Annual Report 1997/98

Introduction to the Annual Report, summarising work performed
April 1997 - March 1998.

1 The European Fusion Programme

The fusion of light nuclei releases energy which could one day be used to generate electricity. Power generation using fusion would have the advantages of plentiful fuel supply, no major accident potential and minimal environmental impact (no emissions of greenhouse gases and little or no waste burden on future generations). It does, however, require very high temperatures at which matter is in the plasma state. To develop an economic power plant, the challenges are to confine the plasma for long enough at high enough density and temperature, and to develop materials and systems that are compatible with this.

Because of its potential advantages, fusion is being investigated as an option for sustainable electricity generation in the twenty-first century. This research is at the European and international level because of the scale of the research and its long-term nature. The current strategy of the European programme is to progress from today's experiments to fusion power plants via three related elements: (a) focusing on the most developed system, the tokamak; (b) exploring concept improvements that may, in the longer term, be more attractive; and (c) developing the technology required for power plants.

The European programme is fully co-ordinated and consists of: (a) programmes at national laboratories such as Culham in the UK; (b) experiments on the Joint European Torus (JET), the world's largest fusion device adjacent to the programme at Culham but managed as a Joint Undertaking; (c) work for ITER including contracts placed directly with European industry; and (d) fusion work at the European Community's Joint Research Centre at Ispra. The JET Joint Undertaking will end in December 1999 - negotiations are under way to decide how the JET facilities could be exploited after this. ITER (the International Thermonuclear Experimental Reactor) is being designed by Europe, Japan, the US and the Russian Federation with the aim of testing sustained, burning plasmas and power plant technology (Canada also participates via the European partner). Much of the work in the programme at Culham is directly relevant to ITER and contributes to it via ITER R&D Tasks and secondments to ITER Teams.

The UK's contribution to the European programme is carried out via a Contract of Association between UKAEA and the European Atomic Energy Community (EURATOM). Most of the research is undertaken by UKAEA at Culham with the remainder by universities and the Rutherford-Appleton Laboratory. The EURATOM/UKAEA fusion programme is funded by the UK Department of Trade and Industry and by EURATOM.

Most of the European research is into confining fusion plasmas using magnetic fields, but there is also "keep-in-touch" work on inertial fusion in which fusion reactions are induced by compressing small, high density fuel pellets using, for example, high energy particle beams or lasers. The EURATOM/UKAEA Association participates in this with work at the Central Laser Facility of the Rutherford-Appleton Laboratory.

2 Programme Strategy and Management

The strategy of the EURATOM/UKAEA programme in 1997 was to contribute as effectively as possible to the European programme by:

• providing close support to JET through Task Agreements, including in the deuterium-tritium campaign DTE1 in which record fusion powers and energies were liberated;
• helping develop the tokamak concept, including ITER, via experiments on the COMPASS-D and START devices at Culham, utilising their high power heating and current drive systems, and via the closely-related
• procuring components for the Mega Amp Spherical Tokamak (MAST), the successor to START scheduled to commence operation in 1998;
• undertaking a programme of fusion power studies concentrating on safety, environmental and socio-economic issues, both for the long-term development of fusion and for input to ITER;
• providing input to the keep-in-touch activities on inertial fusion;
• helping industry to benefit from the fusion programme, including assistance with identifying spin-offs into other markets.

The programme is managed in five programme areas corresponding to different aspects of the strategy, namely:

• Support for JET (mainly work on JET Task Agreements),
• Tokamak Development (COMPASS-D, START and MAST)
• Fusion Power Studies
• Theory and Modelling
• Industry

External relations and other activities are now summarised briefly, followed by a description of the relationship of the EURATOM/UKAEA Fusion programme to the European and international programmes and a summary on the UK contribution to the European "keep-in-touch" work on inertial fusion. (Note, there is also a very small amount of keep-in-touch work on much more speculative approaches, eg. muon-catalysed fusion and bubble fusion.)

3 External Relations and Other Activities

Public acceptability is an important aspect of fusion and must be given appropriate attention in parallel to the scientific research; this section covers the areas devoted to this, eg. contributions to public understanding of science, safety, quality assurance, training etc.

3.1 Public Affairs

During the past year there have been several international visitors to Culham Science Centre as well as those from UK and the rest of Europe. These have included Dr Gordon Adam MEP, Vice President of the Committee for Research, Technological Development and Energy (CRTDE) of the European Parliament; Mr James Elles MEP for Buckinghamshire and Oxford East; Members of the UK Parliament including Dr Evan Harris MP, Liberal Democrat member for Oxford West and Abingdon; as well as senior members of other fusion programmes such as Professor R Goldston, Director of Princeton Plasma Physics Laboratory and Dr Richard Bolton, Director of External Affairs for the Centre Canadien de Fusion Magnétique (Varennes, Québec).

Culham Science Centre has been open for a number of events including Industry Days, an Education Day, and an Open Day for the general public which was held as part of the UK's Science, Engineering and Technology SET 98 week and featured the European mini-Expo on fusion.

3.2 Safety

Public acceptability of fusion will only be won if we are seen to operate in a safe manner, minimising lost-time accidents and ensuring no hazardous emissions. The Assistant Director was responsible, through the Fusion Director and the Chief Executive, to the Chairman of UKAEA for safety activities. This responsibility for safety devolved through the line management function to individual staff. The Fusion Safety and Environment Committee has provided advice and made recommendations to the Director on matters affecting safety and the Fusion Plant Safety Panel (FPSP) has continued to advise the Director and facilities' managers on the technical safety of plant and adequacy of safety documentation. In addition, the FPSP approved the pre-commissioning Safety Report for the MAST facility. There was one 1-man-day lost-time accident to Fusion personnel following a fall from a ladder, which resulted in increased training on the safe use of ladders.

3.3 Quality Assurance

UKAEA has continued to operate to its ISO9001 certified quality system and the new and revised procedures controlling fusion activities, introduced in March 1997, have helped to maintain and improve the quality of the output from the research programme.

The operation of the quality system has been checked by the ongoing series of internal audits and by the regular surveillance visit from Lloyds Register Quality Assurance (LRQA) in November 1997. The number of non-compliances found by the internal audits has fallen and the LRQA visit recorded no non-compliances. A drive is now under way to ensure that any non-compliances that are identified are closed off within a short period of the audit.

3.4 Continuous Improvement

The UKAEA Continuous Improvement initiative continued to gain impetus within UKAEA Fusion. Many suggestions for improvement were successfully implemented by some ten enthusiastic teams and new groups were identified to look at safety incident reporting and stores management. The COMPASS Physics team successfully introduced improvements to data access and computational facilities while the Administration team made significant progress in the areas of training, teamwork and standardisation of procedures. Results of a survey carried out during the year indicated that Culham staff considered the Continuous Improvement programme to have brought substantial benefits in terms of improved efficiency and team-working.

3.5 Staff Training - Investors in People

UKAEA is fully committed to the principles of Investors in People (IiP) and was successful in achieving IiP accreditation in December 1997, at the first attempt. Training and development of Fusion staff is co-ordinated and reviewed by the Culham Training Panel which is responsible for formulating and managing the Culham Training and Development Plan. A number of key initiatives have been launched during the last year including, for example, a training programme to develop more staff who are skilled to undertake operational duties on the COMPASS-D tokamak.

As well as UKAEA and EURATOM staff, visiting scientists (including Marie-Curie fellows) and contractors, many students - from pre-university to post-graduate level - are involved in the programme. This provides valuable training for their future involvement in the international fusion programme or for careers in industry or in academic research.

3.6 The Culham Summer School

A large number of post-graduate students from EURATOM fusion laboratories, and from universities and other research institutes throughout Europe, attended the 1997 Culham Summer School. Lecturers from Cadarache, Culham, Gothenburg, JET, UK universities and the Commission gave scientific talks. Now in its 36th year, the Summer School continues to make an important contribution to the training of young scientists.

3.7 Collaborations

UKAEA collaborates with numerous institutes as part of the European and international fusion programmes. These include: the NET and ITER Teams; JET; IPP Garching and FZ Jülich (Germany); IST Lisbon (Portugal); ENEA Frascati (Italy); CRPP Lausanne (Switzerland); NFR, Chalmers (Sweden); CIEMAT (Spain); General Atomics in San Diego, Oak Ridge National Laboratory, University of Wisconsin at Madison, Massachusetts Institute of Technology, Institute of Fusion Studies at Austin, Princeton Plasma Physics Laboratory, and University of California at San Diego (US); INPE, Brazil; Kurchatov and TRINITI Institutes, Moscow; Efremov Institute and Ioffe Institute, St Petersburg; Moscow State University; KAERI and KBSI Korea; University of Tokyo and JAERI (JAPAN); University of Flinders (Australia).

There are also collaborations with several UK universities - Imperial College, University of Manchester Institute of Science and Technology, University College London, Oxford, St Andrews, Glasgow, Strathclyde, Essex, Nottingham, Warwick and Sussex - and links with Irish Universities via the EURATOM-DCU Association, including Cork, Queens University Belfast, University College Dublin.

The work for ITER involves collaborations with a number of other institutions. In addition much of the safety, environmental and socio-economic work is performed collaboratively in Europe through the Safety and Environmental Assessment of Fusion Power (SEAFP-2, for which UKAEA is co-leader) and Socio-Economic Research in Fusion (SERF) programmes.

Links to basic science research are strengthened through collaborations involving institutes such as CEA Saclay, the Max Planck Institute for Nuclear Physics in Heidelberg, the Rutherford Appleton Laboratory (UK), and the Dublin Institute for Advanced Studies.

4 Relation to the International Fusion Programmes

The EURATOM/UKAEA research on magnetic fusion is exclusively on the most developed system, the tokamak. [Other European countries, as part of the concept improvement component of the European strategy, conduct research into alternative systems like the stellarator (Germany, Spain) and the reverse field pinch (Italy, Sweden).] COMPASS-D is a conventional tokamak with similar geometry to JET and ITER, and START is a low aspect ratio tokamak in which the plasmas appear almost spherical. Plasma physics experiments on START stopped at the end of March 1998 to allow assembly of MAST.

In the following paragraphs the main issues and recent progress in magnetic fusion research are summarised, focusing on the tokamak and emphasising UKAEA contributions. A lot of the work is focused on contributing to the ITER line, but UKAEA also play a leading role in the development of the spherical tokamak and long term fusion technology issues.

4.1 ITER

A. Background

The Engineering Design Activities (EDA), which commenced in 1992 and are scheduled to end in July 1998, have resulted in the ITER Final Design Report. A three year extension to the EDA is likely, in which site-specific design adaptations; preparation for licensing application; prototype testing; physics R&D; design modification to minimise cost, including broader options will be pursued. Over the six years of the EDA the design has evolved and uncertainties reduced as a result of an internationally co-ordinated programme of research and development.

The EURATOM/UKAEA programme has contributed to this, particularly with experiments on COMPASS-D, JET (through Task Agreements) and related theory, providing data to ITER databases and participating in a number of ITER Expert Groups. COMPASS-D is a small, but technologically advanced, member of the European "family" of ITER-shaped devices - ASDEX-Upgrade (Germany) and JET are the others in increasing size - which permit scaling experiments to ITER conditions. The spherical tokamak studies, discussed mainly in Section 4.2, have contributed to ITER by providing data which can be used to test models and theories of tokamak behaviour in new regimes. The Fusion Power Studies programme has contributed, particularly in the areas of safety and neutronics modelling, and by secondment of staff to the European ITER Home Team in Garching and the ITER Joint Central Team in San Diego. UKAEA has participated in Expert Groups and European reviews of ITER, and has a European member of ITER's Technical Advisory Committee. The following describes some of the technical issues for ITER in more detail.

B. Heating and Confinement

In power plants and in ITER the plasma would be heated by the fast a -particles born in fusion reactions - the other fusion products, neutrons, will escape from the plasma and be used to (a) generate tritium required for fuel, and (b) provide the heat required to generate electricity. Experiments with deuterium-tritium mix plasmas on JET in 1997 (the DTE1 campaign) confirmed that a -particle heating is as expected from classical models of a-particle behaviour. Plasmas in most of today's experiments are pure deuterium (or hydrogen) and have many fewer fusion reactions: these are heated by either high power radio-frequency waves (as on COMPASS-D) and/or by beams of energetic neutral particles (as on START and later MAST). These heating methods can also be used to drive currents in plasmas - see Section E below.

The size and cost of a power plant will be determined by how efficient the plasma is at retaining heat and particles - its confinement properties. Losses of heat and particles are largely determined by turbulent processes which are not fully understood. Predictions of the confinement performance of future devices like ITER mainly come from empirical scaling laws constructed to fit data from numerous tokamak plasmas with varying size, density, magnetic field, etc., though theory-based models are becoming increasingly successful at reproducing experimental data. Confidence in the empirical scaling laws can be further improved by comparing discharges on different-sized tokamaks, arranged to have equal dimensionless parameters.

There are different confinement regimes, notably the L- and H-mode regimes (low and high confinement). The reference regime for ITER is the ELMy H-mode (see below), but this can only be accessed if the power heating the plasma exceeds a threshold value. This power threshold is a key question for ITER and an encouraging result from the DTE1 campaign was that the power threshold reduces if the fuel contains tritium (most other experiments use deuterium or hydrogen fuel and have a higher threshold power). On a more negative note, scaling laws for this threshold power, that fit most experiments, fail for the lowest density plasmas on COMPASS-D, when it becomes increasingly difficult to access H-mode.

A key issue for the confinement performance of ITER is the nature of the insulating layer at the edge of H-mode plasmas which manifests itself as density and temperature "pedestals". Some models of ITER performance are sensitive to assumptions about the amplitude and width of the pedestal. The large pressure-gradient in the pedestal region can induce plasma instabilities (so-called Edge Localised Modes, or ELMs) which can deposit large bursts of heat at the target plates where the plasma meets a material surface, limiting their lifetime. On the other hand, ELMs have the beneficial effect of limiting the plasma density and purging the plasmas of impurities which, if allowed to accumulate, could both dilute the fuel and lose plasma heat via radiation. For this reason, it is anticipated that ITER will operate with small ELMs (ie. the ELMy H-mode).

An operating mode with good confinement and density control and no ELMs has been observed on the circular cross-section tokamak TEXTOR at Jülich (Germany). This Radiating Improved Mode (RIM) has the advantage that most of the power is radiated and so the peak power loading on material surfaces is reduced. It is important to establish whether this RIM mode of operation is an option for ITER geometry and conditions.

C. Stability

A tokamak's performance is usually limited by the onset of plasma instabilities which can limit the plasma pressure (and therefore its performance), and can result in plasma "disruptions" in which control of the plasma is lost and damage to surrounding structures can result, particularly in larger devices. Other instabilities have different consequences, such as ELMs (see above) which are localised at the edge.

"Neoclassical" instabilities have been observed to limit the pressure in experiments such as COMPASS-D, ASDEX-Upgrade and JET, and are of concern as they might set the pressure limit in ITER. Theoretical models have been developed at Culham and elsewhere which explain these instabilities in terms of "magnetic islands" that can develop at higher pressures, partly due to the influence of the "bootstrap current" which is driven by the plasma pressure gradient.

Small imperfections in the confining magnetic field, due to coil misalignments for example, produce "error fields" which can trigger instabilities and lead to disruptions. For several years, Europe's work on error field effects has been led by UKAEA with studies on COMPASS-D and (via a Task Agreement) JET, and collaboration in work on the US tokamak DIII-D. The latest experiments confirm that the imperfections that could be tolerated in ITER are so small that a compensation system is required, and UKAEA is helping with the design of such a system (possibly supported by a means to rotate the plasma, which reduces the plasma's sensitivity to error fields).

As tokamak performance limits (e.g. on density and current) are approached there is an increasing risk of a plasma disruption. One effect of a type of disruption known as a vertical displacement event is that a "halo" current flows from the plasma into the material components of the tokamak, giving large forces on these components which might threaten the structural integrity of large devices. These halo currents are generally not toroidally symmetric, and this exacerbates the problem, resulting in large localised forces. Results on halo currents from JET and the large Japanese tokamak JT-60U have indicated that they are, in relative terms, not as big as in smaller machines like COMPASS-D, and therefore may be less of a risk for ITER than originally thought. Nevertheless, they remain a major consideration for the design, and improved understanding of these effects and how they scale is desirable. Another feature of disruptions is the generation of fast beams of "runaway" electrons which might drill holes in the components of ITER. Mitigation techniques such as injection of "killer pellets" have been proposed.

Fast particles (e.g. fusion-generated a-particles) can modify the properties of plasma instabilities, and also generate new ones. A particular concern for ITER is whether these fast particle instabilities, e.g. the toroidal Alfvén eigenmode (TAE), can result in the premature loss of a-particles before they have a chance to heat the plasma. It is encouraging that there was no clear evidence for a-driven TAE modes observed during the JET DTE1 campaign.

D. Plasma Exhaust

ITER is designed to operate in a "divertor" configuration, in which heat and particles (including cold a-particles, the helium "ash") are exhausted into a narrow layer around the plasma, the scrape-off layer or SOL. From here the energy flows to target plates to be removed from the system. An important question for ITER is the power loading that the target plates need to withstand, and this is determined to a large extent by the width of the SOL. A number of theoretical models exist for this width, and there is a wide range in their predictions for ITER. Data from COMPASS-D and other devices has been used to select the best models - these predict a rather narrow SOL width for ITER. The power that reaches the target plates is reduced if a higher fraction of power is radiated - this can be induced by the deliberate injection of impurities, as in the RIM operating mode described above in Section B.

E. Steady State Scenarios

To improve economic viability, fusion power plants should operate continuously, rather than pulsed as in most existing tokamaks. This relies on a large fraction of the plasma current being provided by the plasma pressure gradient (the "bootstrap" current), which in turn requires good confinement and high normalised pressure. The current and plasma flow profiles are believed to play key roles in achieving these conditions. Current drive experiments using radio frequency waves have been performed (e.g. on COMPASS-D, JET and TORE SUPRA in France) to investigate whether they can provide the required current profile control, with encouraging results. A particularly promising mode of operation is the Optimised [magnetic] Shear mode being developed on JET and elsewhere. The ITER design includes the capability for long pulse by exploiting such a regime. Key outstanding issues include whether the walls of the vessel can stabilise instabilities ("resistive wall modes") for long timescales, and whether high confinement, high pressure regimes are compatible with plasma exhaust methods.

F. Technology, Safety and Environment

The ITER EDA has seven major projects on the manufacture and testing of prototype components such as superconducting coils, divertor cassettes and blanket modules. Other important ITER technology issues include: the optimum design of the blanket and shield; the size of inventories of tritium and erosion dust in the vacuum vessel, and safety and the validation of safety-relevant data and models. The tritium and dust issue was highlighted by the tritium retention in the vessel and components following the DTE1 campaign on JET: work to understand this, and improve methods for removal, are under way. UKAEA has made a significant contribution to the validation of safety-relevant data and models, with comparisons between activation calculations and results from Japanese experiments, showing that materials exposed to fusion neutrons generally activate in the manner predicted theoretically.

4.2 Spherical Tokamaks

As part of Europe's concept improvement programme (cf. Section 1.1), the UK has been exploring the spherical tokamak (ST) concept with experiments on the world's first hot ST, START. This work both investigates the potential of STs for fusion applications, such as materials testing and power generation, and adds to the tokamak database thereby increasing the general understanding of tokamaks. While the ST is still a much less developed concept than the conventional tokamak, results from START have been very promising, with the tokamak world record for the value of plasma pressure divided by the magnetic pressure (b = 40%); good stability properties; and confinement and operating space at least as good as conventional tokamaks, including evidence for an improved confinement regime akin to the H-mode. High b was an expected result - that and compactness are the ST's main advantages - but perhaps more significant was the very high value of normalised b that has been achieved: b_N ~ 6 which is higher than on most other tokamaks. These successful experiments have benefited greatly from the loan of neutral beam heating equipment from the US programme and a pellet injector loaned by ENEA Frascati, and built by the Danish Fusion Association.

The encouraging results from START led to the decision to proceed with MAST, and have helped stimulate other countries to build spherical tokamaks (e.g. the US, the Russian Federation, Brazil and Japan). The objectives of MAST are to provide aspect ratio scaling for the tokamak data base, determine the plasma performance in a much larger, higher current device than START, and test the potential for steady-state operation of STs, which theoretical studies have shown should be possible. MAST will also use heating and current drive equipment loaned by the US programme.

4.3 Technology and Power Plants

Fusion Technology comprises the non-plasma-physics aspects of the design of, and R&D for, neutron-producing fusion devices (ITER and beyond), and the analysis and optimisation of their safety, environmental and economics properties. The subject is strongly materials-oriented, especially relating to the effects of fusion neutrons on the properties of materials and issues of materials compatibility. In recent years, world effort has been dominated by the ITER EDA (see Section 1.4.1A,F), though Europe and Japan have significant programmes on additional work needed for a demonstration power station (DEMO). UKAEA hosted the IAEA Technical Committee Meeting on Fusion Power Plant Design in March. There were over 40 participants from overseas, as well as UKAEA staff.

Work needed for DEMO includes the development of neutron-resilient, low-activation materials and reliable components, and studies of the design measures and materials choices needed to ensure safety and environmental (S&E) attractiveness and economic viability. UKAEA continues to make important input to European studies of these topics. Work by UKAEA, in collaboration with other European fusion research institutes, is showing that most - perhaps all - of the main S&E advantages of fusion can be attained using low-activation martensitic steels. More detailed work on improving the physics basis of systems codes for studies of conventional aspect ratio and spherical tokamak power plants has confirmed UKAEA's earlier conclusion that the development of highly reliable, neutron-resilient materials is the technological key to fusion power plant economics, while the physics key is the attainment of high normalised pressure, b_N.

5 Keep-in-Touch work on Inertial Fusion

A. Introduction

Inertial Fusion Energy (IFE) is an alternative approach to generating electricity by thermonuclear fusion. In the UK this activity is centred at the Rutherford Appleton Laboratory's Central Laser Facility (CLF) which is home to VULCAN, presently the most powerful laser in Europe. The work performed with VULCAN over the last year which is relevant to IFE is described below. EURATOM is providing some modest funding in the frame of the Community's keep in touch activities with this alternative approach.

B. VULCAN Petawatt Upgrade

The phase I of the VULCAN Petawatt Upgrade was completed this year (this upgrade was not part of the EURATOM keep in touch programme, but is relevant to the experiments reported below). The enhanced VULCAN facilities now allow all of the VULCAN beams to be deployed for studies of direct relevance to both conventional IFE and the Fast Ignition scheme. The project consisted of four phases: (a) the construction of a new target chamber to support the high irradiance science programme; (b) the installation of a new large area pulse compression and focusing system to allow higher irradiances on target; (c) the installation of a new short pulse oscillator to the front end of VULCAN; and (d) new laser instrumentation to assist the diagnosis of the high irradiance conditions achieved. The most visible enhancement to the Facility was the installation of the new target chamber and beam handling optics.

C. Fast Ignition Physics: Ultra short pulse propagation

The study of the propagation of ultra-short, intense laser pulses through low density, pre-formed plasmas is of fundamental importance, both for its own scientific merit and for its application to the Fast Ignition scheme for IFE. A low intensity, frequency quadrupled laser pulse was used to probe the channel formed by the ultra-intense laser pulse. Frequency quadrupling the probe beam gave a significant improvement in resolution and image quality compared to previous experiments. In addition, the propagation of intense laser pulses through capillary tubes was undertaken. This experiment simulated pulse propagation through the pre-drilled channel, an essential component of the Fast Ignition scheme. These results are presently under analysis.

D. Fast Ignition Physics: Fast electron transport

This experiment utilised the new ultra-intense interaction facilities of VULCAN. The transport of a beam of energetic electrons through both metals and insulating materials was studied using optical shadowgraphy. A rear surface plasma was observed to form almost immediately after the interaction beam that was both smaller than the focal spot and directly on the laser beam axis. This exciting result has far reaching implications for Fast Ignition: previous analyses had assumed a finite cone angle for the fast electron flow. Numerical modelling of these observations indicated that the electron beam was collimated in the solid density plasma by the self generated magnetic field. New features were observed in the side scattered optical channels, which may be indicative of new and unusual behaviour of the parametric instabilities in relativistic laser-plasmas. The collimated electron beam results have been prepared for publication, and further analysis of the parametric instabilities is in progress.

E. Direct Drive: Laser Imprint seeding of Rayleigh Taylor Instability

One of the major impediments to the direct drive scheme is the seeding of hydrodynamic instabilities by the initial imprint of non-uniformities of the laser focus. A high brightness x-ray laser beam was used as a face-on backlighting source, giving high spatial resolution. When accelerated foils were imaged onto a CCD camera, changes in the backlighting source brightness allowed the growth of mass perturbations to be measured. A samarium x-ray laser operating at 7.3nm was used as the backlighting source, and the Rayleigh-Taylor growth caused by the initial imprint on plastic foils of 2mm thickness were recorded. The results are under analysis.