping systems are integrated and connected to the EDS for tritium removal. The
length and volume of exhaust gas lines from LHD to EDS are estimated to beapproximately 100 m and 1 m3, respectively. The EDS as a tritium recovery system
mainly consists of the catalytic reactors for the hydrogen conversion to water vapor,molecular sieves columns for the recovery of water vapor and temporally gas storagesystem for the regeneration gas from cryosorption pump, etc. The detail process flow
diagram, the operation, and the specifications of the EDS are described in Ref. 14.
The tritium monitoring system has been developed and installed at the inlet of
the EDS as shown in Fig. 2. The tritium monitoring system consists of the water
bubbler system for the measurement of total tritium amount and the discrimination oftritium chemical forms, and the ionization chamber for the observation of tritium
exhaust behavior. These detection limits were about 10-7 MBq/m3 and 4 x 10-2 MBq/m3,
respectively. The detail tritium monitoring system is described in Ref. 16.

2.3. Operation of LHD in the first deuterium plasma experiment
The first deuterium plasma experiment in LHD has started on March 7th, 2017. The
plasma operation using deuterium gas (D2), “D phase”, was carried out until June 30th,
2017. Then the plasma operation gas was changed to hydrogen gas (H2), “H phase”, andcarried out until August 3rd, 2017. The total amount of produced tritium was evaluatedby three neutron flux monitors [17, 18]. The uncertainty of the neutron flux monitor was
estimated to be about 10% on the calibration factor [17]. The total amount of produced
tritium in the first deuterium plasma campaign was estimated to be about 6.4 GBq. The
detail deuterium plasma operation of LHD with NBI heating is described in Ref. 12.
The LHD plasma experimental operation in a week was from Tuesday to Friday.
The operation gas was exhausted by the vacuum pump systems. After the plasma
experiment in a day, the cryosorption pumps for CHD were usually regenerated andthen the released hydrogen isotope gas contained with tritium was directory exhausted
via the turbo molecular pump in the vacuum pump system for LHD vacuum vessel.
After the plasma experiment on Friday, the large volume cryosorption pumps for the
NBI system were regenerated at night. Since a large volume of hydrogen isotope gas

was released from the NBI cryosorption pumps for a few hours, the released hydrogenisotope gas was temporally stored in a buffer tank in the EDS. After the regeneration of
NBI cryosorption pumps, the stored hydrogen isotope gas was introduced into theoxidation catalyst reactor by a controlled flow rate to be 0.4% of hydrogenconcentration at the inlet of the reactor. At the same time, the cryosorption pumps in themain vacuum pumping system were regenerated one after another at certain intervals.

3. Results and discussion
3.1. Exhaust behavior of tritium
3.1.1. LHD deuterium plasma operation
The behavior of tritium in the exhaust gas from the LHD vacuum vessel was measured
by the ionization chamber. The volume of the ionization chamber and the sampling gas
flow rate were 10-2 m3 and 1.2 m3/h, respectively. Figure 3 shows the variation oftritium concentration in the exhaust gas during the LHD deuterium plasma experimental
campaign. In the first two months as the initial D phase, the deuterium plasma
experiment was conducted using deuterium plasma heated by two deuterium beams(NBI: #4, #5) and three hydrogen beams (NBI: #1, #2, #3). Thus, produced tritium
amount by D-D reactions was small, and observed tritium concentration in the exhaustgas was low. The maximum tritium concentration was 0.183 MBq/m3. Then the numberof deuterium beams was increased week by week and the deuterium plasma operation as
full D phase was started on May 23rd, 2017. The amount of produced tritium in the coreplasma increased as the number of deuterium beams. The maximum tritium
concentration was reached at 1.28 MBq/m3 on June 21st, 2017. Then, the plasma
operation gas and neutral beams were changed to hydrogen from the beginning of Julyas the H phase. Even though tritium was not produced in the core plasma in thehydrogen plasma operation phase, a tritium concentration of more than 0.1 MBq/m3 wasobserved in the exhaust gas. Since produced tritium was remained in and on the plasmafacing components, it would be released from the components during these hydrogenplasma operations.
Figure 4 shows an example of the variation of tritium concentration for 10 days.
Tritium in the exhaust gas was not observed in the term of the deuterium plasma
operation, because the main vacuum pumps were consisted of cryosorption typepumping system and produced tritium was not exhausted directly. After the plasma
operation in a day, the gate valves of the cryosorption pumps in the main vacuum pumpsystem were closed and then the cryosorption pump for CHD in the vacuum vessel was
regenerated at night. Tritium was desorbed from the CHD cryosorption pump andexhausted by turbomolecular pumps. Thus, the peaks of tritium concentration wereobserved on the night of each day. Some of the deuterium glow discharge cleaning
operations were conducted after the regeneration of CHD cryosorption pumps and thepeak of tritium concentration was also observed separately. The cryosorption pumpsystem in the NBIs was regenerated on Friday night. At the same time, the regenerationof two cryosorption pumps in the LHD main vacuum pump system was also started.
These regeneration gasses were temporally stored in the gas storage system as shown in
Fig. 2. After the gas storage operation, the stored gas was introduced gradually into the
EDS process line in a weekend. The base tritium concentration in a weekend wascaused by the stored gas. On the same weekend, all cryosorption pumps in the LHD
main vacuum pump system were regenerated. Therefore, some of the peaks of tritium
concentration were observed on the base tritium concentration. The base tritium
concentration and each observed peak signal indicate the pathway of tritium releasefrom the vacuum vessel. Therefore, the tritium exhaust pathway can discriminate by theanalysis of these signals. The tritium exhaust pathway from the LHD vacuum vesselusing the data is discussed in section 3.2.

3.1.2. Wall conditioning operation: a glow discharge cleaning
The wall conditioning operations were conducted after deuterium plasma operation in a
day or a weekend. The typical tritium exhaust behavior by glow discharge cleaning
operations in the initial D phase is shown in Fig. 5. The glow discharge operations ofhelium and deuterium gas were carried out from 3:00 to 5:00 and from 9:00 to 14:00 on
May 3rd, 2017, respectively. Each glow discharge operation condition is summarized inFig. 5. In the case of helium glow discharge, tritium release from the vacuum vessel was
almost undetectable by the ionization chamber. However, it was determined less than
0.04 - 0.05 MBq/m3 measured by a proportional counter in the helium glow discharge
with Boronization operation. On the other hand, tritium release could be detected by the
deuterium glow discharge operation in spite of after the helium glow discharge. The
tritium release amount was estimated to be about 2.8 MBq. The average tritium release
rate was calculated to be 0.56 MBq/h. Although the mass of gas species was the same inboth glow discharge operations, tritium release behavior was different. It would becaused by the different tritium release mechanism. It is well-known that the hydrogenisotope exchange reaction on the surface by the hydrogen isotope glow discharge
operation is more effective to remove tritium rather than physical sputtering by the He
glow discharge operation.
Figure 6 shows the behavior of tritium release by H2 glow discharge cleaning
operation in the H phase. The glow discharge cleaning operation condition issummarized in fig. 6. The glow discharge cleaning operations were conducted twotimes at an interval of 12 hours on July 10th and 11th 2017. The plasma experiment wasnot carried out between two glow discharge operations. The tritium release amount by
first and second glow discharge cleaning operations was estimated to be about 13.7
MBq and 1.9 MBq, respectively. The average tritium release rates were 1.1 MBq/h and0.31 MBq/h, respectively. The peak of tritium concentration was observed at theinitiation of glow discharge in the first glow discharge operation. Then the tritium
concentration was gradually decreased and approaching the constant value. In thesecond H2 glow discharge cleaning operation, the peak of tritium concentration was notobserved at the initiation of glow discharge. It would indicate that the tritium on thesurface was almost removed by the first glow discharge cleaning operation and thenreleased tritium by the second glow discharge operation would be migrated from thebulk of plasma-facing components. Therefore, the glow discharge cleaning operation
using hydrogen isotope gases is an effective tritium removal method, however, the long
operation time is not useful because the tritium release rate decreases with the operationtime.
The tritium on the surface is easily released by an isotope exchange reaction. On
the other hand, since the maximum energy of triton produced by d(d, t)p reaction is 1.01
MeV, the tritium would be implanted to a depth of few m in the first wall and divertor
tiles. [19, 20]. Most of the tritium implanted in the bulk must be diffused to the surface
for tritium release. The carbon is mainly deposited on the surface of the first wall near
the divertor tiles [21, 22]. On the other hand, the surface of the first wall at the center ofthe saddle portion is eroded by plasma. Also, the outer torus side is depositiondominant, while the inner torus side is primarily erosion-dominant. Therefore, thetritium release behavior from the LHD vacuum vessel would be a diffusion-limited
process in bulk metal as well as in the deposition layer on the plasma facing materials.

3.1.3. Wall conditioning operation: wall baking
The wall baking operation was carried out following to deuterium glow dischargecleaning operation as shown in Fig. 5. Figure 7 shows the variation of tritium
concentration in the exhaust gas, vacuum vessel temperature, gas species measured by amass spectrometer, and total pressure in the mass spectrometer chamber during thebaking operation. The gas species and total pressure were measured by a quadrupole
mass spectrometer (MKS Instruments, e-Vision2 RGA) and an ionization vacuum
gauge (ULVAC, GI-M2). The flat top of the wall temperature at 368 K was kept for 66
hours. The tritium concentration increased with temperature rise and then fluctuatedduring the flat top. According to the data of a quadrupole mass spectrometer installed on
the LHD vacuum pump system, the hydrogen isotope gases (m/e=2, 3, 4) increased withtemperature rise and then gradually decreased during the constant temperature. The
component of water (m/e=18) also increased with the temperature rise and wasmaintained at a constant during the constant temperature. Therefore, the first peak oftritium concentration in Fig. 7(a) would be caused due to the releases of both tritiatedhydrogen gases and water. After that, the tritiated water would become the mainoutgassing from the vacuum vessel. However, the fluctuation of the tritium
concentration during the constant temperature in the baking operation is not clear.
The tritium release amount during wall baking operation was estimated to beabout 17.4 MBq, although the deuterium glow discharge cleaning operation was carriedout prior to wall baking operation. The average tritium release rate was calculated to be
0.27 MBq/h. The tritium release rate was lower than that of glow discharge cleaning
operation, however, the tritium release amount was larger because the baking time
became longer. The baking operation becomes one of the effective tritium removal
methods in the LHD vacuum vessel.

In the case of wall baking operation, the hydrogen isotope gases or plasma did
not present in the vacuum vessel. Therefore, the mechanism of tritium release duringwall baking operation is a diffusion process from the bulk in the vacuum vessel.
The chemical forms of released tritium during the LHD plasma experiment andwall conditioning operation are discussed in the next section.

3.1.4. Tritium chemical forms in the exhaust gas
Tritium is one of the hydrogen isotopes and it can form various chemical compounds
such as water vapor and hydrocarbons by the plasma surface interactions [23]. Thus, thetritium chemical forms in the exhaust gas should be discriminated in order to therealization of tritium behavior. The water bubbler system installed at the inlet of EDS
has the function of the discriminating chemical forms of water vapor, hydrogen gas, andhydrocarbons. Figure 8 shows the weekly average tritium concentration discriminatedchemical forms measured by the water bubbler system from March 6th, 2017 to August
21st, 2017. Three chemical forms were observed in the exhaust gas during the wholemonitoring period. Since the main operation gas was hydrogen or deuterium gas, tritiumin the vacuum vessel become tritiated hydrogen isotope gas and they were major
components gas which constituted about 96.4% in the exhausted tritium chemical
forms. As the second major gas, the tritiated hydrocarbons were observed andconstituted about 3.2% in the exhausted tritium chemical forms. Since the divertor tileas the plasma-facing components in LHD was made of carbons, the chemical forms ofhydrocarbons were produced by the chemical sputtering in divertor plasma. Although
there was not much data of tritiated hydrocarbons in the exhaust gas, Kaminaga et al.
have reported that the ratio of tritiated hydrocarbons in JT-60U made up 4.3% and 6.3%
[6]. The ratio of tritiated hydrocarbon in the LHD exhaust gas was about two-thirds ofthat in JT-60U. The plasma-facing materials in LHD mainly consist of stainless steel as
the first wall and a carbon material as a divertor tile. Thus, tritium behavior is supposed
to be different from JT-60U based on carbon materials and other fusion test devices.
The metal first wall would reduce the tritiated hydrocarbons as the refractory organic
matter in the exhaust gas.
Tritiated water vapor was minor chemical forms and constituted about 0.4%
because the average dew point in the exhaust gas was less than 253 K whichcorresponds to the water vapor concentration of 0.1 %. In JT-60U, the range of the ratioof tritiated water vapor in the exhaust gas was from 0.2% to 0.5% [1-4, 6]. The ratio oftritiated water vapor in the exhaust gas was almost the same between LHD and JT-60U.
It would indicate that the release of tritiated water vapor was the same source between
two devices. The vacuum chamber contains water in general. The water would be
released from the vacuum vessel during the operation of the plasma experiment andwall conditioning. The hydrogen atom in water in the vacuum chamber would exchangewith tritium atom. Thus, the source of tritiated water vapor would be the vacuum vessel.
Figure 9 shows the relationships between the concentrations of tritiatedhydrocarbons, water vapor, and tritiated hydrogen gas. The concentration of tritiatedhydrocarbons was proportion to that of tritiated hydrogen gas. It suggested that a part ofproduced tritium in core plasma slowed down and transported to the divertor region
along with the scrape-off layer. Then the divertor plasma contained tritium reacted withthe carbon and tritiated hydrocarbons were generated. Also, tritiated hydrocarbons
would be generated by hydrogen or deuterium glow discharge operation. On the other
hand, the behavior of tritiated water vapor might not seem to be related to that oftritiated hydrogen gas. The source of tritiated water vapor would be from the metal wall
because the vacuum vessel contained a small amount of water in the metal wall. Thus,tritiated water vapor was increased during the long-term wall baking operation. Besides,
hydrogen isotopes and oxygen on the stainless steel as the first wall would be splutteredand released by the glow discharge operation. Then the tritiated water vapor was
generated via the chemical reaction between hydrogen isotope and oxygen during theglow discharge operation. Therefore, the behaviors of tritiated water vapor would be
independent of that of tritiated hydrogen gas.

3.2. Tritium balance in LHD and exhaust pathway of tritium
3.2.1. Tritium exhaust rate from LHD vacuum vessel
Figure 10 shows the variation of the amount and the ratio of exhausted tritium andtritium inventory in the LHD vacuum vessel during the plasma experiment. The amountof produced tritium was estimated from the results of neutron measurement. The
amount of exhausted tritium was measured by a water bubbler system. The tritium
exhausted rate, which is defined as the ratio of the exhausted tritium and produced
tritium, was 5.1% in the first week and then gradually increased with the progress of thedeuterium plasma experiment. It suggested that the produced tritium was implanted in
the vacuum vessel and not released from that in the initial phase of the deuterium
plasma experiment. As mentioned in 3.1.2, produced tritium has high energy and it is
implanted into the deep of few m. Therefore, implanted tritium would be held in thebulk of plasma-facing materials in the initial phase of the deuterium plasma experiment.
Then performing the number of the deuterium plasma experiments and the wallconditioning operations, tritium in the materials would be diffused along with theoperation time, and the tritium exhausted rate was gradually increased toward the end of
the plasma experiment. The implanted tritium has been continuously released in the Hphase. The total amount of exhausted tritium was 32.8% of produced tritium at the end

of the plasma experimental campaign. It suggested that two-thirds of produced tritium
would be implanted and remained in the vacuum vessel after the first deuterium plasma
experimental campaign. Since the tritium inventory was increasing even at the end of
the D phase as shown in Fig. 10, it is likely that the tritium inventory in the vacuum
vessel is not yet saturated. The tritium inventory in the vacuum vessel is determined by
the balance of release, decay, and implantation of tritium. The tritium balance will be
continuously observed to evaluate the time it takes for the tritium inventory in thevacuum vessel to reach saturation. Besides, for the investigation of the tritium amountand the distribution of tritium inventory, the analysis of the material samples installed in
LHD vacuum vessel has been started [24].
The deuterium plasma experiment has been conducted in various fusion test
devices. Some of the large fusion test devices have evaluated the tritium balance in thevacuum vessel. The tritium exhaust rate in JET, JT-60U, and LHD are summarized inTable 2. The plasma-facing materials and wall conditions are also indicated in Table 2.
These data of JET were evaluated in the deuterium plasma phase prior to thePreliminary Tritium Experiment and the Deuterium–Tritium Experiment. In the
deuterium plasma experiment in the JET graphite phase, about 20% of tritium was
exhausted from the vacuum vessel [8]. On the other hand, the tritium exhausted rate in
the beryllium phase of JET became two times larger than that in the carbon phase. In thecase of tritium balance in JT-60U which has the carbon-based first wall and divertor
tiles, 28% of produced tritium was exhausted and about 50% was retained in thevacuum vessel [2]. In the case of LHD, the tritium exhausted rate was achieved to be
32.8%. The ratio of exhaust tritium during the plasma experiment in LHD was about 1.2
times and 1.6 times larger than that of JT-60U and in the carbon phase of JET,
respectively. However, the tritium exhausted rate in LHD was low in comparison withthe results of the beryllium phase of JET. The first wall plates in the vacuum vessel of
LHD are stainless steel, while graphite plates are used for the divertor. The area ofstainless steel is 700 m2 and the graphite area is 30 m2 which constitutes about 5% of
the total plasma facing area [22]. Besides, a part of the first wall near the graphite plates
would be covered by sputtered carbon as mentioned in the section 3.1.2. Thus, the ratioof the primary metal wall area in LHD would have been reduced by carbon deposition.
These results suggest that the larger the ratio of the metal wall as the plasma-facing
components in the vacuum vessel, the larger the tritium exhausted rate. It is well-known
that the carbon material is sputtered by plasma and the hydrogen isotopes of H, D, andT are incorporated into the carbon redeposition layer [25]. Thus, the tritium inventory inthe vacuum vessel increases when the carbon tile is used as the plasma-facing
components. It is considered that the metal wall would reduce the tritium inventory by
the carbon co-deposition in the vacuum vessel.

3.2.2. Tritium release pathway from LHD vacuum vessel
As mentioned in 3.1.1, we could observe the tritium release pathway from LHD. Figure
11 shows the exhausted tritium pathway from the vacuum vessel at the end of theplasma experiment. The NBI cryosorption pumps were the main tritium exhaust
pathway during plasma operation and held more than 50% of the total exhausted
tritium. Since the NBI is installed evenly over the entire torus vacuum vessel and the
NBI cryosorption pump has a high pumping speed, produced tritium in the LHD
vacuum vessel would be transported and absorbed in the NBI cryosorption pump via the
NBI vacuum ports. On the other hand, the rate of tritium exhaust pathway via the LHD
cryosorption pumps as the main vacuum pumping system and the CHD in the vacuum
vessel were merely one-third of the total exhausted tritium because they are installedlocally in the torus as shown in Fig. 1. The total amount of exhausted tritium via thecryosorption pumps for NBIs, LHD and CHD corresponded to 28.1% (about 1.8 GBq)of produced tritium. Besides, the ratio of tritium released by wall conditioning operation
including the He, H2, D2 glow discharge cleaning, the wall baking at 368 K, and theboronization operation was only 4.7% (about 0.3 GBq) of produced tritium thought the
total time of wall conditioning operation was more than 1100 hours as summarized inTable 1. In the deuterium plasma operation, the implanted tritium in the vacuum vesselis hardly removed by the wall condition operation in the case of LHD.

4. Conclusion
The tritium exhaust behavior and tritium balance in the LHD vacuum vessel were
investigated in the first deuterium plasma experiment. The tracer tritium produced by DD reactions in LHD vacuum vessel revealed that (i) tritium on the surface of the firstwall and divertor tiles as plasma facing components was removed by hydrogen isotope
exchange reaction and the tritium release from the bulk of the plasma-facing
components was the diffusion-limited process, and (ii) about one-third of produced
tritium was exhausted from the LHD vacuum vessel during the plasma operation andthe large portion of produced tritium was still retained in the LHD vacuum vessel as
tritium inventory. The estimation of tritium inventory was necessary to clarify thetritium balance in the LHD.
The tritium exhaust ratio in LHD using the stainless steel as the first wall and thecarbon as the divertor tile was larger than that in JT-60U based on the carbon material
and less than that in the case of JET using beryllium as the plasma-facing components.
Also, the ratio of tritiated hydrocarbons in the exhaust gas of LHD reduced incomparison with the case of JT-60U. Thus, the selection of metal wall would be one of

the key factors to control the tritium inventory and to reduce the tritium compounds inthe exhaust gas.

Acknowledgements
We are grateful to Prof. N. Akata of Hirosaki University and Ms. C. Iwata of NIFS for
their technical supports of tritium measurement. We are also grateful to Dr. K. Ogawa
of NIFS for his helpful supports of neutron data. This work was supported by the NIFS
budget ULAA023, ULAA708, and JSPS KAKENHI Grant Number 17K06998.

Disclosure statement
No potential conflict of interest was reported by the authors.

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Figure captions

Figure 1. The layout of the LHD vacuum pump, NBI, CHD systems, and glow
discharge electrode.
Figure 2. Schematic diagram of the LHD vacuum pumping system and the exhaust
detritiation system.
Figure 3. Variation of tritium concentration measured by an ionization chamber in theexhaust gas during the plasma experiments.
Figure 4. An example of tritium exhaust behavior from LHD in ten days.
Figure 5. The effect of gas species on the tritium release behavior by the glow dischargecleaning.
Figure 6. The tritium release behavior on the repetition glow discharge operation.
Figure 7. The hydrogen isotopes release behavior on the wall baking operation at 368 K:(a) the operating temperature of LHD vacuum vessel and tritium concentration at the
inlet of EDS, (b) the gas species measured by a mass spectroscopy and the total pressurein the mass spectrometer (Q-mass) chamber.
Figure 8. The behavior of tritium chemical forms in the exhaust gas during the plasma
experiment.
Figure 9. The relationships between tritiated hydrogen gas and tritiated hydrocarbons,water vapor in the exhaust gas.
Figure 10. Accumulated tritium amount and the ratio of exhaust and inventory of tritium
during the first deuterium plasma experiment.
Figure 11. Tritium exhaust pathway from LHD during the first deuterium plasma
experiment.

Figure 1. The layout of the LHD vacuum pump, NBI, CHD systems, and glow
discharge electrodeFigure 2. Schematic diagram of the LHD vacuum pumping system and the exhaust
detritiation system

Figure 3. Variation of tritium concentration measured by an ionization chamber in theexhaust gas during the plasma experiments.

LHD plasma operation: 9:30 ~ 18:45 (Tuesday ~ Friday)

Tritium concentration [MBq/m 3]

10

CHD cryopump
regeneration gas

CHD cryopump
regeneration gas

Glow discharge

1

0.1

0.01
6/20

LHD, CHD and NBI
cryopump regeneration gas

6/22

6/24

6/26

6/28

Month / day

Figure 4. An example of tritium exhaust behavior from LHD in ten days.

6/30

Tritium concentration [MBq/m 3]

0.12
0.11
0.1

Gas Voltage Current Pressure
He 234 V
15 A
3.2 PaD
393
V
10A
0.9 Pa2

0.09

D2 glow dischargecleaning [4 hours]

0.08
0.07
0.06

He glow dischargecleaning [2 hours]

0.05

BG: 0.04 MBq/m3

0.04
0:00

3:00

6:00

9:00

12:00

15:00

18:00

21:00

Figure 5. The effect of gas species on the tritium release behavior by the glow
discharge cleaning.

Tritium concentration [MBq/m 3]

0.2
0.18
0.16

1st H2 glow

No.
1st
2nd

discharge cleaning

Voltage Current
330 V
10 A
336 V
10 A

Pressure
0.9 Pa
0.9 Pa

0.14
0.12
0.1

2nd H2 glow

0.08

discharge cleaning

0.06
0.04
12:00

BG: 0.04 MBq/m3

18:00

0:00

6:00

12:00

18:00

0:00

6:00

Figure 6. The tritium release behavior on the repetition glow discharge operation.

0.08

340

0.07

320

0.06

300

0.05

280
BG: 0.04 MBq/m3

0.04

Intensity [a.u.]360

66 hours

(b)

260
m/e=2
m/e=3

m/e=4
m/e=18

Total pressure

10

-5

10

-6

10

10

-7

10

10

-8

10

10

-9

Measurements
start

-1

10

-2

-3

-4

17/5/4

Vacuum vessel temperature [K]

0.09

380

(a)

17/5/517/5/6
Year / month / day

17/5/710
17/5/8

Total pressure in Q-mass chamber[Pa]Tritium concentration [MBq/m3]

0.1

-5

Figure 7. The hydrogen isotopes release behavior on the wall baking operation at 368
K: (a) the operating temperature of LHD vacuum vessel and tritium concentration at the
inlet of EDS, (b) the gas species measured by a mass spectroscopy and the total pressurein the mass spectrometer (Q-mass) chamber.

Weekly average3

tritium concentration [MBq/m ]

10

0

10

-1

10

-2

10

-3

10

-4

10

-5

10

-6

Detection limit: 10-7 MBq/m

3

Wall conditioning(Glow discharge,
wall baking)Tritiated water vapor
Tritiated hydrogen gas
Tritiated hydrocarbon-7

10
17/3/1

17/4/1

17/5/1
17/6/1
Year / month / day

17/7/1

17/8/1Figure 8. The behavior of tritium chemical forms in the exhaust gas during the plasma
experiment.

10

-2

Tritiated water vapor [MBq/m 3]

Tritiated hydrocarbon [MBq/m 3]

Hydrocarbon
Water vapor
10

-3

10

-4

10

-5

10

-6

10

-4

10

-3

10

-2

10

-1

10

Tritiated hydrogen gas [MBq/m3]
Figure 9. The relationships between tritiated hydrogen gas and tritiated hydrocarbons,water vapor in the exhaust gas.

0

Accumulated tritium amount [Bq]

7x10

9

6x10

9

5x10

9

4x10

9

3x10

9

2x10

9

1x10

9

Exhausted tritium from LHD
Produced tritium in LHD
Tritium inventory in LHD

0

Ratio of exhausted tritium
and tritium inventory [%]

(a)

(b)

80
60

Tritium inventory

40
20
0
17/3/1

Exhausted tritium
17/4/1

17/5/1

17/6/1

17/7/1

17/8/1

Year / month / day

Figure 10. Accumulated tritium amount and the ratio of exhaust and inventory oftritium during the first deuterium plasma experiment.

Others [Baking, etc]:
Boronization:
9.2 %Glow dischrage
0.3 %cleaning: 2.8 %CHD cryosorption
pump: 13.4 %LHD cryosorption
pump: 17.2 %NBI cryosorption
pump: 57.2 %(LHD exhaust gas waspartially contained)

Figure 11. Tritium exhaust pathway from LHD during the first deuterium plasma
experiment.

Table 1. Operating conditions for glow discharge, Boronization, and baking.
Wall

Operation

Discharge

Discharge

Operation

Total

conditioninggas

current

voltagepressure

operation(Constant(Average)(Average)

operation*

current)
Glow discharge

time(minutes)

He

15 A

227 V

3.0 Pa

840

H2

10 A

336 V

0.9 Pa

3600

D2

10 A

370 V

0.9 Pa2040

Boronization

B2D8+He

15 A

237 V

2.2 Pa350

Baking (368 K)

-

-

-

< 10-3 Pa

63300*** The pressure was measured by a diaphragm gauge.
** The total baking operation time contained with the temperature rise and fall time about 37200
minutes.

Table 2. Comparison of tritium exhaust rate in the initial deuterium plasma phaseamong the large fusion test devices.
Device First
Divertor Wall conditioning operation Sampling orwallmaterial Baking
monitoring period
Discharge
material
temperature cleaning and
JET

Be

Be

C

C

Be

-*

Tritium Operation
exhaust temperature on
rate
vacuum vessel

Remarks

Referencewall coating
Be evaporation Shot by shot sampling (600 sec) 12~45% 573K or 623 K

Limiter: Be[7], [8]-20%Limiter: C

BeLong term samplingBe evaporation (over a period of many hours)

40%

Limiter: CBeBe

Be evaporation

39%

Limiter: Be

JT-60U C

C

573 K

28%

LHD

C

368 K

Tyler or glow July 1991 to October 1993
discharge (He),Tokamakdischarge,
Boronization
Glow discharge March 2017 to August 2017
(H2, D2, He)
Boronization

SUS* No information in ref. 8.

573 K

32.8% 293~303 K

Produced tritium: [2]31 GBqProduced tritium: This
6.4 GBq
study