In order to support EAST to realize the main mission of long-pulse operation lasting 1000s, the existing full active multijunction (FAM) launcher will be upgraded to a passive active multijunction ...(PAM) for the 2.45 GHz lower hybrid current drive (LHCD) system. The RF components were designed and optimized with consideration of the port size, power density inside the waveguides to avoid RF breakdown and power spectrum for efficient current drive. The designed PAM has a power handling capability of 4 MW CW at 2.45 GHz with matched load. The coupling properties and power spectrum were calculated using the ALOHA code and a comparative analysis with the old FAM was carried out. The main feature of this new launcher is that the density for optimum coupling and power spectrum directivity (ne ˜ 0.3 × 1017/m3) lies below the cut-off value (ne_co ˜ 0.74 × 1017/m3), which is favored for long-pulse steady-state operation on EAST. It has a capability to deliver 3.6 MW of power during real operation (ne ˜ 2 × 1017/m3) and 2 MW during large ELMs (ne = 10 ne_co). For the nominal phasing of 180 °s between modules, the power spectrum is peaked at N// = 1.8 and a maximum power directivity of 71.2% and reflection coefficient less than 1% can be obtained via simulations. Weaker hot spot phenomenon is expected with the designed PAM than with the FAM for their respective optimum densities. Finally, a thermal analysis was performed and the maximum temperature at the front of the antenna is expected to be ˜ 260 °C which is safe enough for operation. The PAM is scheduled to be installed on EAST in 2021.
Lower hybrid current drive (LHCD) experiments with line-averaged density varying between 1.5 × 1019 and 6 × 1019 m−3 are performed on the Tore Supra tokamak under quasi-steady-state conditions with ...respect to the fast electron dynamics. The LHCD efficiency is analysed from the fast electron bremsstrahlung (FEB) and electron cyclotron emission (ECE). The effect of plasma equilibrium and particle fuelling is documented. It is concluded that the fast decay of FEB with plasma density could be consistent with simple scaling of the current drive efficiency and FEB. Plasma edge measurements are presented looking for the effect on fast electron emission. In a specific case of particle fuelling, an anomalous decay of the hard x-ray and ECE signals suggests deleterious interaction of the wave with edge plasma.
Mitigation of Edge localized mode(ELM) has been achieved with different external actuators such as lower hybrid wave (LHW), mixture supersonic molecular beam injection(SMBI), and laser blow-off(LBO) ...impurity seeding on HL-2A. During these experiments, the pedestal turbulence enhancement is commonly observed, which is found closely related to ELM mitigation. The turbulence enhancement is caused by the externally driven the velocity shear rate without change of the turbulence correlation length, but correlated to its radial wavenumber spectral shift. A common plausible mechanism for the ELM mitigation with different external source input seems to be involved. A modified theoretical model based on the turbulence radial wavenumber spectral shift is used and successfully interprets the experimental observations. The simulation suggests that a critical growth rate of the most unstable mode, also identified by the experimental results, survives in the competition of the velocity shear rate, enhancing the turbulence intensity. An example of the LHW case is used and good agreements have been found between the experimental results and simulation results.
A new concept of lower hybrid antenna for current drive has been proposed for ITER (Bibet
et al
1995
Nucl. Fusion
35
1213–23
): the passive active multijunction (PAM) antenna that relies on a ...periodic combination of active and passive waveguides. An actively cooled PAM antenna at 3.7 GHz has recently been installed on the tokamak Tore Supra. This paper summarizes the comprehensive experimental characterization of the coupling properties of the PAM antenna to the Tore Supra plasmas. In this paper, the electromagnetic properties of the antenna are measured at a reduced power (<1 MW) to allow a systematic comparison with linear wave coupling theory and the associated modelling based on the linear ALOHA code. In a wide range of edge electron densities at the antenna aperture (spanning a factor 20 from 0.5 ×
n
c
to 10 ×
n
c
where
n
c
is the slow wave density cut-off,
n
c
= 1.7 × 10
17
m
−3
at 3.7 GHz) and antenna phasing, the ALOHA simulations reproduce the experimental results observed on Tore Supra. In addition, reduced power reflection coefficients (<5%) are measured at a low edge density, close to
n
c
, i.e. in the range 0.5–3 ×
n
c
. Measurement and analysis with ALOHA of the antenna–plasma scattering matrices provide explanation of the good coupling properties of the PAM antenna close to
n
c
by highlighting the crucial role of the slow wave intercoupling between active and passive waveguides through the plasma edge. This detailed validation of the coupling modelling is an important step towards the validation of the PAM concept in view of further optimizing the electromagnetic properties of the future ITER antenna.
In the frame of a R&D effort conducted by CEA towards the design and the qualification of a 5 GHz LHCD system for the ITER tokamak, two 5 GHz 500 kW/5 s windows have been designed, manufactured and ...tested at high power in collaboration with the National Fusion Research Institute (NFRI). The window design rely on a symmetrical pill-box concept with a cylindrical beryllium oxide ceramic brazed on an actively water cooled copper skirt. The ceramic RF properties have been measured on a test sample to get realistic values for guiding the design. Low power measurements of the manufactured windows show return losses below -32 dB and insertion losses between -0.01 dB and -0.05 dB, with an optimum frequency shifted toward lower frequencies. High power tests conducted at NFRI show unexpected total power loss for both windows. The ceramic temperature during RF pulses has been found to reach unexpected high temperature, preventing these windows to be used under CW conditions. A post-mortem RF analysis of samples taken from one window shows that the dielectric properties of the ceramic were not the ones measured on the manufacturer sample, which partly explain the differences with the reference modelling.
It has been demonstrated that lower hybrid current drive (LHCD) systems play a crucial role for steady-state tokamak operation, owing to their high current drive (CD) efficiency and hence their ...capability to reduce flux consumption. This paper describes the extensive technology programmes developed for the Tore Supra (France) and the KSTAR (Korea) tokamaks in order to bring continuous wave (CW) LHCD systems into operation. The Tore Supra LHCD generator at 3.7 GHz is fully CW compatible, with RF power PRF = 9.2 MW available at the generator to feed two actively water-cooled launchers. On Tore Supra, the most recent and novel passive active multijunction (PAM) launcher has sustained 2.7 MW (corresponding to its design value of 25 MW m−2 at the launcher mouth) for a 78 s flat-top discharge, with low reflected power even at large plasma-launcher gaps. The fully active multijunction (FAM) launcher has reached 3.8 MW of coupled power (24 MW m−2 at the launcher mouth) with the new TH2103C klystrons. By combining both the PAM and FAM launchers, 950 MJ of energy, using 5.2 MW of LHCD and 1 MW of ICRH (ion cyclotron resonance heating), was injected for 160 s in 2011. The 3.7 GHz CW LHCD system will be a key element within the W (for tungsten) environment in steady-state Tokamak (WEST) project, where the aim is to test ITER technologies for high heat flux components in relevant heat flux density and particle fluence conditions. On KSTAR, a 2 MW LHCD system operating at 5 GHz is under development. Recently the 5 GHz prototype klystron has reached 500 kW/600 s on a matched load, and studies are ongoing to design a PAM launcher. In addition to the studies of technology, a combination of ray-tracing and Fokker-Planck calculations have been performed to evaluate the driven current and the power deposition due to LH waves, and to optimize the N spectrum for the future launcher design. Furthermore, an LHCD system at 5 GHz is being considered for a future upgrade of the ITER Heating and Current Drive systems, with a power capability of 20 MW coupled to the plasma using a PAM launcher. An R&D programme is being conducted at CEA/IRFM to develop a BeO vacuum window which is a safety critical component of the transmission line. In addition, a mock-up of a TE10-TE30 mode converter at 5 GHz, designed for a rectangular transmission line, has been manufactured and successfully tested on Tore Supra at low RF power.
In the frame of a R&D effort conducted by CEA towards the design and the qualification of a 5GHz LHCD system for the ITER tokamak, two 5GHz 500kW/5s windows have been designed, manufactured and ...tested at high power in collaboration with the National Fusion Research Institute (NFRI). The window design rely on a symmetrical pill-box concept with a cylindrical beryllium oxide ceramic brazed on an actively water cooled copper skirt. The ceramic RF properties have been measured on a test sample to get realistic values for guiding the design. Low power measurements of the manufactured windows show return losses below −32dB and insertion losses between −0.01dB and −0.05dB, with an optimum frequency shifted toward lower frequencies. High power tests conducted at NFRI show unexpected total power loss for both windows. The ceramic temperature during RF pulses has been found to reach unexpected high temperature, preventing these windows to be used under CW conditions. A post-mortem RF analysis of samples taken from one window shows that the dielectric properties of the ceramic were not the ones measured on the manufacturer sample, which partly explain the differences with the reference modelling.
Tritium (T) retention constitutes an outstanding constraint for ITER. It has been proposed that the end of the discharge could be used for reducing the amount of tritium trapped in the device by ...switching to He or H2 injection during the ∼200 s of plasmas following the burning phase (power and plasma current ramp down). Thanks to the long discharge capabilities of Tore Supra, long pulse experiments (> mn) have been carried out to evaluate the effectiveness of such a scenario in reducing the tritium inventory during plasma operations. Starting with the device operated only in D2, series of changeover experiments from D2 to He and from D2 to H2 have been carried out in Tore Supra. The results demonstrate that with He the amount of D recovered after 130 s is limited to 0.8 × 1022 D whilst no further gain is foreseen. From these experiments, it is demonstrated that He injection will not contribute to the drop of the tritium inventory in the vessel. In contrast, with H2 injection the amount of D recovered after 250 s is ∼4.2 × 1022 D with no limitation observed in the amount that could be removed from the vessel. The higher efficiency in removing D from the vessel by H2 injection compared to He is attributed to the H charge-exchange (CX) flux (four to six times larger than the He CX flux) allowing for a significantly stronger plasma wall interaction with carbon deposition and layer areas. In Tore Supra, since most of the D retention through co-deposition with eroded material (C) takes place in these areas, H plasmas result in a better removal efficiency of D(T) from these regions. These experimental observations are supported by the results obtained using the EIRENE code for evaluating both the ion and CX fluxes for He and H plasmas. Finally, the consequences of removing D(T) from the vessel for the next discharges are unfavourable for both the He and H2 removal methods. Indeed, in both cases, twice the amount of D(T) removed through the isotope exchange has to be re-injected since co-deposition of the re-injected D(T) will also take place in addition to the plasma wall isotope exchange. In these conditions, the low efficiency of the H2 gas injection for controlling the plasma isotopic ratio inhibits a recovery of the initial plasma isotopic ratio over a time scale in the range of 200 s.
High-Power CW Klystron for Fusion Experiments Beunas, A.; Marchesin, R.; Bellemere, J.C. ...
IEEE transactions on electron devices,
05/2009, Letnik:
56, Številka:
5
Journal Article
Recenzirano
A 3.7-GHz 700-kW klystron in continuous-wave (CW) operation has been developed to upgrade the lower hybrid RF plasma heating power in a tokamak up to 10 MW. The klystron is equipped with a diode gun, ...a five-cavity RF structure, two BeO-disk RF windows, and a large-size X-ray-shielded hypervapotron collector. The output power is recombined in a four-port junction which we also developed. The tube is designed to deliver 620-kW CW RF power with a mismatched load (VSWR = 1.4) and 700-kW CW with a matched load. Several prototypes have been built with successive design improvements. The major improvement was to change one single RF output into two RF outputs. The most recently built prototype meets all design specifications at 73.1 kV and 20.7 A, with an efficiency of 47% on a matched load and 40% with a 1.4 : 1 VSWR load, worst case phase. The power losses dissipated in the body have been measured as low as 17 kW, which corresponds to the RF heating and implies low beam interception. The measured temperatures of the output cavity noses and collector wall have been kept below 130degC and 200degC, respectively, which results in large thermal margin.
The power handling capability of the JET Lower Hybrid Current Drive (LHCD) system is examined using the long-term database. The limitations, in particular in H-mode plasmas, are discussed and the ...performance compared with other LHCD experiments using multijunctions as power dividers. Although the power density of 25 MW m−2 has been exceeded in L-mode and almost obtained in ELMy H-mode (on 1/6th of the antenna), it is concluded that the RF conditioning performed on JET does not allow us to exceed an electric field of ∼5.5 kV cm−1 which is generally not sufficient under the rather weak coupling conditions of the JET H-mode. Modelling of an arc occurring in a waveguide indicates that rather small variations of the reflected wave (amplitude and phase) may occur, rendering arc detection based on RF measurements difficult in some cases. The JET bolometry diagnostic with four chords viewing the antenna front is found to be an efficient tool to detect an arc. In L-mode plasmas, a very good correlation between the amplitude of the bolometry signals and the iron spectroscopic lines is found. In H-mode the arc detection is clearly more difficult with enhanced radiation during the ELM but is still possible when the bolometry signals are properly processed.
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