J. M. Moller

Motional Stark effect diagnostic expansion on DIII-D for enhanced current and Er profile measurements

The motional Stark effect (MSE) diagnostic on DIII-D has been expanded to take advantage of a change in the neutral beam geometry, adding 24 new MSE channels viewing a beam injected counter to the plasma current. When data from these channels are used with those from two older MSE arrays viewing a different beam, the overall radial resolution improves near the magnetic axis at least a factor of 2, and the uncertainty in calculations of vertical magnetic field and radial electric field decreases in the edge at least a factor of 4. The new design uses two optical systems mounted on the same vacuum port with a common shutter and shielding.

Generation of high power 140 GHz microwaves with an FEL for the MTX experiment

We have used the improved ETA-II linear induction accelerator (ETA-III) and the IMP steady-state wiggler to generate high power (1-2 GW) microwaves at 140 GHz. The FEL was used in an amplifier configuration with a gyrotron driver. Improved control of energy sweep and computerized magnetic alignment in ETA-III resulted in small beam corkscrew motion (<1.5 mm) at 6 Mev, 2.5 kA. Reduction of wiggler errors (<0.2%), improved electron beam matching, and tapered wiggler operation resulted in peak microwave power (single-pulse) of up to 2 GW. These pulses were transported to the MTX tokamak for microwave absorption experiments. In addition, the FEL was run in a burst mode, generating 50-pulse bursts of microwaves; these results are discussed elsewhere.<<ETX>>

Test results of the 100 kWh SMES model coil – AC loss performance

Abstract In order to establish a technology needed for a small-scale 100 kWh SMES device, an SMES model coil was fabricated. Performance tests were carried out at the Japan Atomic Energy Research Institute (JAERI) in 1996. After that, the coil was installed in the Lawrence Livermore National Laboratory (LLNL) facility and tested in 1998, in collaboration between Japan and the United States. The AC losses measured at LLNL were in good agreement with those measured at JAERI. It was reconfirmed that the coupling loss of the coil could be expressed in two components: one with a short and another with a long coupling time constant. We found out from the Hall probe signals that the loop currents with long decay times were induced in the CIC conductor by varying magnetic field. These currents resulted in additional AC loss in the coil. To develop a concept of CIC with low AC loss, we made a sub-scale CIC conductor of strands coated with CuNi. We fabricated a small coil out of this conductor and measured the AC loss. The measured AC loss in this coil was about 1/6 of that in the SMES model coil conductor per strand volume. Thus, the CuNi coating of the strands was demonstrated to be effective to reduce the AC loss in the coil.

Energy confinement and magnetic field generation in the SSPX spheromak

The Sustained Spheromak Physics Experiment (SSPX) [Hooper et al., Nuclear Fusion 39, 863 (1999)] explores the physics of efficient magnetic field buildup and energy confinement, both essential parts of advancing the spheromak concept. Extending the spheromak formation phase increases the efficiency of magnetic field generation with the maximum edge magnetic field for a given injector current (B∕I) from 0.65T∕MA previously to 0.9T∕MA. We have achieved the highest electron temperatures (Te) recorded for a spheromak with Te>500eV, toroidal magnetic field ∼1T, and toroidal current (∼1MA) [Wood et al., “Improved magnetic field generation efficiency and higher temperature spheromak plasmas,” Phys. Rev. Lett. (submitted)]. Extending the sustainment phase to >8ms extends the period of low magnetic fluctuations (<1%) by 50%. The NIMROD three-dimensional resistive magnetohydrodynamics code [Sovinec et al., Phys. Plasmas 10, 1727 (2003)] reproduces the observed flux amplification ψpol∕ψgun. Successive gun pulses are...

Transport and fluctuations in high temperature spheromak plasmas

Higher electron temperature (Te>350eV) and reduced electron thermal diffusivity (χe<10m2∕s) is achieved in the Sustained Spheromak Physics Experiment (SSPX) by increasing the discharge current=Igun and gun bias flux=ψgun in a prescribed manner. The internal current and q=safety factor profile derived from equilibrium reconstruction as well as the measured magnetic fluctuation amplitude can be controlled by programming the ratio λgun=μ0Igun∕ψgun. Varying λgun above and below the minimum energy eigenvalue=λFC of the flux conserver (∇×B=λFCB) varies the q profile and produces the m∕n=poloidal/toroidal magnetic fluctuation mode spectrum expected from mode-rational surfaces with q=m∕n. The highest Te is measured when the gun is driven with λgun slightly less than λFC, producing low fluctuation amplitudes (<1%) and 1∕2<q<2∕3. Transport analysis shows a reduction in χe as Te increases, differing from Bohm or open field line transport models where χe increases with Te. Detailed resistive magnetohydrodynamic sim...

Burst mode FEL with the ETA-III induction linac

Pulses of 140 GHz microwaves have been produced at a 2 kHz rate using the ETA-III induction linac and IMP wiggler. The accelerator was run in bursts of up to 50 pulses at 6 MeV and greater than 2 kA peak current. A feedback timing control system was used to synchronize acceleration voltage pulses with the electron beam, resulting in sufficient reduction of the corkscrew and energy sweep for efficient FEL operation. Peak microwave power for short bursts was in the range 0.5-1.1 GW, which is comparable to the single-pulse peak power of 0.75-2 GW. FEL bursts of more than 25 pulses were obtained.<<ETX>>

Characterization of the separatrix plasma parameters in DIII-D

Characterization of the plasma density and temperature at the last closed flux surface (the separatrix) of a tokamak requires accurate knowledge of the location of the separatrix. In this paper we discuss the effect of inaccuracy in the separatrix location on the measured parameters in DIII-D [Luxon et al., International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), p. 159] An uncertainty in the separatrix position of ±0.5 cm, as expected in this device using magnetic reconstruction to determine the location of the separatrix, leads to unacceptably large uncertainty in the plasma parameters. Several techniques to improve the accuracy obtained from magnetic reconstruction are discussed. A new technique that is based on a characterization of the electron temperature profile is proposed. A comparison of the separatrix location defined in this manner with that obtained using magnetic reconstruction techniques suggests a systematic error in the r...

Remote control of alcator C-mod from Lawrence Livermore National Laboratory

Operation ofa tokamak from a remote site has been demonstrated for the first time. The Alcator C-Mod tokamak, located at the Massachusetts Institute of Technology, was operated over the Internet from a remote control room set up at Lawrence Livermore National Laboratory in California. Prescription of the physics parameters such as plasma current, density, shape, heating power, and active diagnostics was accomplished entirely from the remote site using the same interface as when operating from the C-Mod control room. Engineering control of subsystems (e.g., vacuum, cooling, and power supply limits) remained under local control, providing appropriate equipment and personnel security. Although the principal purpose for running this experiment from a distance was to demonstrate the remote operation, it was planned as a productive physics run. The operation was highly successful; important new physics data were obtained, and valuable insight was gained into the potential of remote operation as well as its limitations.

DIAGNOSIS OF EDGE LOCALIZED MODE EVOLUTION IN DIII-D USING FAST-GATED CID AND INFRARED CAMERAS

The tangentially viewing visible and vertically viewing infrared cameras systems on DIII-D were upgraded to permit emission measurements during edge localized modes (ELMs) with integration times as short as 1 and 100 {micro}s respectively. The visible system was used to obtain 2-D poloidal profiles of CIII (465 nm) and D{sub {alpha}} (656.3 nm) emission with 20 {micro}s integration during various stages of ELM events in the lower DIII-D divertor. The infrared (IR) system was used to measure the heat flux to the divertor targets at 10 kHz with 100 {micro}s exposure. Upgrades to the data processing and storage systems permitted efficient comparison of the temporal evolution of these measurements.

AC loss performance of the 100 kWh SMES model coil

AC loss tests of the SMES model coil for 100 kWh SMES pilot plant were carried out at Lawrence Livermore National Laboratory (LLNL) in 1998, in collaboration between Japan and the USA. The AC loss results at LLNL were in good agreement with those obtained at the Japan Atomic Energy Research Institute (JAERI) in 1996. The coupling loss in the coil could be described by two components with a short time constant (0.22s) and a long time constant (30s). The short time constant was in good agreement with that measured in a short sample. The signals of Hall probes, mounted on the surface of the coil, revealed that the induced loop currents in the conductor decayed with long time constants. At least two long time constants were observed: about 4s and 100s. The long time constant was also identified by the observation of voltage decay after the coil discharge. These loops result in the additional AC loss in the coil. Effect of lateral force in the cable on losses was studied as well. An improved conductor aiming to reduce the AC loss was designed, fabricated and wound in a small coil. The measured AC loss in the small coil made of the improved conductor was about 1/6 of the SMES model coil per strand volume.