Lower Hybrid Current Drive Antenna Research Articles

Neutronics assessments of LHCD antenna system for CFETR

To maintain a long-duration plasma current and obtain steady-state operation for CFETR, a non-inductive Heating and Current Drive (H&CD) system is currently under study. Lower Hybrid Current Drive (LHCD) is an important auxiliary heating system, that has good current drive efficiency and improves plasma confinement by changing the current distribution.Neutronics calculation was conducted to study the nuclear performance and shielding effectiveness. This study was performed on a CFETR 22.5° model integrated with an upper-port LHCD antenna structure. The detailed 3D neutronics model was built using the computer-aided modeling platform cosVMPT, and particle transport was performed using MCNP5 with the nuclear data library FENDL-2.1. With the support of high performance variance reduction technique, “on-the-fly” GVR method, global neutron and photon flux distributions with reasonable relative error were obtained. The irradiation damage to components in the antenna system was assessed, including DPA, gas production and nuclear heat density. The total nuclear heat for the transmission structure was 1.192 MW, and the peak irradiation damage was located at the front end of the Passive-Active Multi-junction (PAM) launcher. Owing to the blanket opening for wave propagation, the induced nuclear responses for VV and TFC were analyzed, and the shielding design was initialized. The results showed that all the parameters were within the design limit after the shielding design. Because part of the blanket module was occupied by the antenna launcher, the TBR of CFETR was evaluated, and it decreased by only 0.22% for the whole fusion reactor.

High field side LHCD launcher study for KSTAR

Upgrading to a lower hybrid current drive (LHCD) system on the high field side (HFS) or top launcher is under consideration for efficient current drive of KSTAR. The HFS or top launcher needs to be compact because of the tight and congested space at the inboard and near the upper divertor. This paper introduces a study of compact LHCD antenna development, and includes a brief review of a traveling wave antenna cold model and comparison with the new design of a resonant type slotted waveguide array in combination with a passive active multijunction, i.e. SWA PAM. The traveling wave antenna has structural simplicity, while SWA PAM has N|| adjustment capability.

Hot spots induced by LHCD in the shadow of antenna limiters in the EAST tokamak

Hot spots induced by lower hybrid wave in experimental advanced superconducting tokamak tokamak have caused high performance experiment disruption and serious damages to the guard limiters. Experimental and theoretical analyses have been carried out to study its physical mechanism. Plasma density scan experiments indicate that the wall temperature within the hot spots enhanced by a factor of 5 and increases with the plasma density near the antenna. A lower hybrid current drive (LHCD)-only density climb experiment shows that the carbon impurity decreases to a minimum value at certain plasma density and then increases with the line averaged plasma density. A model has been developed to explain the mechanism of sputtering of graphite tiles due to hot spots as the plasma density near the LHCD antenna and the time increases. A theoretical scaling of the heat flux driven by LHCD is also presented and is consistent with the experimental scaling in the Tore Supra tokamak. The simulation results show that the total sputtering flux density has a minimum at a certain plasma density and gradually increases as the plasma density increases or decreases away from the minimum value, and the increase in parallel heat flux near the antenna would enhance the sputtering flux density. The sputtering flux density trend is qualitatively consistent with the density scan experiments. The simulated temporal evolution of sputtered flux implies that the chemical sputtering could be a candidate for the carbon impurity explosion.

Analysis of double stub tuner control stability in a phased array antenna with strong cross-coupling

Active stub tuning with a fast ferrite tuner (FFT) has greatly increased the effectiveness of fusion ion cyclotron range of frequency (ICRF) systems (50–100MHz) by allowing for the antenna system to respond dynamically to changes in the plasma load impedance such as during the L–H transition or edge localized modes (ELMs). A high power waveguide double-stub tuner is under development for use with the Alcator C-Mod lower hybrid current drive (LHCD) system at 4.6GHz. The amplitude and relative phase shift between adjacent columns of an LHCD antenna are critical for control of the launched n|| spectrum. Adding a double-stub tuning network will perturb the phase and amplitude of the forward wave particularly if the unmatched reflection coefficient is high. This effect can be compensated by adjusting the phase of the low power microwave drive for each klystron amplifier. Cross-coupling of the reflected power between columns of the launcher must also be considered. The problem is simulated by cascading a scattering matrix for the plasma provided by a linear coupling model with the measured launcher scattering matrix and that of the FFTs. The solution is advanced in an iterative manner similar to the time-dependent behavior of the real system. System performance is presented under a range of edge density conditions from under-dense to over-dense and a range of launched n||. Simulations predict power reflection coefficients (Γ2) of less than 1% with no contamination of the n|| spectrum. Instability of the FFT tuning network can be problematic for certain plasma conditions and relative phasings, but reducing the control gain of the FFT network stabilizes the system.

Modelling of power limit in RF antenna waveguides operated in the lower hybrid range of frequency

Multipactor, the well-known electron avalanche phenomena occurring in waveguides when the amplitude of the RF electric field exceeds a threshold, is modelled for lower hybrid current drive (LHCD) antennas. The electric field threshold departures significantly from the expected linear scaling with the frequency when the width of the waveguide along the electric field increases. For 5 GHz waveguides, the multipactor does not occur when this width exceeds 26 mm, in very good agreement with analytical formulation. Effect of static magnetic field with typical values of tokamak environment is examined. It is found that the threshold is weakly reduced (−6%) for the front face of the antenna featuring the rows of narrow waveguides. Strong reduction in the threshold is found where the wave frequency equals the electron cyclotron frequency (0.178 T at 5 GHz) but this condition will not be met on ITER. After baking at 240 °C, wide waveguides at the antenna input (29 mm on ITER) have a threshold of ∼7 kV cm−1, assuming a homogeneous magnetic field. However, preliminary results indicate higher threshold when the gradient of the magnetic field along the waveguide is taken into account and no limitation, from this section, is expected for the ITER LHCD antenna.

Influence of gas puff location on the coupling of lower hybrid waves in JET ELMy H-mode plasmas

Reliable coupling of the lower hybrid current drive (LHCD) to H-mode plasmas in JET is made feasible through a dedicated gas injection system, located at the outer wall and magnetically connected to the antenna (Pericoli Ridolfini et al 2004 Plasma Phys. Control. Fusion 46 349, Ekedahl et al 2005 Nucl. Fusion 45 351, Ekedahl et al 2009 Plasma Phys. Control. Fusion 51 044001). An experiment was carried out in JET in order to investigate whether a gas injection from the top of the torus, as is foreseen for the main gas injection in ITER, could also provide good coupling of the LH waves if magnetically connected to the antenna. The results show that a top gas injection was not efficient for providing a reliable LHCD power injection, in spite of being magnetically connected and in spite of using almost twice the amount of gas flow compared with the dedicated outer mid-plane gas puffing system. A dedicated gas injection system, set in the outer wall and magnetically connected to the LHCD antenna, is therefore recommended in order to provide the reliable coupling conditions for an LHCD antenna in ITER.

Measurement of LHCD antenna position in Aditya tokamak

To drive plasma current non-inductively in ADITYA tokamak, 120 kW pulsed Lower Hybrid Current Drive (LHCD) system at 3.7 GHz has been designed, fabricated and installed on ADITYA tokamak. In this system, the antenna consists of a grill structure, having two rows, each row comprising of four sub-waveguides. The coupling of LHCD power to the plasma strongly depends on the plasma density near the mouth of grill antenna. Thus the grill antenna has to be precisely positioned for efficient coupling. The movement of mechanical bellow, which contracts or expands up to 50mm, governs the movement of antenna.In order to monitor the position of the antenna precisely, the reference position of the antenna with respect to the machine/plasma position has to be accurately determined. Further a mechanical system or an electronic system to measure the relative movement of the antenna with respect to the reference position is also desired. Also due to poor accessibility inside the ADITYA machine, it is impossible to measure physically the reference position of the grill antenna with respect to machine wall, taken as reference position and hence an alternative method has to be adopted to establish these measurements reliably.In this paper we report the design and development of a mechanism, using which the antenna position measurements are made. It also describes a unique method employing which the measurements of the reference position of the antenna with respect to the inner edge of the tokamak wall is carried out, which otherwise was impossible due to poor accessibility and physical constraints. The position of the antenna is monitored using an electronic scale, which is developed and installed on the bellow. Once the reference position is derived, the linear potentiometer, attached to the bellow, measures the linear distance using position transmitter. The accuracy of measurement obtained in our setup is within +/− 0.5 % and the linearity, along with repeatability is excellent.

Development of cold isostatic pressing graphite module for a heat-resistant lower hybrid current drive antenna

Development of a plasma facing module using Cold Isostatic Pressing Graphite (CIPG) was successfully achieved for use in driving a heat-resistant Lower Hybrid Current Drive (LHCD) antenna. A thin stainless film (10μm), a molybdenum film (10μm), and a copper film (50μm) were overlaid on the CIPG materials, the CIPG surfaces were successfully coated with the Cu-layer by diffusion bonding method. This module whose length is 206mm, has four waveguides, and a water cooling channel. High rf-power long pulse operations with water cooling were successfully tested up to 250kW (125MW∕m2)∕700s, a stationary temperature and vacuum pressure was performed. But the maximum rf transmission power was limited to 47MW∕m2 for a plasma facing module using Carbon Fiber Composite (CFC) due to poor Cu-plating, a power density large by more than a factor 2.5 was achieved with the CIPG module. The rf power density which meets the requirement of the rf electric field of 5kV∕cm for a design of the LHCD antenna in International Thermonuclear Experimental Reactor (ITER) was successfully performed.

Outgassing of plasma facing antenna front for lower hybrid wave launcher

A 3.7 GHz mock-up antenna module using carbon fiber composite (CFC) was fabricated and tested for the development of a heat-resistive front of the lower hybrid current drive (LHCD) antenna. This module has four waveguides and a water cooling channel, the length is 206 mm. The CFC surface was coated with a thin titanium layer and was plated with copper in order to reduce RF losses, to bond rods and septum plates and to assemble them with cooling channel. The RF losses and the outgassing rates of this CFC module at high RF power were measured during long pulses. When the injected power varies between 30 and 100 kW, the RF losses measured by calorimetery, were found to be in the range of 1.0–1.2%. It is found that this experimental value is 2.5–3.0 times higher than the theoretical value of pure copper. Stationary operation of the CFC module with water cooling is performed at the RF power density of 45 MW m −2 during 1000 s. The outgassing rates from the CFC module are in the range of 0.93∼1.3×10 −6 Pam −3 s −1 m −2 at the module temperature of 120°C, it is low enough for an antenna material. No significant bonding defects occurred during the steady-state operation. It is assessed that a CFC module is an attractive candidate for a heat-resistive front of LHCD antenna.

Development of plasma facing component for LHCD antenna

A mock-up module of an antenna tip made of carbon fiber composite (CFC) was fabricated for the development of a heat-resistive front of the lower hybrid current drive (LHCD) antenna. The CFC surface was coated with a thin Ti layer and was plated with Cu. The mock-up module has three waveguides and is designed to simulate the front tip of the 8 GHz LHCD antenna. The module has an active water cooling channel and the length is 200 mm, the mouth dimension is 19.5×50 mm 2. The mock-up module was tested with heat fluxes up to 20 MW m −2 due to electron beam irradiation. No damage of the waveguide mouth was observed up to the heat flux of 3.2 MW m −2 for 2 min, though a maximum temperature of 762°C was measured at the septum plate. The obtained heat load of 3.2 MW m −2 is about 10 times higher than the steady-state surface-averaged heat load of the first wall required in the ITER design.

Outgassing measurement of the waveguide module for a steady state LHCD antenna

An outgassing rate is measured using a waveguide module during a long pulse r.f. injection at 3.7 GHz in order to operate a lower hybrid current drive (LHCD) antenna in steady state. The waveguide module consists of four sub-waveguides which are made of dispersion-strengthened copper and copper-plated stainless steel with cooling channels to control its working temperature between 100 and 500 °C. The waveguide module shows the high power capability of power density up to 200 MW m −2 after short term conditioning. A low outgassing rate of about 2 × 10 −7 Pa m 3 s −2 m −2 is obtained during r.f. injection up to 150 MW m −2 at a working temperature of T=300 °C after 450 °C baking. Long pulse r.f. injection is effective for reduction of the outgassing by about 1/100. Outgassing during r.f. injection depends on the working temperature but is independent of the r.f. power density after sufficient conditioning. The calculation code taking desorption, adsorption and diffusion processes into account can show time behavior of outgassing in the waveguide module. A quasi-constant outgassing of about 10 −7 Pa m 3 s −1 m −2 is observed at a power density of 150 MW m −2 in 1800 s injection while keeping a saturated temperature at the center of the module below 120 °C by using water cooling. The outgassing properties obtained indicate that steady state operation of an LHCD antenna is feasible without a large pumping system.

Outgassing of the lower hybrid current drive antenna on JT-60U

Abstract The outgassing properties of the lower hybrid current drive (LHCD) antenna during r.f. operation were studied on the JT-60U tokamak. The antenna is not equipped with a differential pumping system to evacuate the outgassing of the antenna. The pressure rise in the antenna during r.f. operation strongly depended on the r.f. power when r.f. conditioning was started. However, the outgassing rate was quickly reduced after several shots, and showed weak r.f. power dependence after sufficient r.f. conditioning. Finally, the outgassing rate of the LHCD antenna was in the range of approximately 10−6 Pa m3 s−1 m−2 after four weeks operation at the working temperature of 250 °C. Then a power density of up to 25 MW m−2 was successfully injected into the plasma without the differential pumping system at the antenna on JT-60U.