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20 years of ISTTOK scientific activity

H. Fernandes, C. Silva, J.A.C. Cabral, C.A.F. Varandas and ISTTOK team

Associação Euratom/IST, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, Portugal

E-mail contact of main author: [email protected]

Abstract . The ISTTOK tokamak commissioning began in January 1990, after the signature of the EURATOM association agreement established with Instituto Superior Tecnico in 1990 on the field of controlled . In 1991 the first article was issue in the Portuguese journal “Gazeta de Física” and the firsts scientific reports were mentioned to the international community on the 17th Symposium on Fusion Technology, Rome 1992. This communication aims to present an overview of ISTTOK scientific activity since its commissioning in the main areas of its activity, namely (i) diagnostics in particular the Heavy Ion Beam (HIBD), (ii) AC current operations, (iii) plasma control, (iv) liquid metal limiters, (v) plasma fluctuation studies and (vi) study of fusion relevant materials. ISTTOK housed several international activities including a Joint Experiments in the framework of the IAEA Coordinated Research Project (CRP) on “Joint Research Using Small ”. Presently ISTTOK is committed within the IAEA CRP “Utilization of a Network of Small Magnetic Confinement Fusion Devices for Mainstream Fusion Research” under the research agreement “Tokamak diagnostics, plasma control and data analysis”. The ISTTOK achievements demonstrate that small tokamaks can play an important role in the fusion plasma physics community as a result of their flexibility, high availability and good opportunity for the development of sophisticated diagnostics and technology tools.

1. Historical Note

The ISTTOK (Figure 1) is a circular cross-section tokamak with a graphite limiter being operated exclusively in inductive way by means of an iron core. ISTTOK was built from the basic structure of the TORTUR Tokamak (support structure, vacuum chamber, copper shell, transformer, toroidal magnetic field coils and capacitor banks, meanwhile dismantled in FOM-Rijnhuizen in Niewegein (Netherlands) by a team of engineers and technicians from the group of Nuclear Fusion (embryo of the former CFN) in 1988. The remaining ISTTOK components (vacuum systems and gas supply, power supplies, diagnostics and system control and data acquisition) were designed and constructed by personnel of the current Association Euratom / IST or awarded to domestic companies (eg toroidal field power supply) [1].

2. Diagnostics

In addition to the basic diagnostics of a small tokamak with low density and low electron temperature, it was developed over the years several diagnostics based in new techniques or with significant technological innovations with respect to traditional techniques, namely: • Heavy ions deflection diagnostic using a cell array collector as a detector; 2 OV/P-08

• Kinetic plasma pressure measurement system based on a pendulum equipped with a Mach probe and a mechanical force sensor; • Different electric probe systems for determining a wide variety of peripheral plasma parameters; • Energy analyzer of the ions in the plasma periphery; • Ion time of flight analyzer based on applying rapid pulses of accelerating voltage; • Linear Cameras (3) for tomographic reconstruction of the plasma emissivity; • High dispersion spectrometer (465nm, 0.0015 nm); • Thomson scattering diagnostic for electron temperature measurements; • Scanning and fixed frequency reflectometer; • Microwave interferometer with differential detection of the quadrature and phase; • Magnetic probes installed inside the vacuum chamber;

Figure 1 - Photograph of ISTTOK tokamak highlighting the transformer core (red) and toroidal field coils (vertical)

3. New concepts

The ISTTOK also served to test various concepts of control and data acquisition systems, opting from its beginning to develop specific hardware due to incipient offer of this type of equipment specific for fusion machines at that time. The continued development of these systems in ISTTOK generated a group in this area which contributes actively to delivery innovative equipments to other laboratories: • The first data acquisition systems were developed over VME and consisted in products of timing (optical and electrical lines) and transient recorders that had several updates in terms of memory and speed of data recording; • After the year 2000 the ISTTOK data acquisition was equipped with PCI boards equipped with DSPs that has allowed the plasma control in real time which greatly improved the plasma performance; • Recently ISTTOK was one of the first fusion machines to adopt the ATCA systems alongside with JET and COMPASS, where communication between boards is done through advanced data interfaces based on PCIexpress and/or gigabit ethernet, allowing (i) a universality for the choice of control parameters of the machine and (ii) execution algorithms 3 OV/P-08 latency much lower, (iii) together with a significant improvement on the software development time. • The real time control system is based on fast switching power supplies developed at IPFN that use the plasma position information obtained from a set of magnetic coils for feedback; recently the use of ATCA allowed to simultaneously integrate a wide range of diagnostics which can serve as references for the control. In particular, this approach has been tested using the real time tomography system to improve the ISTTOK operation. Tomography in ISTTOK is performed with the use of three linear cameras with 30 lines of sight (total) measuring the full light intensity generated in the optical path and allows to reconstruct the profile of the plasma emissivity. • A liquid gallium jet was recently installed to test the concept of a liquid limiter and has been proven its compatibility with the plasma. However, it was found that the jet is deflected by the interaction with the plasma and therefore free-flying jets are not suitable for tokamak operation. • In the last years, studies of plasma facing components have been pursuit were several functional tungsten materials were developed, produced and tested. This line culminated in the study of nanostructured tungsten with micro-fibers of tantalum (Figure 2).

Figure 2 - Full densified W-Ta materials and image showing cracks in W being diverts by Ta microfibers

4. Plasma physics studies

Despite being a small tokamak, the physical studies performed in ISTTOK produced a positive impact on the Euratom fusion program, since typical parameters of the ISTTOK plasma column core are of the same order of magnitude of those occurring at the periphery of tokamaks of medium and large size. The main results were: • First demonstration that the Reynolds stress induced by fluctuations can generate poloidal rotation in a tokamak [8]. The radial gradient of the Reynolds stress was determined using the radial and poloidal electric field measured by electrical probes, concluding that its magnitude is equal or even superior to other possible sources of plasma rotation; 4 OV/P-08

• Obtaining alternate discharges of long duration (~ 250 ms and recently ~500ms) based on the development of fast switching power amplifiers with digital control and real time control systems [2]; These discharges were initially very difficult to obtain due to the absence of the active control in ISTTOK and are currently only limited by recycle of impurities and hydrogen from the walls and the physical limit of the duration of the magnetic field. Recently the use of ATCA and the improvements on power supplies allowed to increase the duration of the discharges even more (Figure 4) achieving a discharge with 0,5s at 4kA. The ISTTOK became the first tokamak operating routinely with multi-cycle flat alternating discharges. (Figures 3 and 4).

Plasma current − Shot #16711 5

0 Iplasma (kA)

−5 0 50 100 150 200 250 Time (ms)

18 x 10 Plasma density 10

8 ) 6 −3

4 n (m 2

0 0 50 100 150 200 250 Time (ms)

Figure 2 - Obtaining long duration AC discharges extending the duration by more than five times its nominal time. Year 2007.

Figure 3 - Obtaining long lasting AC discharges. Year 2012. • Control of the electric field profile and particle confinement using an electrode emissive [3- 5]. The emissive electrode developed in ISTTOK proved to be a valuable tool for robust 5 OV/P-08 control of the radial electric field in both polarities, allowing detailed investigation of the role of E×B flow shear in the control of fluctuations in the periphery of the plasma and in the global transport of the plasma (Figure 5). 3.0

2.5 No Bias ) α 2.0 /(n/H 1.5 Bias )

α Negative bias Negative bias: ICEs 1.0 Positive bias (n/H Positive bias:ICEs 0.5 0 10 20 30 40 ExB flow shear (105 s-1)

Figure 4 - Dependence of change in particle confinement induced by electrode bias on the E×B flow shear.

• Observation of large-scale low-frequency potential fluctuations (zonal flows) and demonstration of its importance in the control of the local turbulent transport [6, 7]. The relationship between large-scale fluctuations and local turbulent transport was investigated in ISTTOK peripheral plasma. It was found that the floating potential fluctuations in the edge plasma exhibit a significant correlation at long distances that is consistent with the geodesic acoustic mode. It is shown that edge plasma fluctuations are dominated by low frequency oscillations with a symmetric structure in the toroidal and poloidal directions. The toroidal correlation is highly intermittent, leading to a similar behavior in the turbulent particle flux. The toroidal correlation was found to be proportional to the GAM amplitude and in phase opposition to the turbulent flux demonstrating that on ISTTOK radial transport is largely regulated by long-range correlations. These findings provide the first direct evidence of multi- scale physics in the regulation of transport mechanisms in the edge of fusion plasmas.

60 60 Poloidal array (a) 40 Radial array 40 20 20 0 0 -20 -20 -40 -40

Floating potential (V) -60 -60 Floating potential (V) 140 (b) 120 100 80 60 40 Frequency (kHz) 20 0 1.0 1.0 (c) 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 Toroidal Correlation 0.0 0.0 FFT (10-25 kHz) (a.u) 20 21 22 23 Time (ms) Figure 5 - Time evolution of: (a) floating potential measured simultaneously in two toroidal positions, (b) toroidal correlation between Vf signals together with the amplitude of the Vf power spectrum in the range 10-25 kHz. 6 OV/P-08

5. Conclusion

Currently the ISTTOK program is performed in strong cooperation with foreign nuclear fusion laboratories mainly with the European counterparts and with countries from other continents through the IAEA, such as Brazil. In particular, during the year 2008 ISTTOK hosted the "IAEA Joint Experiment" involving 29 scientists from 14 countries. Much of the diagnostics developed in ISTTOK have been migrated to other larger tokamaks, being example of that the heavy ion bean diagnostic and the reflectometer, as well the introducing of new concepts of data acquisition and real-time control. After 20 years of activity the ISTTOK has demonstrated a continued innovation in the development of scientific findings and technological developments, following the cutting- edge research in these areas.

6. Acknowledgments

This work, supported by the European Communities and “Instituto Superior Técnico”, has been carried out within the Contract of Association between EURATOM and IST. Financial support was also received from “Fundação para a Ciência e Tecnologia” in the frame of the Contract of Associated Laboratory and from IAEA technical contract under the CRP on the Joint Research Using Small Tokamaks. The opinions expressed by authors do not necessarily represent the positions of the European Commission.

7. References

[1] C. A. F. Varandas et al., Fusion Technology, 29, 105 (1996) [2] FERNANDES, H. et. al., Fusion Technology, 29 (1996), 977 [3] SILVA, C. et. al., Plasma Phys. Control. Fusion, 46 (2004) 163 [4] SILVA, C. et. al., Nuclear Fusion, 44 (2004) 799 [5] SILVA, C. et. al., Plasma Phys. Control. Fusion, 48 (2006) 727 [6] SILVA, C. et. al., Phys. Plasmas, 15 (2008) 120703 [7] SILVA, C. et. al., Plasma Phys. Control. Fusion, 51 (2009) 085009 [8] HIDALGO, C., SILVA, C., et. al., Phys. Rev. Lett., 83 (1999) 2003 7 OV/P-08

Appendix 1: ISTTOK technical data and main parameters of the machine

TABLE I: ISTTOK technical data and main parameters of the machine

Parameter Value Larger radius 0.46 m Minor radius 85 mm Maximum toroidal magnetic field 0.8 Tesla Avaiable flux (primary transformer) 0.25 Vs Plasma current ~ 7 kA Duration of the discharge (DC/AC) ~ 40/500 ms Plasma density @ r=0 ~5x10 18 m -3 Electron temperature @ r=0 ~120 eV Ion temperature @ r=0 (CIII ) ~100 eV Energy confinement time ~0.8 ms Beta @ r=0) ~0.6% Safety factor q(0)~1,q(a)~5