Plasma and Fusion Research: Regular Articles

Volume 14, 3401098 (2019)

Eﬀect of Substrate Temperature on the Hydrogen Reflection atMetal Surfaces ∗)Jhoelle Roche M. GUHIT, Kenta DOI and Motoi WADA
Graduate School of Science and Engineering, Doshisha University, Kyotanabe, Kyoto 610-0394, Japan(Received 10 January 2019 / Accepted 8 April 2019)
Doppler-broadening of the Hα emission spectra from hydrogen reflected at surfaces of palladium and tungsten sheet metal surfaces held at clamp holders were presented in this study. The reflected hydrogen atoms wereobserved with velocity components produced by the back scatterings of H+3 , H+2 and H+ ion incident regions. Palladium exhibited higher spectrum emission intensities than tungsten for both clamp structures. Modification of
the clamp structure should produce cooler surface condition due to direct contact of metal surface to the substrateholder thus having H atoms adsorbed more on the palladium sheet.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: hydrogen reflection, Doppler-broadening mechanism, palladium, tungsten
DOI: 10.1585/pfr.14.3401098

1. Introduction

2. Experimental Methods

Hydrogen plasma interaction on metal wall surfaces
causes surface permeation or erosion and plasma impurity emission in which investigating and analyzing these
phenomena are significant in the advancement of fusion
research and device development. The phenomena must
be studied before fusion devices will be put into operation with the long life-time. It is consequently of interest
to examine and understand the underlying mechanisms ofhydrogen isotope permeation and particle release during
plasma-surface interactions [1, 2]. Permeation of hydrogen through metallic materials like palladium is of vital
concern to control transport and mass separation of hydrogen isotopes in the system. Palladium (Pd), in comparison to tungsten (W), has a strong absorbance of hydrogenmolecules and can show unique behavior in a hydrogenplasma environment due to the nature of rapid hydrogenisotope transport [3]. Thus, a comparative study in hydrogen reflection between Pd and W has been made using asmall plasma exposure device [4].
The material temperature is an important parameterthat determines the hydrogen transport in the bulk media,
while it also aﬀects the amount of hydrogen atoms adsorbed on the surface. Thus, the impact due to the target
surface cooling for palladium and tungsten during plasma
exposure in a magnetized hydrogen plasma is presented inthis paper. With two diﬀerent target design structures, thedata on velocity distribution functions of hydrogen atoms
reflected at metal surfaces will be discussed.

The plasma bombardment experimental device setup
is shown in Fig. 1. This apparatus irradiated palladium (Pd)and tungsten (W) metal sheets by a linear magnetized hydrogen plasma. These metal materials are 0.1 mm thick,
24 mm wide and 25 mm long in size. An 18.7 mT magnetic field is produced by three pancake coils. Each coilcurrent input is 5 A and connected in parallel around thedevice to create linear magnetic-force lines that extended
the electrons from the tungsten filament cathode to the target substrate holder. The embedded circuit diagram for thefilament region indicates two DC power supplies, in seriesconnection, are used to extract electrons from the filamentand generate the hydrogen gas discharge. Both power supplies are maintained with discharge current of I2 = 1.0 A
and 70 V for both V1 and V2 for applied potentials. Hydrogen gas pressure is set at 1.2 Pa. These conditions generate
a hydrogen cylindrical-shaped magnetized plasma with adiameter is Φ = 15 mm. A viewing port of BK7 (borosolicate glass) is fitted with an ICF70 flange for the alignmentof the substrate holder and hydrogen reflection measurement.
The floating potential, V f , was approximately −8 V

author’s e-mail: cyjb3303@mail4.doshisha.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

Fig. 1 Hydrogen bombardment device with embedded circuit
diagram for the filament discharge region.

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c 2019 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 14, 3401098 (2019)

Fig. 3 (a) Endpoint and (b) Modified Clamp.

Fig. 2 Plasma diagnostic observation area from the metal sheet.

and −12 V with respect to the chamber wall that was fixedat the anode potential for Pd and W, respectively. Negative
bias voltages were applied to the substrate holder startingfrom −100 V at an increment of −100 V to −600 V maximum through a DC power supply. The −600 V is themaximum limit for the current device configuration. The
molecular and atomic spectra at the vicinity of the bombarded metal sheets of Pd and W were observed througha 0.23Å wavelength resolution Czerny-turner monochromator. The wavelength used for Hα observation wasset around 656.28 nm to monitor the atomic emissions.
A continuously cooled CCD detector, S7034-1007S ofHamamatsu Photonics, with 60 seconds integration time
recorded the output spectra.
Figure 2 illustrates the observation area for the plasma
diagnostic for the hydrogen emission spectra. Ions in thehydrogen plasma, bombarded the center of the metal sheet
target positioned at a 45-degree angle. The location C isdenoted as the center of the metal substrate holder. Thiscenter is aligned with the epicenter of the view port througha guide laser. The substrate target holder position is adjusted in the x-axis right direction through the manipulator by 9 mm from the center, C. The spot S, indicated bya filled circle, is the observation of interest to detect andinvestigate the Doppler-broadening due to the velocity distribution of hydrogen reflection at the tungsten and palladium metal sheets. By setting the line of sight outside of
the plasma column, all lines emissions from molecules directly excited by plasma electrons are excluded in the spectrum [4]. The first mirror, m1 , is perpendicular throughthe z-axis of the spot. It is moveable at the y-axis direction where l1 = 9 mm. A second mirror, m2 , is placed
below and not moveable. Both m1 and m2 are angled at45 degrees. The light from the plasma is guided to themonochromator through m1 and m2 mirrors outside thechamber. The adjustments on the manipulator and positioning of m1 are made to observe the emission spectra
outside the plasma boundary. This mirror is set to observe perpendicularly the area of interest. The S regionis perpendicular from the metal surface and approximately
12.7 mm in measurement.
An endpoint fixture structure and a ring clamp struc-ture were utilized to hold the metal sheet target in positionof the water-cooled target holder (Fig. 3 (a) and Fig. 3 (b)).
In the endpoint fixture structure or the endpoint clamp,
small rectangular stainless-steel washers press the metaltarget to the water-cooled body by the screws. The second
structure of modified clamp press the peripheral part of themetal target evenly with the ring fixture to realize a better
heat transfer from the thin metal target to the surface of thewater-cooled target holder (Fig. 3 (b)). A continuous waterflow at the backside of the holder was maintained to avoid
excessive heating of the target, as temperature exceeded
1100 K during a hydrogen plasma bombardment onto thetarget maintained with the structure of Fig. 3 (a) [5]. Meanwhile, the surface temperature of the target mounted by
the modified clamp cannot be evaluated as the temperature was below 800 K, which was the detection limit for
the infrared radiation thermometer. Eﬀect on the Dopplerbroadening of the hydrogen plasma on tungsten and palladium sheet metal targets are investigated between these
two target holder structures.

3. Experimental Results
Hydrogen reflection of tungsten and palladium havebeen compared between endpoint and modified clamp
structures. The reflection characteristics were recognizable under a floating potential and increasing bias voltages
with values of −100 V, −200 V, −300 V, −400 V, −500 V,
and −600 V. Figures 4 (a) and 4 (b) shows the normalized intensity of the Balmer-α line spectrum emission of
the S spot region of the palladium target using an endpoint clamp in linear and in semi-logarithmic scale, respectively. Figure 4 (a) illustrates an expansion of theDoppler-broadening due to the velocity distributions of hydrogen atoms excited by the collisions with H2 molecules
to the surface. Comparing the −600 V and at the floating potential, −8.76 V, the former exhibits a wider line
spectrum width than the latter. This indicates that the velocity distributions, from the dissociative excitation of H2molecules, is prominent at increasing negative bias voltages. Figure 4 (a) also displays the blue and red wings ofthe Doppler shift in the negative and positive region of Δλ,respectively. Taking the spectra at −600 V, the blue andred wing under the endpoint clamp shows minimal diﬀerence. As the target bias was raised from −8.76 V floating

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(a)

(b)
Fig. 4 Doppler-broadening spectrum, in (a) linear and (b) semilogarithmic scale, of the magnetized hydrogen plasma on
palladium target under an endpoint clamp. The spectrum
broadens with increasing negative bias voltages.

potential up to −600 V, the spectrum displays enlarged redand blue wings corresponding to the incident kinetic energies of hydrogen ions. In addition, the ion species reflected
are H+ , H+2 , and H+3 can be observed (Fig. 4 (a)). However, to exhibit a clearer view of the reflected ions, a semilogarithmic graph is displayed (Fig. 4 (b)). Ion species
present in the plasma are H+ , H+2 , and H+3 with the fraction composition changed due to particle recycling process
at the target surface.
The regions of the incident ions can be separated into
three according to the general equation of wavelength shift,Δλ, calculated by the following equation.
λαΔλ =c

2eEb.
M(1)

Here, λa is the wavelength of Hα or 656.28 nm, e is thecharge of the electron, Eb is the bias voltage at −600 V, M
is the mass of the ion and c is the speed of light. Equation
1 determines the regions where contributions from whichions are present. For example, at −600 V Eb , only the hydrogen atoms produced by reflection of H+ ions at the target forms the spectrum in the wavelength from 0.52 nm to0.74 nm. The cut-oﬀ energies of atoms formed by the reflections of H+2 ions, and those of H+3 ions determine thewavelength range from 0.43 nm to 0.52 nm as the part constituted by reflections of H+ and H+2 . The dissociative excitation of the background H2 molecules produce light emission forming the sharp central peak with Δλ < 0.5 nm.
Figure 4 (b) shows the wavelength spectrum of lightemissions from hydrogen atoms H+ , H+2 , and H+3 reflections at Pd surface for the corresponding wavelength range.
Intensities are normalized at the central peak. At the highest bias voltage of −600 V, the Doppler-broadening shoulder extended outward up to 0.7 nm, close to the value corresponding to 600 eV kinetic energy proton, due to excitation collisions with hydrogen molecules.
Figures 5 (a) and (b) exhibits the palladium’s hydrogen emission spectrum taken with the modified clamp at
normalized intensity shown in linear and semi-logarithmic
scale, respectively. From floating potential to −600 V bias
voltage, a widening of spectrum is seen due to prominent presence of H2 molecules dissociative excitation atincreasing negative bias voltage (Fig. 5 (a)). The blue andred wing regions of the palladium target using the modified
clamp demonstrates minute diﬀerence.
From equation 1, velocity distribution regions of H+3 ,+H2 , and H+ are seen in Fig. 5 (b). Similar cut-oﬀ energies
at Eb = −600 V and expansion of the Doppler-broadening
shoulder up to Δλ = 0.7 nm is present.
Hydrogen ion reflection from tungsten and palladium metal sheets with both manifold clamps are seen inFigs. 6 (a) and (b). Velocity distributions of the ion reflection is separated by bias voltages of Eb = −100 V andEb = −600 V. From Eq. 1, at Eb = −100 V, the wavelength shift for H+3 , H+2 , and H+ are |Δλ| ≤ 0.17 nm,0.17 nm ≤ |Δλ| ≤ 0.21 nm, and |Δλ| ≥ 0.30 nm, respectively. On using either the endpoint or modified clamp,
there is no remarkable change on the velocity distributions
H+2 and H+ regions on tungsten surface. There is minimal change to no significant change in the H+3 region for
Eb = −600 V and Eb = −100 V, respectively, on tungsten
surface when using either of the two clamps.
On the other hand, Fig. 6 (b) displays the comparisonof the eﬀect of both endpoint and modified clamps on thehydrogen reflection on the surface of the palladium metal
sheet with bias voltages Eb = −100 V and Eb = −600 V.
At Eb = −100 V, there is a prominent gap region wherein
there is an observed lower emission spectrum on the endpoint clamp region relative to the modified clamp. Palladium with bias voltage Eb = −600 V also exhibited higheremission spectra, under the modified clamp, relative to the3401098-3

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Volume 14, 3401098 (2019)

(a)
(a)

(b)
Fig. 5 Doppler-broadening spectrum, in (a) linear and (b) semilogarithmic scale, of the magnetized hydrogen plasma on
palladium target under the modified clamp. The spectrum
broadens with increasing negative bias voltages.

reflection distribution when endpoint clamp was used. Palladium show some enhanced signal on the H+ and H+3 incident ion region when using a modified clamp. Modified
clamp’s impact on the emission spectra for palladium may
indicate that the surface temperature on the metal sheets
during bombardment also changed due to the full peripheral clamping of the second manifold. Placing a modified
clamp, instead of an endpoint clamp, over the palladium
metal created a direct contact with the surface backside
cooling water. On the other hand, the endpoint clamp, were
there is less cooling contact of the metal target, may create a mechanism where there are more adsorb moleculeson the surface which may result to lesser hydrogen protonreflection. When using the infrared radiation thermometer,(b)Fig. 6 Doppler-broadening spectrum (a) Tungsten and (b) Palladium in both endpoint and modified clamp. The spectral graph of −100 V and 600 V negative bias voltage arecompared with each other using both clamps.

temperature was not completely detected for the modified
clamp due to the 800 K detection limit for the infrared radiation thermometer.
Figure 7 shows the hydrogen reflection on palladium
metal sheet under modified clamp in certain cases wherein
there is noticeable diﬀerence in hydrogen reflection fordiverse surface conditions. Conditions where comparedfrom a spectra captured during continuous plasma process
and after substrate cooling eﬀect were applied. After substrate water cooling, higher emission spectrum relative tothe continuous plasma process is observed. There is achange in spectra with regards to relative temperature and3401098-4

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Fig. 7 Hydrogen reflection on palladium after substrate cooling and exposure to continuous plasma under modified
clamp.

condition of the metal surface target.

4. Conclusions
The endpoint and modified clamp eﬀect on the hydrogen reflection mechanism for both tungsten and palladium
were discussed. Reflection characteristics were recogniz-able under a floating potential and at increased bias voltages applied to metal targets under endpoint and modified
clamp. Blue and red wings on the spectra were also observed to be balanced on both metal targets under bothmanifold holders. Hydrogen reflection and ion velocity
distributions on the tungsten surface have minimal diﬀerence when using either an endpoint or modified clamp.
This phenomenon was observed at diﬀerent bias voltages
also.
Meanwhile, hydrogen velocity distributions for palladium surface showed a diﬀerence between emission spectrum for both manifolds. Modified clamp provided higher
velocity distributions relative to the endpoint clamp at different bias voltages. It is suggested that better cooling waspresent through direct contact of the metal substrate on
the substrate holder. This mechanism may provide lessadsorbed hydrogen molecules and have higher Dopplerbroadening distribution of emission spectra relative to using an endpoint clamp.
It is also interesting to relate results to thermocouple
data diagnostic to measure the surface temperature sincecurrent setup do not contain aforementioned mechanism.
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[2] A.P. Zakharov et al., J. Nucl. Mater. 241-243, 52 (1997).
[3] B. Sakintuna et al., Int. J. Hydrog. Energy 32, 1121 (2007).
[4] K. Doi et al., IEEE Trans. Plasma Sci. 46, 482 (2018).
[5] K. Doi et al., Fusion Eng. Des. 136, 100 (2018).

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