# High heat flux testing of actively cooled graphite- and tungsten-armoured JT-60SA flat tile divertor mock-ups

Daniel Dickes, Bernd Böswirth, Katja Hunger, Gabriel Peyron, Quentin

Tichit, Marianne Richou, Johann Riesch, Mehdi Firdaouss, Valerio

Tomarchio, Rudolf Neu

► To cite this version:

Daniel Dickes, Bernd Böswirth, Katja Hunger, Gabriel Peyron, Quentin Tichit, et al. High heat flux testing of actively cooled graphite- and tungsten-armoured JT-60SA flat tile divertor mock-ups. Nuclear Materials and Energy, 2026, 46, pp.102067. 10.1016/j.nme.2026.102067. cea-05478015

HAL Id: cea-05478015

https://cea.hal.science/cea-05478015v1

Submitted on 26 Jan 2026

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High heat flux testing of actively cooled graphite- and tungsten-armoured JT-60SA flat tile divertor mock-ups

Daniel DICKES¹, Bernd BÖSWIRTH¹, Katja HUNGER¹, Gabriel PEYRON², Quentin TICHIT², Marianne RICHOU², Johann RIESCH¹, Mehdi FIRDAOUSS², Valerio TOMARCHIO³, and Rudolf NEU¹,⁴

¹ Max-Planck-Institut für Plasmaphysik, 85748 Garching b. München, Germany

² CEA, IRFM, 13108 Saint-Paul-Lez-Durance, France

³ Fusion for Energy, 85748 Garching b. München, Germany

⁴ Technische Universität München, 85748 Garching b. München, Germany

Abstract

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During the process of developing actively cooled divertor plasma-facing components for JT-60SA, a fusion experiment in Japan built within the framework of the “Broader Approach Agreement” between the European Union and Japan, three small-scale divertor mock-ups have been manufactured. The mock-ups follow the flat-tile design, i.e., have plasma-facing armour tiles joined to an actively cooled heat sink. One mock-up has tungsten (W) armour tiles, and two mock-ups have carbon (C) armour tiles, with the heat sink material being the molybdenum alloy TZM. The joining was realized via diffusion bonding with a titanium interlayer. In this work, the thermo-mechanical behaviour of the mock-ups is assessed with the high heat flux test facility GLADIS in order to qualify the joining technology. This includes screening tests up to a heat flux of 15 MW/m² and cyclic loading with a heat flux of 10 MW/m² for 10 s. During high heat flux testing, pyrometer and thermocouple temperature measurements, digital camera images, and thermographic imaging were used to monitor the mock-ups. In addition, comparative infrared thermography tests and visual characterizations before and after high heat flux testing were performed, including the preparation of cross-sections for scanning electron microscopy. Debonding of the armour tiles did not occur during high heat flux testing, indicating that the diffusion bonding process is suitable. However, this work outlines challenges like a potentially decreasing heat removal capability of the TZM heat sink during cyclic loading or the occurrence of detrimental deep cracking in the case of W armour tiles.

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1. Introduction

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JT-60SA is a superconducting tokamak [1,2], which is built within the framework of the “Broader Approach Agreement” between the European Union and Japan, aiming to complement the ITER and DEMO fusion reactor projects [1,3]. According to the initial JT-60SA research plan [2], the operation was foreseen to start using a divertor with graphite (C) based plasma-facing components, followed by an operation phase with tungsten (W) plasma-facing components. The divertor is intended to withstand a steady-state power deposition of up to 10 MW/m² for up to 100 s and transients of up to 15 MW/m² for 5 s [1–4]. A total of 13000 long pulses, 3000 of which ending in fast transients, is expected. This implies the need for actively cooled plasma-facing components. In this context, the European Union’s procurement agency “Fusion for Energy” (F4E) developed a series of actively cooled small-scale divertor mock-ups. These mock-ups were tested within the EUROfusion research
program [5]. Both common divertor mock-up designs, the flat-tile and the monoblock design [6], which in the meantime has been selected as the preferred option for the JT-60SA divertor target, were considered. This paper focuses on flat-tile mock-ups. Flat-tile mock-ups consist of plasma-facing armour tiles that are joined to an actively cooled heat sink material. Given the JT-60SA research plan, small-scale mock-ups with W and C armour tiles have been manufactured as a transition step in the shift from an inertially cooled C divertor to an actively cooled divertor. TZM alloy, a molybdenum alloy containing titanium and zirconium [7], has been selected as heat sink material, as proposed already in the 90s [8]. Diffusion bonding has been chosen to join the armour tiles on the heat sink because it has been demonstrated to be a promising technique for joining C and TZM [9]. In the process of developing reliable plasma-facing components for fusion reactor experiments like JT-60SA, dedicated high heat flux testing is important to assess the thermo-mechanical behaviour of such mock-ups [10] and can be used to qualify the relevant manufacturing and joining technologies. In this paper, we therefore report on the behaviour of W- and C-armoured TZM small-scale divertor mock-ups, under thermal loads using the Garching LArge DIvertor Sample high heat flux test facility GLADIS [11,12], with a special focus on qualifying the joining technology, for the armour tile-heat sink joint. This is important, as an insufficient joining between the flat tiles and the heat sink can lead to progressive tile debonding during cyclic thermal loading, which leads to a reduced heat removal capability from the tile surface into the cooling structure and thus overheating of the tiles, or even a complete detachment and hence a loss of the armour function of the tiles.

## 2. Materials and Methods

### 2.1 Mock-up design and manufacturing

Three small-scale mock-ups have been manufactured by RTC engineering (Slovakia), two mock-ups with C armour tiles (named SSM C1 and SSM C2) and one with W armour tiles (named SSM W1). Each mock-up consists of a pre-machined TZM heat sink structure, divided into four segments by three milled segmentation grooves of 1 mm width and a curved baseline of 35 mm radius, leading to a minimum groove depth of 0.5 mm in the center of the cooling structure and 4.37 mm at the sides (Figure 1 b) and c)). One armour tile was joined to each of the segments. Before joining, the tiles were prepared from the W respectively C material specified in Table 1. The tile dimensions were 30 mm x 32 mm x 3 mm (length x width x height). In the case of C tiles, the sides to be joined to the TZM heat sink structure were prepared with a groove pattern by milling (groove distance 0.3 mm, groove depth $0.1 \ \mu m$) to increase the surface area during the subsequent diffusion bonding process (see Figure 9), while the W tiles had a ground surface with $Ra \le 0.2 \ \mu m$. The diffusion bonding process was implemented with a Dr. Fritsch SPS/FAST hot press. In this press, the tiles were placed on the TZM heat sink structure with a thin titanium foil in between. The diffusion process was then conducted by exposing the components to a temperature of 1320 °C for 2.5 h at a pressure of 5 MPa in an argon atmosphere, with a heating and cooling rate of 50 °C/min. Figure 1 a) and b) show one C- and one W-armoured mock-up as obtained from RTC engineering. It is visible that four thermocouples (TC1-TC4) were attached to the component and subsequently covered by metal sheets as protection. Figure 1 c) details the position of temperature measurement, and shows that underneath the metal shields, 2 mm deep channels are present at one side of the mock-ups halfway along the 30 mm length direction of each tile. In these channels, the thermocouples are guided and subsequently
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contacted to the mock-up 8 mm away from the plasma-facing surface with an estimated positioning accuracy of ±0.5 mm. Figure 1 c) furthermore shows that the cooling channel in the TZM heat sink was equipped with a massive swirl, allowing for a larger cooling channel diameter of 18 mm to reduce the distance between the cooling water and the plasma-facing surface while keeping the cross-section the same as for the ITER-like design, being a 12 mm cooling channel with only a swirl tape.

<table><caption>Table 1: Overview of the deployed materials.</caption><thead><tr><th>material</th><th>C</th><th>W</th><th>TZM</th></tr></thead><tbody><tr><th scope="row">description</th><td>isostatic graphite</td><td>forged bar</td><td>machined bar</td></tr><tr><th scope="row">specification</th><td>Grade R7710 [13]</td><td>ITER material specification [14]</td><td>ASTM B387 type 364 [7]</td></tr><tr><th scope="row">manufacturer</th><td>SGL Carbon, Germany</td><td>Luoyang Hypersolid Metal Tech Co., China</td><td>Union Ocel, Czechia</td></tr></tbody></table>

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<figure><img src="image_2.png" /><figcaption>Figure 1: Small-scale flat-tile mockups with a) graphite and b) tungsten armour tiles, and c) a partial cross-section view of the mockup (left) revealing the uncovered thermocouple (TC) position, the segmentation groove dimensions, and the swirl inside the cooling channel (right). The red lines in a) and b) represent the contours of pieces A to D that were cut out of the mock-ups for a post high heat flux exposure characterization, with the arrows indicating the cut surfaces of interest.</figcaption></figure>

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2.2 High heat flux testing
For high heat flux (HHF) testing, the high heat flux test facility GLADIS [11,12] at the Max Planck Institute for Plasma Physics in Garching, Germany, was used. The test facility was operated with one of its two 1.1 MW beam sources, which has a Gaussian beam profile that has a full width at half maximum of approx. 150 mm [12]. The intended test program for each mock-up consisted of a screening phase and a cyclic loading phase. During the screening, the heat flux and the duration of beam exposure, i.e., the pulse length, were increased stepwise from a heat flux of 6 MW/m² up to 15 MW/m². Cyclic loading was conducted with 10 MW/m², a pulse length of 10 s, and a pulse repetition rate of 60 s. Table 2 provides an overview of the realized test program. Repeating the test program of mock-up SSM C1 with mock-up SSM C2, plus an additional screening and continued cyclic loading of mock-up SSM C2 after an intermediate inspection, was to confirm the observations made for mock-up SSM C1. For all tests, the cooling water flow was kept at 0.8 l/s with an inlet water temperature of 16 °C. Beam scrapers were used to protect the pipe connection of the mock-ups from exposure to the neutral hydrogen GLADIS beam and thus any incident heat flux.

<table><caption>Table 2: High heat flux test program as realized for the two C-armoured small-scale mock-ups (SSM C1 & C2) and the W-armoured mock-up (SSM W1).</caption><thead><tr><th rowspan="2"></th><th colspan="5">screening 1</th><th colspan="5">cyclic loading 1</th><th colspan="5">screening 2</th><th colspan="5">cyclic loading 2</th></tr><tr><th>6</th><th>8</th><th>10</th><th>12</th><th>15</th><th>10</th><th>6</th><th>8</th><th>10</th><th>12</th><th>15</th><th>10</th><th>6</th><th>8</th><th>10</th><th>12</th><th>15</th><th>10</th></tr></thead><tbody><tr><td>heat flux in MW/m²</td><td>6</td><td>8</td><td>10</td><td>12</td><td>15</td><td>10</td><td>6</td><td>8</td><td>10</td><td>12</td><td>15</td><td>10</td><td>6</td><td>8</td><td>10</td><td>12</td><td>15</td><td>10</td></tr><tr><td>pulse length in s</td><td>2 to 10</td><td>5 to 10</td><td>5 to 15</td><td>10</td><td>5</td><td>10</td><td>2 to 10</td><td>5 to 10</td><td>5 to 15</td><td>10</td><td>5</td><td>10</td><td>2 to 10</td><td>5 to 10</td><td>5 to 15</td><td>10</td><td>5</td><td>10</td></tr><tr><td>pulses on mock-up SSM C1 (sum per heat flux)</td><td>8</td><td>8</td><td>6</td><td>3</td><td>3</td><td>101</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>pulses on mock-up SSM C2 (sum per heat flux)</td><td>6</td><td>5</td><td>7</td><td>3</td><td>3</td><td>151</td><td>6</td><td>5</td><td>7</td><td>3</td><td>3</td><td>151</td><td>6</td><td>5</td><td>7</td><td>3</td><td>3</td><td>151</td></tr><tr><td>pulses on mock-up SSM W1 (sum per heat flux)</td><td>8</td><td>6</td><td>9</td><td>3</td><td>2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>
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Several diagnostics were used to monitor the behavior of the mock-ups during HHF testing.

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This includes photographic imaging using a Nikon D80 digital camera, thermographic

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imaging using an Infrantec VarioCam HD and an Infrantec ImageIR 8300 hs, surface

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temperature measurements via a Maurer QKTRD two-color pyrometer (spot size Ø ≈ 10 mm)

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and a Kleiber 740 one-color pyrometer (spot size Ø ≈ 20 mm), and temperature recording via

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the attached type K thermocouples. By default, the pyrometers were focused on tile three

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(with respect to the cooling water inlet) during cyclic loading and used to calibrate the

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thermographic images. Based on earlier work [15], we assume a measurement uncertainty of

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±5% for the surface temperature determination, accounting for, e.g., minor emissivity

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changes due to alternating surface properties, which, however, are not expected in the present

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case. In addition to the temperature measurements, the operating parameters of the GLADIS

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test facility, e.g., the beam and cooling water parameters, were recorded.

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## 2.3 Pre- and post-high heat flux exposure characterization

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After HHF-testing, the surface of each mock-up was inspected using a Keyence VHX digital

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optical microscope. Stitched maps covering the plasma-facing surface of all tiles were

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generated. In addition, the tiles were inspected from the sides, including the interface region

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with the TZM heat-sink material. As the surface inspection via digital optical microscopy is

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non-destructive, a similar analysis has already been carried out before the HHF testing,

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allowing a comparison between the pre- and post-HHF exposure state. This also applies to

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infrared thermography testing using the SATIR facility at CEA-IFRM, France. In these tests,

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with a setup similar to the one shown in Garita et al. [5], the mock-ups were connected to a

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cooling water system (10 bar), which allowed to rapidly decrease the temperature from

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100 °C to 10 °C. By monitoring and comparing the thermal response of the tiles with infrared

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cameras, expressable via a time constant deduced from the surface temperature cool down

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curves, this method can reveal defects like, e.g., a local increase in thermal contact resistance

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due to debonding of the tiles or defects within the material itself. As no accompanying

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numerical simulations and calibrations were performed in this case, the assessment of the

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location and dimensions of potential defects herein is of a qualitative nature.

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For a detailed characterization of microstructural features, dedicated pieces (A-D, see Figure

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1) were cut from the mock-ups SSM C2 and SSM W1 after HHF testing using electrical

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discharge machining. Surfaces of interest were then prepared with metallographic techniques

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like grinding and polishing, and subsequently analyzed with a Keyence VHX digital

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microscope or a Thermo Fisher Scientific Axia ChemieSEM scanning electron microscope.

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# 3. Results

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## 3.1 C flat-tile mock-ups

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Figure 2 a) shows the surface temperature evolution, determined with the two-colour

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pyrometer, during the initial screening campaign of the mock-ups SSM C1 and SSM C2. The

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surface temperature increases linearly with increasing heat flux. Both mock-ups show a

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consistent behavior and reach comparable surface temperatures at a given incident heat flux.

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In Figure 2 b), the surface temperature evolution during cyclic loading at 10 MW/m² is

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depicted. Despite a constant incident heat flux, the surface temperature increases with

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increasing cycle number by approximately 0.5 °C/cycle. This is reproducible for both mock-

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ups. Even after an intermediate inspection of mock-up SSM C2, which involved a complete
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un- and remounting of the mock-up in GLADIS and hence a reset of the test facility, the surface temperature increase continues.

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<figure><img src="image_3.png" /><figcaption>Figure 2: Surface temperature measured with the two-colour pyrometer during a) the screening testing (stepwise increase of heat flux, see Table 2) and b) cyclic loading at 10 $MW/m^2$, 10 s, of C flat-tile mock-ups SSM C1 and C2 in line with Table 2.</figcaption></figure>

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The thermocouple measurements shown in Figure 3 a) reveal a similar trend. The temperatures at the respective measurement positions increase with increasing cycle number. The temperature readings of the thermocouples TC2 and TC3 are higher than those of thermocouples TC1 and TC4. This is plausible according to the ANSYS simulation shown in Figure 3 b) and caused by the actually realized Gaussian heat flux profile (Figure 3 c)), the component geometry, and the cooling channel geometry, leading to different, position-dependent temperatures. For the ANSYS simulation, steady-state, a Gaussian heat flux profile with a peak load of 10 $MW/m^2$ in the mock-up center, and a one-sided heated pipe with a swirl in the subnucleate boiling regime as a cooling channel are assumed. Accordingly, the highest surface temperatures are reached at the outer edges of the two center tiles (tile two and tile three) close to the positions of thermocouples TC2 and TC3. The lowest surface temperatures occur at the two outer tiles (tile one and tile four) where thermocouples TC1 and TC4 are positioned. The numbering of the tiles and thermocouples follows the cooling water flow, i.e., tile one is the tile close to the water inlet on the right side, while tile four is the tile close to the water outlet on the left side (Figure 3 b)). The differences between the two mock-ups SSM C1 and SSM C2 with respect to TC1 to TC4 can be caused by inaccuracies during the thermocouple positioning process and are not relevant herein, as the trend of an increasing temperature with increasing cycle number is confirmed nonetheless.
<figure><img src="image_4.png" /><figcaption>Figure 3: a) Maximum thermocouple temperature reading during cyclic loading at 10 MW/m² of C flat-tile mock-ups SSM C1 and C2, b) the corresponding thermocouple positions on the mock-up, including the temperature distribution for a peak heat flux of 10 MW/m² simulated with ANSYS (half mock-up), and c) the experimentally determined actual Gaussian heat flux profile realized on the mock-ups at a nominal heat flux of 10 MW/m².</figcaption></figure>

Figure 4 a) and b) show thermography images of mock-up SSM C2 taken during the first and 150th pulse of the cyclic loading campaign at 10 MW/m². Despite the absolute temperature increase, there is no apparent qualitative difference with respect to the surface temperature distribution. Moreover, the surface temperature distribution qualitatively agrees well with the simulation-based expectation (Figure 3 b)). Corresponding digital camera images taken
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during the first and 150th pulse are shown in Figure 4 c) and d). They also agree well with the qualitative surface temperature distribution shown in the thermography images.

<figure><img src="image_5.png" /></figure>

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<caption>Figure 4: Thermography images of mock-up SSM C2 during the first (a)) and 150th (b)) pulse at 10 MW/m² and the corresponding digital camera images after the first (c)) and 150th (d)) pulse (tiles one to four from right to left).</caption>

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Figure 5 compares the surface appearance of mock-up SSM C2 before HHF testing (a)) and after 150 cycles of cyclic loading at 10 MW/m² (b)). There are no cracks visible in the C tiles, even at higher magnification. The darker appearance on the left and right sides of the mock-up is a consequence of a partial shielding of these sides from the beam due to the beam scrapers that were used to protect the pipe connection from beam exposure.

<figure><img src="image_6.png" /></figure>

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<caption>Figure 5: Top view of the mock-up SSM C2 a) before HHF testing, and b) after 150 cycles of loading at 10 MW/m² made using the digital optical microscope (tiles one to four from right to left).</caption>

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Figure 6 shows C tiles, including the interface with the TZM heat sink, of SSM C2 viewed from the side before HHF testing (a)) and after 150 cycles of cyclic loading at 10 MW/m²
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(b)). A comparison of both does not indicate any debonding. Only for one out of all C tiles

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(SSM C1 and C2), a crack in the C tile parallel to the beam exposed surface is identifiable

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(Figure 5 c)).

<figure><img src="image_7.png" /><figcaption>Figure 6: Representative selection of side views of tiles one to four (from the top to the bottom) of the mock-up SSM C2 a) before HHF testing and b) after 150 cycles of loading at 10 MW/m² made using the digital optical microscope. In tile one only, a crack occurred in the C flat-tile during HHF testing (c)).</figcaption></figure>

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However, cracks in the TZM material have been identified in all segmentation grooves between the tiles, as exemplified in Figure 7. The cracks are oriented perpendicular to the cooling channel and extend from one side of the mock-up to the other, i.e., over the full width.

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<figure><img src="image_8.png" /><figcaption>Figure 7: Top view of the mock-up SSM C2 after 150 cycles of loading at 10 MW/m², revealing cracks in the segmentation grooves between the tiles.</figcaption></figure>

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The thermal response of the mock-ups SSM C1 and C2 after 100, respectively 150, cycles of loading at 10 MW/m² remains unaffected by these cracks, as the comparative SATIR tests depicted in Figure 8 a) show. The visualized position-dependent graphic time constants, as obtained during SATIR testing, are not significantly different before and after HHF testing. Therefore, further local defects or a debonding of the tiles are not indicated. Figure 8 b) shows the surface temperature-time evolution of the four C tiles of mock-up SSM C2 after 150 cycles of HHF testing at 10 MW/m² during SATIR testing. The temperature curves match each other and imply that there is no difference between the tiles despite the fact that the two center tiles reached higher temperatures during HHF testing (Figure 3).

<figure><img src="image_9.png" /></figure>

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Figure 8: Thermal response of mock-ups SSM C1 and C2 represented via position-dependent graphic time-constant (visualized via a color scale bar) as obtained during SATIR testing before and after 100, respectively 150 cycles of loading at 10 MW/m² (a)) (tiles one to four from right to left), and b) the temperature-time evolution of the surface temperature of the four tiles of mock-up SSM C2 during SATIR testing after 150 cycles of loading at 10 MW/m².

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In Figure 9, back-scattered scanning electron microscopy images of cross sections prepared from mock-up SSM C2 after 300 cycles of HFF loading at 10 MW/m² are shown. Regardless of the tile position on the mock-up (piece A versus piece B, cut along the cooling channel direction, see Figure 1) or the position within the same tile, i.e., in the center of the mock-up or at the edges (piece C, cut perpendicular to the cooling channel direction, see Figure 1), an additional phase of comparable thickness is present between the C tile and the Ti interlayer despite the different, location dependent temperatures reached during HHF testing. Figure 9 f) shows that such an additional phase is also present in a test sample provided by RTC engineering that has not been exposed to HHF testing. In all cross-sections, there is no indication of a detachment or crack formation along the interfaces. Figure 9 g) shows the microstructure near the cooling channel (piece B). There is no indication of a cooling channel deterioration or the formation of additional phases or layers on the cooling channel wall.
<figure><img src="image_10.png" /><figcaption>Figure 9: Back-scattered scanning electron images of the interface between C armour tiles and the TZM heat sink of mock-up SSM C2 after 300 cycles of HHF loading at 10 MW/m² taken on a) piece A (see Figure 1), b) piece B, c) close to the left edge on piece C, d) in the center of piece C, e) close to the right edge of piece C, f) on a reference sample not exposed to any HHF, and g) of the region near the cooling channel (piece B).</figcaption></figure>

3.2 W flat-tile mock-ups

The surface temperatures determined for SSM W1 during the HHF screening tests up to 15 MW/m², as specified in Table 2, are presented in Figure 10 a). With increasing heat flux, the surface temperature linearly increases. The obtained surface temperatures are lower compared to the C flat-tile mock-ups, which is expected according to the higher thermal conductivity of W [16] compared to the used C [13]. After the screening tests at 15 MW/m² a leakage that prevented further HHF testing occurred. The thermographic image and the digital camera image taken during the second 15 MW/m² pulse are shown in Figure 10 b) and c). The qualitative surface temperature distribution matches the one implied by the simulation (Figure 3 b)).
<figure><img src="image_11.png" /></figure>

Figure 10: Surface temperature measured in the center of tile two with the one- and two-colour pyrometer during the screening testing of the W flat-tile mock-up SSM W1 (a)) and b) a thermographic image with c) the corresponding digital camera image during the second pulse at 15 MW/m² (tiles one to four from right to left).

Figure 11 shows the surface of the mock-up before HHF testing and after the screening up to a heat flux of 15 MW/m². Cracks oriented parallel to the cooling channel direction are visible in the middle of all tiles, while they are especially pronounced in the two center W tiles (tile two and tile three).

<figure><img src="image_12.png" /></figure>

Figure 11: Top view of mock-up SSM W1 made using the digital optical microscope a) before HHF testing and b) after screening until a heat flux of 15 MW/m² with apparent cracks parallel to the cooling channel direction (tiles one to four from right to left).
Figure 12 a) shows a cut piece of mock-up SSM W1 (piece D, see Figure 1) with the cut plane being in the segmentation groove. According to the corresponding digital microscopy image in Figure 12 b), the cracks identified in Figure 11 initiated in the W tiles and can extend down to the TZM material and even to the cooling channel. The interface between the W tile and the TZM is intact and does not deflect the crack. In addition to the cracks in the W tiles, cracks in the segmentation grooves between the tiles, as observed for SSM C1 and SSM C2, are present. Figure 12 c) shows an image of such a crack. Based on Figure 12, there is no indication that these cracks in the segmentation grooves also extend down to the cooling structure.

<figure><img src="image_13.png" /><figcaption>Figure 12: Images of a) cut piece D (see Figure 1) of mock-up SSM W1 after HHF testing, depicting b) a deep crack from the W tile to the cooling channel, and c) the side edge of piece D showing crack formation in the segmentation groove.</figcaption></figure>

4. Discussion
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The HHF tests performed on two C flat-tile mock-ups and one W flat-tile mock-up reveal common features related to the design, the joining technology, and the nature of the TZM heat sink, but outline differences regarding the thermo-mechanical performance depending on the tile material.

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All mock-ups exhibited cracks in the TZM material located in the segmentation grooves between the armour tiles (Figure 7, Figure 12 c)). During thermal loading, there is a significant difference in thermal expansion of the beam exposed side compared to the cooler back side of the mock-ups, which causes repetitive bending during cyclic HHF tests and leads to fatigue cracking [17]. As the segmentation grooves act like a notch, with notches leading to local stress concentrations [18], the localization of such fatigue cracks in the segmentation grooves is plausible. The cracks in the segmentation grooves do not affect the thermal response of the mock-up (Figure 8) and, therefore, are not critical unless they impact the structural integrity or leak tightness of the component upon prolonged cyclic loading. This is not the case for the cycle numbers realized in this work.

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For all mock-ups, the surface temperature increased linearly with increasing heat flux during the screening HFF tests (Figure 2 a), (Figure 10 a)) as expected. However, the continuous temperature increase observed for mock-ups SSM C1 and C2 during cyclic loading at 10 MW/m² requires further discussion. As this temperature increase is reflected in the pyrometer surface temperature measurements (Figure 2 b)) and the thermocouple measurements approximately eight millimeters away from the surface (Figure 1 c)), a systematic drift of the measurement systems can be excluded. The fact that the surface temperature increase continues even after mock-up SSM C2 has been un- and remounted in GLADIS for an intermediate inspection allows to furthermore exclude a systematic heat-up or parameter drift of the GLADIS device during cyclic operation. Instead, the temperature increase must be due to an alteration of the mock-ups. The C tiles themselves, except for one singular event of a small crack at the side of one tile, remain unaffected by the HHF testing and do not show any apparent defects (Figure 5, Figure 6). The comparison of the thermography images and the digital camera images (Figure 4) before and after 150 cycles of thermal loading at 10 MW/m² does not reveal any local defects like a partial debonding. Such a debonding would be visible as hot region deviating from the qualitative temperature distribution expected according to the simulation (Figure 3 b)). The side views of the C tiles bonded to TZM (Figure 6) confirm that there is no visible detachment of the C tiles. This is in line with the SATIR results (Figure 8), which stress that, regardless of the actual heat flux imposed on the tiles due to the Gaussian beam profile of GLADIS, there is no significant difference in the thermal response between the tiles. The comparison of cross-sections of the interface between the C tiles, the Ti interlayer, and the TZM heat sink at different positions of mock-up SSM C2 (Figure 9 a)-e)) shows that there are no position-dependent differences. The formation of an additional phase at the C-Ti interface is observed in all cross-sections in equal measure, and there are no indications of a bonding failure. Such an additional phase at the interface between the C tiles and the Ti interlayer is also observed for a reference sample (Figure 9 f)) and therefore not caused by HHF testing. Instead, this phase forms during the 2.5 h long diffusion bonding process performed at 1320 °C. Given the contact partners, the phase formed is TiC, which is observed elsewhere [19,20]. The temperature increase of the mock-ups during cyclic HHF loading is therefore neither related to the C armour tiles nor the interface, and must be related to the heat removal capability of the TZM heat sink. This
includes possible changes of the TZM thermal material properties or changes related to the cooling channel and swirl. According to Figure 9 g), there are no apparent features in the cooling channel that indicate a cooling channel deterioration, leading, e.g., to a surface roughening of the channel walls, or the formation of additional phases or layers, e.g., a corrosive layer, which would alter the heat transfer from the mock-up to the cooling water. However, as the focus of this work was on the joining technology, the destructive investigation of the tile-heat sink interface was prioritized, meaning that a dedicated monitoring of the cooling channel state and the swirl throughout the HFF tests is not given and cannot be reconstructed in retrospect. With respect to the scope, it can be concluded that the process of joining C armour tiles on a TZM heat sink with a Ti interlayer via diffusion bonding can be considered as suitable for manufacturing reliable flat-tile mock-ups.

Other than the C flat-tiles, the W flat-tiles developed cracks during the screening testing. The cracks are oriented in the direction of the cooling channel and extend from the surface down to the cooling structure (Figure 11b), Figure 12 b)), causing a leakage. This is in contrast to W armoured flat-tiles on a CuCrZr heat sink with a soft Cu interlayer that have been reported to develop cracks in the W tiles perpendicular to the cooling channel direction during cyclic HHF testing [21]. Instead, the occurrence of cracks in the direction of the cooling channel is similar to cracks forming in W monoblocks [22], which is referred to as deep cracking [23]. The fact that under similar HHF loading conditions, such cracks only initiated in W and not in the C flat-tiles, suggests the conclusion that the brittle nature of W is critical for the chosen mock-up design. The bonding between the W flat-tiles and the TZM heat sink, as such, is good, as the cracks did not deflect at the interface between the W tiles and the heat sink. This proves the capability of the chosen diffusion bonding process. Nevertheless, the resulting W-armoured TZM flat-tile mock-up would require further development to avoid detrimental deep cracking during HHF testing, which, to the author's knowledge, is currently not intended.

Since the detrimental deep cracking during screening testing prevented further testing, a surface temperature increase during cyclic loading due to possible changes of the TZM thermal material properties or changes related to the cooling channel and swirl, as observed for the C-armoured mock-ups, could not be revealed. However, a similar behavior of the W- and C-armoured mock-ups in this respect can be expected due to the identical heat sink design.

## 5. Summary & Conclusion

Three small-scale flat tile mock-ups were tested in the high heat flux test facility GLADIS. Two mock-ups had C armour tiles, and one mock-up had W armour tiles, which were diffusion bonded to a TZM heat sink with the help of a Ti interlayer.

The mock-ups with C armour tiles showed a comparable performance during screening testing up to a heat flux of 15 MW/m² and during cyclic loading. Cyclic loading was performed at 10 MW/m² with a pulse length of 10 s and a repetition of up to 300 cycles. Based on visual inspections before and after HHF testing, thermographic images during cyclic loading, and SATIR thermal response tests, there is no indication of relevant failure of the C flat-tiles or their bonding to the TZM heat sink. Scanning electron images on cross-sections prepared after HHF testing underline the good bonding between the C tiles, the Ti interlayer, and the TZM heat sink, with TiC forming between the C tiles and the Ti interlayer.
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during the diffusion bonding process. A continuous surface temperature increase observed
during cyclic HHF testing with increasing cycle number is not related to the C flat-tiles or the
bonding and indicates a potentially decreasing heat removal capability of the TZM heat sink.

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Furthermore, cracks perpendicular to the cooling channel direction developed in the
segmentation grooves between the tiles.

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The joining process of W armour tiles to TZM also works well. However, a leakage of the W-armoured mock-up developed during screening testing at 15 MW/m². Detrimental deep cracks, oriented in the direction of the cooling channel, initiated in the middle of the W tiles and extended down to the cooling channel. In addition, cracks perpendicular to the cooling channel direction developed in the segmentation grooves between the tiles.

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Despite a potentially decreasing heat removal capacity of the TZM heat sink during cyclic
HHF testing and detrimental crack formation in the W-armoured mock-up, the applied
diffusion bonding process itself, including the use of a Ti interlayer, is well suited for
manufacturing flat-tile mock-ups.

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## 6. Acknowledgements

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The authors thank T. Vavro and P. Liska from RTC engineering, Slovakia, for providing samples and information about the manufacturing process.

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This work has been carried out within the framework of the EUROfusion Consortium, funded
by the European Union via the Euratom Research and Training Programme (Grant
Agreement No 101052200 — EUROfusion). Views and opinions expressed are however
those of the author(s) only and do not necessarily reflect those of the European Union or the
European Commission. Neither the European Union nor the European Commission can be
held responsible for them.

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## 7. Conflicts of interest

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Co-author Marianne Richou is listed as an editorial board member. There are no further
conflicts of interest to disclose.

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## 8. Data availability

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All relevant data are included in the article. Additional data can be obtained from the authors
upon reasonable request.

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