Dott. ClaudioVerona Dip. Ingegneria Meccanica Università di Roma “Tor Vergata” , Italy E-mail: claudio.verona@uniroma2.

1. CVD single crystal diamond photodetectors for UV plasma diagnostic at JET tokamak Dott. ClaudioVerona Dip. Ingegneria Meccanica Università di Roma “Tor Vergata” , Italy E-mail: claudio.verona@uniroma2.it

2. Outline • Motivation • SCD V-UV detectors • Device characterization • Results at JET • Conclusions

3. Magnetic Confinement Nuclear Fusion Joint European Torus (JET) Culham (UK) • Plasma temperature: (T) 100-200 million Kelvin • Central density in plasma: (n) 1-2 x 1020 particles m-3 • Most relevant emissions for plasma diagnostic in extreme-UV / soft-X ray spectral range

4. E-UV diagnostic at JET • temperature • density • particle and energy confinement timescale • impurities dynamics • atomic collision rates • plasma-wall interaction • plasma dynamics • characterization of the electron fluid • atomic structure of highly ionized atoms Required features: • Radiation hardness • Low sensitivity to neutrons and  rays Diamond detectors • Fast response • Visible blind • Good sensitivity in the EUV/soft-X spectral range

5. Single crystal diamond deposition Typical growth parameters Plasma composition H2-CH4 Temperature 650 - 750 °C Microwave power 500 - 700 W Pressure 100 - 150 mbar Gas flow rate 1- 100 sccm Thickness rate 1μm/h • Substrates • (100) HPHT type Ib(Element 6, Sumitomo, others)

6. Single Crystal Diamond UV detector UV + Vbias Cr/Al contact electrode is grid-shaped CVD intrinsic Ag contact CVD B-dopd A Grid-shaped Cr/Al electrode The detector active area is about 6 mm2 HPHT substrate • The diamond photodetector operated in transversal configuration. • The detector can work with no applied bias voltage (photovoltaic regime). • The photocurrent is measured between the p-type diamond layer and ground. No signal due to secondary electrons.

7. Devices characterization Experimental Setup 1 Experimental Setup 2 Stepper motor • EUV toroidal grating vacuum monochromator, with 5Å wavelength resolution • 10 - 100 nm spectral range • DC He and He/Ne gas discharge radiation sources • Tunable laser source (210 – 2400 nm) • 5 ns pulses

8. Devices characterization: I-V curve

9. Devices characterization: I-V curve The device shows a photocurrent response even at zero voltage bias, exploiting the internal junction electric field The best signal to dark current ratio (SDR) performance is obtained at zero bias voltage

10. Devices characterization:Temporal response Shutter ON Shutter OFF

11. Devices characterization: linearity He UV He II and He I Vbias =0Volt

12. Devices characterization He and He-Ne spectra measured by the SCD detector • All spectral lines are clearly observed with high signal to noise ratios, demonstrating the good photodetection capabilities of CVD single crystal diamond in the extreme UV spectral region. • the low intensity lines of the He spectrum in the wavelength range 22 – 27 nm are easily resolved.

13. Devices characterization The spectral response shows a visible/UV rejection ratio of about 6 orders of magnitude Iph = qFo η μ E τ /d = qFo η G R= Iph /Popt EQE = 1240•R/ λ[nm]

14. Measurements at JET One of those CVD diamond detectors was permanently installed at JET and connected to the main on-line data acquisition system They are continuously recording every JET shot VUV broadband spectroscopy • Det. n. 1 Feb. 2008 – broadband VUV KS6 KT2

15. Measurements at JET The detector has been installed inside the KS6 vacuum chamber Diamond photodetector 40 m vacuum pipe to the reaction chamber 2 mm diameter collimator was placed in front of the detector The electronic used consist in a variable-gain low-noise current amplifier FEMTO model DLPCA-200 and a 500Hz sampling rate ADC

16. Measurements at JET JET shot #72428: Ne Blips The detectors inside KT2 is a multi channel plate based on phosforus. The primary function of it is to monitor the impurity content of the JET plasma. Practically every impurity (intrinsic or extrinsic) in the plasma produces some spectral line emission in the KT2 wavelength range (approximately 100 - 1100Å).

17. Measurements at JET Shot #72273: Nickel ablation experiment

18. Measurements at JET Neutron insensitivity JET shot acquired by keeping closed the UV entrance shutter Shot #71898 Shot #71902 SCD Neutron monitor

19. Conclusions • CVD single crystal diamond UV detectors are fabricated in our University laboratory in a p-type/intrinsic/metal layered structure. • Our detectors can operate at zero bias voltage and show fast response time with no memory effect (priming) and good signal to noise ratio. • One of those detectors was permanently installed at Joint European Torus and connected to the main on-line data acquisition system. • First data acquired at JET have evidenced fast response time, good sensitivity and extremely low sensitivity to the neutron background. • Two plasma physics research groups at JET are currently using our device in order to measure the dynamic of impurities in the plasma.

20. Contributors Università di Roma “Tor Vergata”, Italy S. Almaviva M. Marinelli E. Milani G. Prestopino A. Tucciarone G. Verona Rinati EFDA-JET, Culham, UK I. Coffey A. Murari Ass. EURATOM/ENEA, Frascati, Italy M. Angelone M. Pillon

21. Thankyou!