Codeposit removal by UV laser irradiation

W.M. Shu, T. Tadokoro*, Y. Oya**, Y. Kawakubo and M.F. Nishi
Tritium Engineering Laboratory, Department of Fusion Engineering Research
Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-1195, Japan

tritium fuel was retained inside the vacuum vessel ofABSTRACT

both TFTR (Tokamak Fusion Test Reactor) and JET
In the development work of an effective method for(the Joint European Torus) after deuterium - tritium

decontamination of tritium retained in codeposits on(D-T) plasma burnings, principally by codeposition

plasma facing components of D-T fusion reactors suchwith carbon eroded from plasma facing components.1-3

as ITER, ultra violet (UV) laser irradiation test was

The amount of tritium retained in future DT machinescarried out using simulated codeposits formed by C2H2

like ITER (International Thermonuclear Experimental

or C2D2 glow discharge. The UV laser applied was anReactor) will scale-up with the long plasma duration

ArF excimer laser with a wavelength of 193 nm, aand high duty-cycle operation.4 From the viewpoint ofpulse energy of 200 mJ and a pulse duration of 25 ns.

the safety, tritium inventory in the vacuum vessel

A removal rate of 13.5 nm/pulse was observed at theshould be controlled not to exceed some amount2

laser irradiation with the energy density of 0.05 J/cm

determined properly. Besides tritium inventory control,

for the simulated codeposits whose atomic ratio oftritium removal from the vacuum vessel is also requiredhydrogen or deuterium against carbon (H/C or D/C) at

to control plasma fuelling and to reduce the tritium

the surface was 0.53. Hydrogen isotopes were removedout-gassing during maintenance activities.

from the codeposits predominantly in the form of gases

Significant progress in tritium removal technologies

of hydrogen isotopes by the UV laser irradiation.

was made in support of the engineering design*

activities of the ITER project.4 Thermo-oxidativeI.

INTRODUCTION

erosion at temperatures above 570 K, or oxygen plasma
discharges have been found to be effective in laboratory

Tritium safety is one of the most important issues forthe development of fusion power. A large fraction ofexperiments to remove tritium from tritium-containingfilms.5-9
have*Present address: Power & Indus. Sys. R&D Lab., HitachiLtd., Hitachi-shi, Ibaraki 319-1221, Japan**Present address: Radioisotope Center, The University ofTokyo, Bunkyo-ku, Tokyo 113-0032, Japan

These techniques using oxygen, however,somedrawbacks,

suchaslengthy

wall

conditioning time to remove oxygen and recover normal
plasma operation, and the formation of large quantities

of tritiated water with four orders greater health hazard

rate of 5 Hz and a beam size of 2.3 cm by 0.7 cm.

than tritium gases (T2, DT and HT). On the other hand,laser energy density through a quartz window was 0.05

ultra violet (UV) irradiation method has been

J/cm2 on the sample.

10-12TheThe released gases during thedeveloped as an advanced detritiatiom technique.

laser irradiation were analyzed by a quadrupole mass

Our previous work showed that tritium could bespectrometer.

removed effectively from the surface of actual plasma
facing components by UV laser irradiation (193 nm, 0.1
2 13J/cm ).

II.C.

Characterization of simulated codeposits

In this study, the application of UV laserirradiation

for the removal

by ERD, RBS, SIMS and TDSof codeposits wasinvestigated.

After the UV laser irradiation, analyses of ERD(Elastic Recoil Detection) and RBS (Rutherford BackII.

EXPERIMENTAL

Scattering) were performed to obtain the hydrogenconcentration at the surface of the simulated codeposits

II. A.

Preparation of simulated codeposits

formed by C2H2 glow discharge. The primary 1.7 MeV
He+ ion beams were directed onto the sample at anTo produce simulated codeposits, a membrane ofangle of 80° to the surface normal.

The typical ion

tungsten or stainless steel as substrate was placed in abeam size was 1 mm in diameter. The recoiled

pretreatment chamber, which was evacuated to less thanhydrogen isotopes were detected by ERD with a silicon-410 Pa.

Then the glow discharge of C2H2 or C2D2 was

surface barrier detector (SSD) at a forward angle of 80°
The fluence of He+ ionscarried out to deposit the hydrocarbons on the substrate.

to the surface normal.

The pressure of C2H2 or C2D2 during the deposition wasbombarded on the sample during ERD measurements

kept at 67 Pa. The glow discharge voltage was set towas monitored simultaneously by means of the RBS

be about 1.2 - 1.3 kV and the current was 5 - 20 mA.

measurements at the angle of 50° to the surface normal.

The simulated codeposits were formed by the deposit ofThe concentration of hydrogen retrained in thethe hydrocarbons, and their thickness was changed bysimulated codeposits was calibrated using the known

adjusting the time of glow discharge from 3 - 60

experimental data of B. Tsuchiya and K. Morita.14

minutes.

The SIMS (Secondary Ion Mass Spectrometry) wasalso applied to analyze the depth profiles of deuterium

II.B.

UV laser irradiation to simulated codeposits

and carbon in the simulated codeposits formed by C2D2
glow discharge, using the primary ion (Cs+) of an

The sample of simulated codeposits on the substrate
was placed in a decontamination chamber, and the-4

accelerated energy of 14.5 keV and a beam current of
10-7 A.

The depth scale for the SIMS profiles waschamber was pumped out to less than 10 Pa. Then, thedetermined from the relation between the sputtering

simulated codeposits was irradiated by a UV laser (antime and the stylus roughness measurement.

ArF excimer laser), which produces UV laser beamof deuterium against carbon (D/C) was corrected with

with a wavelength of 193 nm (corresponding photon

the experimental results (H/C) of RBS and ERD

energy: 6.3 eV), a pulse energy of 200 mJ, a repetition

analyses assuming that the isotope effect can beThe ratio

5×10 -10

The simulated codeposits formed by C2D2 glow
discharge was analyzed by thermal desorption (TDS).
The signals of quadropole mass spectrometer (QMS)
for the mass numbers (m/e) of 4, 20, 28 and 44 were
measured as functions of the increasing temperature.

Intensity (arb.unit)

neglected in the deposition process.

UVUV-laser
laser irradiation
irradiated
4×10 -10
M/e=3(HD)M/e=4(D2)HD

3×10 -10

D2

2×10 -10
1×10-1000III.

5RESULTS AND DISCUSSION

III. A.

1015202530Time (min)Removal of simulated codeposits

Fig. 2.

Mass spectra monitored at mass number of 3and 4 before and during UV laser irradiation.

Figure 1 shows simulated codeposits formed by
C2D2 glow discharge after 300 pulses’ UV laserirradiation.

As shown in this figure, codeposits of thearea irradiated were clearly removed. A removal rate
was 13.3 nm/pulse at the laser energy density of 0.05
J/cm2.

The photon energy of the UV laser is 6.3 eV,that is high enough to break the C-H bonds (about 4.5eV)15 with one photon absorption. However, the energy
is not sufficiently high to cut C=C bond (about 7.2 eV)
15by one photon absorption. In the previous study using

a UV lamp, 11-12 it was reported that only C-H bonds on
the surface of the codeposits were decomposed by theUV lamp irradiation.

On the contrary, all thecodeposits were removed by the UV laser irradiation inthepresentexperiment.

Thisindicatesthatmultiple-photons’ absorption may contribute to thecodeposit removal during the UV laser irradiation.
Simulated codeposits (4 µm thick) formed by
C2D2 glow discharge on SS 304 substrate

The released gases during the UV laser irradiation
were compared with the residual gases in the vacuum
chamber before the irradiation.

The QMS signals forthe mass numbers of 3 and 4 are plotted in Fig. 2 asfunctions of the time. The partial pressures of HD andD2 showed a rapid increase immediately after the UV
laser irradiation (Fig. 2). In comparison, the partialpressures of HDO and D2O only showed a slightincrease during the UV laser irradiation.16 This suggeststhat the UV laser irradiation in vacuum removes
hydrogen isotopes from the codeposits predominantlyin the form of gases of hydrogen isotopes.

Thisfeature of the UV laser irradiation technique is very
attractive from the viewpoint of both radiation safety
and tritium processing, because the radiation hazard oftritiated water is 4 orders greater than that of tritium gas
and tritium recovery process from hydrogen isotope
gases is much simpler than that from tritium oxide.
III.B.

Characteristics of simulated codeposits

25 mm

Figure 3 and 4 depict ERD and RBS spectra ofArea irradiated by the UV laser for 300 pulses

simulated codeposits

formed by C2H2 glow discharge,

respectively. The ratio of hydrogen against carbon atFig. 1.

Simulated codeposits after laser irradiation.

the top surface was calculated by the correlation of the0.6

5000Counts

0.5

4000

0.4

3000

D/C

Counts

3450

2000

0.3

removed by UV-laser

0.2

afterpulses’
irradiation
after 50
UV-laserirradiatedbefore UV-laser
irradiatedbeforeirradiation

1000
0.1

00

100

200

300

400

50000

600

100

200300Channel Number

Fig. 3.

400

500

600

700

800

Depth (nm)

ERD spectrum of C-H codeposits.

Fig. 5.

Depth profile of C-D codeposits withand without UV laser irradiation.

RBS and ERD counts.

A standard sample of H/C ratioof 0.4 gives ERD counts of 555 and RBS counts 99. 14

Simulated

codeposits

formedbyC2D2

glowSo, the ratio of hydrogen concentration over carbon

discharge were analyzed by TDS, in which the QMS(H/C) at the top surface of the codeposits was

signal traces for the mass numbers of 4, 20, 28 and 44

calculated to be 0.53 from the ERD counts of 3450 andwere determined as shown in Fig. 6. The peaks for

RBS counts of 465 (the difference of 727 and 262).

mass numbers of 28 and 44 appeared around 500 K,

Using this value, the hydrogen concentration inand the most probable candidates for these masssimulated codeposits was obtained by correcting thenumbers were C2D2 and CO at mass number of 28 andSIMS depth profile.

As shown in Fig. 5, the simulated

C3D4 and CO2 at mass number of 44. Since the total

codeposits were removed by 680 nm by 50 pulses’ UV

oxygen content in the sample was low, the dominate

laser irradiation (a frequency of 5 Hz and an irradiation

gases for the mass number of 28 and 44 weretime of 10 seconds).

The removal rate of theconsidered to be C2D2 and C3D4, respectively. This

simulated codeposits was calculated to be 13.6

assignment was supported by the presence of peaks of

2

nm/pulse at the laser energy density of 0.05 J/cm .

the mass number of 20 (CD4).

In comparison,

theThis value agrees well with the results mentioned
6×10 -6of the simulated codeposits was estimated to be 13.5
nm/pulse at the laser energy density of 0.05 J/cm2.
2000
CountsCounts

1500

1000

C-edgeIntensity (arb.unit)

above (4 µm/300 pulses). So the averaged removal rate-6M/e=44×10-6

M/e=20M/e=28M/e=44

3×10-6

2×10-6

1×10-6

C3 D4 ,CO20400

727 Counts

D2CD 4C2 D2 ,CO600

800

1000

1200

1400

Temperature (K)500
262 Counts

0200

5×10

250

300

350

Fig. 6.
400

450

500

550

600

Channel Number

Fig. 4. RBS spectrum of C-H codeposits.

Thermal desorption spectra of C-D codeposits
monitored at mass number of 4, 20, 28, 44
at the ramp rate of 50 K/min.

releaseofD2(massnumber:4)

occurredattemperatures much higher than those in the forms ofdeuterium carbides.

Release of deuterium molecule

occurred from 800 K and it peaked at around 1050 K.
This suggests that the breaking of C-D bonds occurredat high temperatures of about 1000 K.
V. CONCLUSION
Simulated codeposits were irradiated with an ultra
violet laser at the laser energy density of 0.05 J/cm2 andthe released gases during the irradiation were measured.
The characterization of the simulated codeposits werecarried out. The following conclusions were deduced:(1) A removal rate of 13.5 nm/pulse was obtained
for the simulated codeposits at the laser energy densityof 0.05 J/cm2.
(2) The partial pressures of HD and D2 showed arapid increase immediately after C-D codeposits wereirradiated by the UV laser.
(3) The atomic ratio of deuterium against carbon at
the surface of the simulated codeposits was determined
to be 0.53.
ACKOWLEDGEMENT
The authors wish to thank Drs. M. Seki, S. Seki andH. Takatsu for their continuous encouragement on thisstudy.

Sincere thanks are also due to Prof. K. Moritaat Nagoya University for his excellent technical supportin the ERD and RBS measurements.
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