K. H. Dippel

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (60)83.65 Total impact

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    ABSTRACT: The Advanced Limiter Test-II (ALT-II) is a large area toroidal pump limiter in the TEXTOR tokamak. Discrete target plates located in channels at eight toroidal positions behind the main limiter surface neutralize a portion of the plasma efflux from the core. The resulting gas is exhausted by eight external pumps. The primary experimental goals of ALT-II are aimed at power loading studies and plasma density control during long pulse (4 s), high power (6 MW) tokamak discharges. It is found that both the power and the plasma flow to the limiter are asymmetric and depend on line density. Peak neutral pressures of 0.8 m torr and removal rates of up to 0.15 torrL/s per pump station are achieved in the Ohmic phase. The projected exhaust efficiency of ALT-II with full pumping is 5-10%. During ion cyclotron resonance frequency heating, the particle removal rate exceeds 0.4 torrL/s per blade, and the exhaust efficiency is 4-5% for power levels up to 2.6 MW.
    Nuclear Fusion 01/2011; 31(6):1067. DOI:10.1088/0029-5515/31/6/005 · 3.06 Impact Factor
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    ABSTRACT: During runaway discharges in TEXTOR, intense infrared (IR) radiation is-emitted in the electron flow direction. This can only be explained by synchrotron radiation of fast electrons. The observed spectral dependence is consistent with electrons of 25-30 MeV energy; the intensity corresponds to about 1016 electrons or to an electrical current of 40 kA. From the spatial structure of the observed IR pattern, new insight into the spatial distribution of the runaway electrons and their perpendicular momentum can be gained. The runaway electrons populate a torus with a diameter of 0.5-0.6 m, which is slightly larger than the plasma radius; the perpendicular momentum is determined from the vertical extent of the IR pattern and amounts to about 5 m0c. The transformation rate of electrons to runaways can be estimated from the time delay of the IR signal as 2 × 10−4 s−1; this agrees with theoretical expectations derived from the ratio of the electrical field strength to the critical field strength. In TEXTOR, runaways are confined up to energies of 50 MeV, which is just below the limit where a phase should exist in which runaways radiate as much energy as they gain per turn.
    Nuclear Fusion 01/2011; 30(5):859. DOI:10.1088/0029-5515/30/5/005 · 3.06 Impact Factor
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    ABSTRACT: The Advanced Limiter Test (ALT) project is the focus of a fruitful and intense International Energy Agreement collaboration on TEXTOR. The pump limiter is a mechanical boundary that is laid out for taking the full heat load of TEXTOR, namely 8 MW (assuming 2 MW radiated power) for 10 s, and provides a pumping efficiency of at least 5% of the working gas. This layout is adopted from the requirements of a fusion reactor: It is mandatory to remove both the full power that is convected to the limiter or divertor and the helium ash that is generated in the fusion process. In order to obtain pumping for all gases, the ALT-II is equipped with turbomolecular pumps. A short description of ALT-II is given, and the power and particle fluxes to the limiter surface and into the exhaust scoops are discussed. Requirements of the helium removal rate for a reactor and relevant measurements are discussed, and particle removal and the power distribution to the limiters are treated. Related topics of the ALT-II program were hydrogen recycling and the measurement of turbulence-induced anomalous particle transport in the plasma edge.
    Fusion Science and Technology 02/2005; 47(2). · 0.49 Impact Factor
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    ABSTRACT: Edge radial electric fields were induced in the edge of the TEXTOR tokamak by means of a polarization electrode in order to study their influence on the plasma edge profiles and its confinement. The studies include the generation of H-mode behaviour with either positive or negative polarity. Particle confinement (τp) of deuterium and of impurity ions as well as energy confinement (τE) are investigated. For positive fields which remain below the threshold for the L-H transition, an interesting regime of reduced particle confinement without noticeable energy confinement loss is found. A strong asymmetry in the edge density profiles with respect to the electric field sign is observed at these low polarization voltages. Above the threshold, H-mode behaviour with increased energy confinement and especially particle confinement can be produced with either polarity of the applied electric field. It is, however, found that, whereas the energy confinement in positive H-modes is at least as good as that in negative ones, the ratio τp/τE is about three times lower in the former case
    Nuclear Fusion 11/2002; 32(5):837. DOI:10.1088/0029-5515/32/5/I10 · 3.06 Impact Factor
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    ABSTRACT: Rapidly changing heat fluxes deposited on the limiter blades are observed during disruptions by infrared (IR) scanners. These scanners are a suitable tool for the analysis of these heat fluxes because they provide both spatial and temporal information with sufficient resolution. Several new features of the power flux to the plasma facing surfaces, during a disruption have been found. The disruptive heat flux occurs on three different time-scales. The fastest ones are for heat bursts with a duration of ≤ 0.1 ms; several of these bursts form a thermal quench of about one millisecond duration, and some of these thermal quenches are found to occur during the current decay phase. Power flux densities of the order of 50 MW/m2 have been observed during a burst. The spatial extent of the area on which this power is deposited during a burst is larger than or equal to the size of half an ALT-II blade, i.e. about 1 m in the toroidal direction. Simultaneous measurements with two cameras show that the correlation length of a single burst is smaller than half the toroidal circumference, probably of the order of half a blade or a full blade length. This is consistent with plasma islands of low mode number. The typical heat deposition patterns at the limiter blades for normal discharges are preserved during a disruption. The magnetic structure near the plasma surface can therefore not be destroyed completely during the thermal quench. The power flux follows the field lines. However, the power e-folding length is about a factor of two to three times larger than under normal discharge conditions
    Nuclear Fusion 11/2002; 32(6):915. DOI:10.1088/0029-5515/32/6/I01 · 3.06 Impact Factor
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    ABSTRACT: Particle exhaust studies have been carried out with the pump limiter ALT-II in the TEXTOR tokamak, under ohmic conditions as well as with NBI and with ICRF auxiliary heating, and the pumping effectiveness is shown to meet the requirements for a fusion reactor. Quantitative measurements of Dα emission, made with a CCD camera, have been used to determine the particle efflux from the plasma. Roughly one third of the Dα emission occurs in a diffuse `halo' that surrounds the limiter belt. The particle confinement time is less than the energy confinement time by a factor of typically 4. Modelling in 2-D of plasma and neutral flows in the TEXTOR boundary has been performed. The source of D+ ions can be related to the Dα emission by a factor that is found to depend on the location of the emission and on the discharge density. The predicted total Dα emission agrees with the measurements within a factor of about 2. Pumping of ALT-II allows for density control; with NBI, the density can be increased well beyond the ohmic limit without the discharge ending in disruption. The plasma particle efflux and the pumped flux both increase with density as well as with heating power. The exhaust efficiency is typically ~2%, with the highest values observed in high density NBI discharges. Higher exhaust rates are observed with NBI than with ICRF. Plasma and neutral flows in the ALT-II scoops have been simulated, making use of a simple plasma model. The scoop may be viewed as a non-linear amplifier of the plasma particle flux; the amplification is found to range from about 2 to 3 for most cases. Flow reversal in the scoop is found in some of the NBI cases and particularly in the highest density case.
    Nuclear Fusion 05/2002; 38(11):1585. DOI:10.1088/0029-5515/38/11/301 · 3.06 Impact Factor
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    ABSTRACT: Four new low field side antennae grouped in pairs have been installed on TEXTOR. It is found that the interaction with the wall (density rise, impurity generation) is significantly reduced when operating each pair out of phase ( pi ) as opposed to in phase (0). The beneficial effect in the pi configuration is obtained without drop in plasma loading. This experimental property is shown, from theory, to be explained by the judicious choice of the geometrical configuration. A further improvement in the wall interaction is made possible by an appropriate choice of wall conditioning (wall carbonization with liner at 400 degrees C or, above all, boronization). As a result record low values of Prad/Ptotal were achieved during ICRH. The large reduction in wall interaction during ICRH allows routine long pulse (>1 s) ICRH operation at the maximum power level available ( equivalent to 2.5 MW).
    Plasma Physics and Controlled Fusion 12/2000; 31(6):921. DOI:10.1088/0741-3335/31/6/004 · 2.19 Impact Factor
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    ABSTRACT: The TEXTOR research programme focuses on the systematic analysis of plasma wall interaction, the development of a suitable wall system and the production of quasi-stationary long-pulse high-temperature plasmas with tolerable impurity concentrations, with well-defined boundary layer and with relevant particle and power fluxes through the boundary. The authors describe the method of wall carbonization developed in Julich and some characteristic features of the TEXTOR plasma obtained with this method for both metal limiters and graphite limiters. Moreover, the effect of ICRH heating (2 MW, 1 sec) on plasma parameters and on the boundary layer is discussed, together with the application of the single head pump limiter ALT-I and of the localized set of magnetic perturbation coils for boundary 'ergodization'.
    Plasma Physics and Controlled Fusion 12/2000; 28(9A):1413. DOI:10.1088/0741-3335/28/9A/018 · 2.19 Impact Factor
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    ABSTRACT: Langmuir probes have been used to characterize the edge plasma of the TEXTOR Tokamak and measure the parameters of the plasma incident on the ALT-I pump limiter during ohmic and ICRH heating. Probes mounted directly on the ALT limiter, and a scanning probe located 90 degrees toroidally from the limiter, provide data for the evaluation of pump limiter performance and its effect on the edge plasma. The edge plasma is characterized by density and flux e-folding lengths of about 1.8 cm when ALT is the main limiter. These scrape-off lengths do not vary significantly as ALT is moved between the normal 42-46 cm minor radii, but increase to over 2.2 cm when ALT is inserted to 40 cm. The flux to probes at a fixed position in the limiter shadow varies by less than 25% for core density changes of a factor of five. This suggests that the global particle confinement time, tau p, scales as the core density. Estimates from the probes indicate that tau p is on the order of the energy confinement time, tau E. The edge electron temperature, Te, typically decreases by a factor of two when the core density is raised from 1 to 4*1013 cm3.
    Plasma Physics and Controlled Fusion 12/2000; 29(4):473. DOI:10.1088/0741-3335/29/4/002 · 2.19 Impact Factor
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    ABSTRACT: Electrostatic biasing experiments using the Advanced Limiter Test (ALT-II) pump limiter in the TEXTOR tokamak have been carried out with the dual goals of: (a) improving the core plasma confinement in the tokamak and (b) enhancing the performance of the pump limiter. The fully toroidal belt limiter has been biased during both ohmic and neutral beam heated discharges. Both polarities of bias have been applied up to a maximum of +/- 500 V with no evidence of impurity accumulation in the central plasma, although applying either polarity of bias to the limiter increases recycling from both the limiter face and the vacuum vessel liner. This in turn results in an increase of the central density. The application of a negative bias to the limiter produces a barrier to radial particle transport in the region between the limiter and the wall. This barrier is not observed in either the no bias or the positive bias case. Neither polarity of limiter bias affects the central plasma energy confinement, apparently because the electric field structure producing the radial barrier is outside the limiter tangency radius. The enhanced recycling, coupled with high edge density, increases the radiated power from the plasma edge and may lower the power flux to the plasma facing surface of the limiter blade. In the case of positive limiter biasing, the pressure in the pumped plasma collection scoops of the limiter increases by approximately 20%, corresponding to a similar increase in the particle removal rate of the pump limiter. The increase in the particle removal rate appears to result from a lower edge electron temperature. This is consistent with the observation of an increase in edge radiated power.
    Nuclear Fusion 07/1994; 34(7-7):975-983. DOI:10.1088/0029-5515/34/7/i05 · 3.06 Impact Factor
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    ABSTRACT: In the TMI tokamak current spikes into elements of the poloidal limiter were observed during disruptions Currents flowed between limiter elements located at the outboard side and inboard side of the torus as well. Simultaneously, the variation of the plasma column vertical position is observed. The direction of this shift coincided with that of a force produced by currents between poloidal limiter elements and the toroidal magnetic field. The correlations between vertical shift, current flowing to limiters and the shift along the major radius were investigated in the small TF tokamak. During major disruptions in the TEXTOR tokamak the currents flowing between the ALT-II toroidal belt limiter and the liner were recorded.
    Journal of Nuclear Materials 12/1992; 196:676-679. DOI:10.1016/S0022-3115(06)80121-2 · 1.87 Impact Factor
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    ABSTRACT: Helium removal experiments were conducted in TEXTOR with a small helium self-pumping module located in a modified ALT-I limiter head. The module contained two heated nickel alloy trapping plates, a nickel deposition filament array, a Langmuir probe, flux probe, and thermocouples. The experiment examined plasma helium removal via trapping of helium ions in the deposited nickel surfaces. Such helium removal was successfully observed, with about 10% of the helium He/D plasma being removed in a â¼1 s period. The module was found to be compatible with overall tokamak operation with essentially no sputtered nickel entering the core plasma. The temperature rise on the ion-exposed inner trapping plate, during a plasma shot, is consistent with a nickel a local sheath potential of â¼3 kT{sub e}. Post-tokamak test examination of the trapping plates shows helium atom concentrations in the deposited nickel consistent with the observed helium removal, and shows very small D concentrations.
    Journal of Nuclear Materials 12/1992; 196-198:664-669. DOI:10.1016/S0022-3115(06)80119-4 · 1.87 Impact Factor
  • K.H. Finken · W.Y. Baek · K.H. Dippel · J.A. Boedo · D.S. Gray · G. Mank ·
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    ABSTRACT: Thermographic measurements at the ALT-II pump limiter on TEXTOR have been performed with the following results. (1) The design of the toroidal pump limiter provides a rather uniform temperature distribution and prevents an overheating of the graphite cover tiles. (2) The local measurements of the convective energy flux to the limiter (IR scanner) and of the radiated power (bolometer) show normally a good power balance; this indicates that the power is leaving the plasma toroidally symmetrically. (3) From the analysis of the temperature distribution in poloidal and toroidal direction along the ALT-II tiles a power decay length of 5–7 mm is derived. (4) The ion temperature at the last closed flux surface is obtained from the energy flux along the magnetic field lines. At low plasma densities, the ion temperature is up to a factor of 2 higher than the electron temperature and at high densities both temperatures converge to the same value.
    Journal of Nuclear Materials 12/1992; 196:220-225. DOI:10.1016/S0022-3115(06)80035-8 · 1.87 Impact Factor
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    ABSTRACT: Experiments have been carried out in the TEXTOR tokamak to investigate the performance of the pump limiter ALT-II at the highest achievable core densities. Neutral beam heating allows the density to be extended beyond the ohmic limit. The use of one beam raises the line-averaged density limit by approximately 50%. Results from discharges with co- and balanced injection are shown. Plasma fluxes to the pump limiter increase with density. Up to 4.5×1021 ions/s (720 A) are collected in the eight segment belt, with pumped particle fluxes of up to 3×1021 atoms/s (60 mbar l/s). Exhaust efficiency determination requires knowledge of the efflux of plasma from the core, which can be estimated from SOL profiles and from Dalpha emission. Both exhaust efficiency estimates increase with density. Experimental results are discussed in terms of predictions from the neutral transport code EIRENE.
    Journal of Nuclear Materials 12/1992; 196:1096-1100. DOI:10.1016/S0022-3115(06)80202-3 · 1.87 Impact Factor
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    ABSTRACT: Comparisons of helium (He) transport and exhaust in L-mode and in an enhanced confinement regime (H-mode), which is induced by a polarizing electrode, have been made for the TEXTOR tokamak. The results show an increased tendency for He accumulation when bulk plasma energy and particle confinement are improved during the polarization-induced H-mode. Since these results imply that a high He pumping efficency may be necessary for H-mode burning plasmas, we have begun exploring He transport in a divertor H-mode, similar to that proposed for the international thermonuclear experimental reactor (ITER). A collaborative program has been initiated to measure He transport and scaling on DIII-D during L-mode, H-mode, and ELMing H-mode plasma conditions. To simulate the presence of He ash in DIII-D, a 25 ms He puff is injected into a DIII-D plasma, resulting in a He concentration of ≈5%. The time dependence of the He2+ density profiles in the plasma core is measured by charge-exchange recombination spectroscopy at 11 radial locations.
    Journal of Nuclear Materials 12/1992; 196:35-44. DOI:10.1016/S0022-3115(06)80009-7 · 1.87 Impact Factor
  • J. Boedo · D. S. Gray · G. R. Tynan · R. Pitts · K. H. Dippel · K. H. Finken · R. Conn ·
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    ABSTRACT: Poloidal asymmetries in the electron density profiles and limiter diagnostics suggest that poloidal and toroidal flows exist in the SOL and edge plasma of the TEXTOR tokamak. A fast reciprocating probe located at the outer midplane provides profiles of the basic plasma parameters including radial electric field and fluctuation driven transport. Based on the probe measurements, we calculate the E(r) X B(t) drift velocities and phase velocities of the turbulence. The measurements are compared to each other. The above measurements are compared to calculations using the SOLXY code for B(t) and I(p) in normal and inverted configuration.
    Journal of Nuclear Materials 12/1992; 196. DOI:10.1016/S0022-3115(06)80085-1 · 1.87 Impact Factor
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    ABSTRACT: The compatibility of radiative edge cooling by neon injection and He exhaust with the pump limiter ALT-II is studied on TEXTOR. It is demonstrated that in plasmas with strong auxiliary heating (2 MW NBI) and with the highest average electron densities (fie = 5.5 x 1019 m -3) the effective confinement time r~, for He has a minimum. This good pumping performance is maintained even for cases in which up to 90% of the heating power is radiated from the plasma boundary (cold radiative edge). The processes inside the scoops of the pump limiter (neutral particle transport, re-ionization) and the variation of particle confinement in the main plasma dominate this behavior.
    Journal of Nuclear Materials 12/1992; 196-198:633. DOI:10.1016/S0022-3115(06)80113-3 · 1.87 Impact Factor
  • K. H. Finken · K. H. Dippel · W. Y. Baek · A. Hardtke ·
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    ABSTRACT: The detection of helium in a deuterium gas environment (both mass 4) is an important task in the field of nuclear fusion physics. To fulfill this purpose a detector has been developed which is based on spectroscopic measurement techniques. For the excitation of the helium and deuterium atoms different electron sources have been tested. Most sources had to be rejected because they either produce a disturbingly high background light level or cannot be operated reproducibly after venting or are mechanically not stable enough. The best line emitting light source has been found to be a Penning gauge. The light is collected in an optical fiber bundle, transferred away from the tokamak experiment to an area more easily accessible, split by interference filters into D α light and the light of a strong He i line, and detected by photomultipliers. Unfortunately broad spectral lines from the deuterium molecule superimpose the strongest He i lines and have nearly the same intensity as the helium lines at a He concentration of about 5%. Therefore, some effort is necessary to deduce the partial pressures of helium and deuterium. A method is described which yields the calibration factors for the observed nonlinear pressure response of the spectral lines. The lower limit for the determination of the helium concentration presently amounts to about 1%–2%; the time resolution of the system is of the order of a few milliseconds.  
    Review of Scientific Instruments 02/1992; 63(1-63):1 - 7. DOI:10.1063/1.1142957 · 1.61 Impact Factor
  • R.W. Conn · K.H. Dippel · W.B. Gauster · A. Miyahara ·
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    ABSTRACT: A toroidal belt pump limiter has been designed, constructed, installed and operated in the TEXTOR tokamak. The Advanced Limiter TEST-II, or ALT-II, is carried out as a partnership under a joint agreement between the European Community, Japan and the United States. ALT-II is designed to withstand the high heat fluxes (greater than 500 W/cm2) intrinsic to the boundary region of tokamak plasmas while simultaneously removing 5% to 10% of the core plasma efflux. The belt consists of eight segments, each 28 cm wide, 150 cm long, and 1.2 cm thick, and each is covered with shaped graphite tiles. Mounted behind each blade is a “scoop” system which captures plasma flowing past the leading edges and behind the blades. This plasma neutralizes on a collector plate and the resulting neutral gas is pumped. During ohmic discharges, 50 to 60 A of current is collected in the scoops and removal rates up to 0.25 Torr-l/s are achieved. The blades are instrumented with Langmuir probes and thermo couples, and other diagnostics include infrared cameras, Hα-monitors, pressure gauges, helium charge exchange spectroscopy, and helium gas diagnostics. The plasma edge and the operation of the pump limiter system are described.
    Fusion Engineering and Design 12/1990; 13(3-13):251-259. DOI:10.1016/0920-3796(90)90045-8 · 1.15 Impact Factor
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    ABSTRACT: In TEXTOR helium removal experiments with the pump-limiter ALT-II have started. To simulate the presence of helium ash in the plasma, helium is injected into the discharge (e.g. at t = 0.7s) as a short pulse of Δt = 20ms. It is found that the He is removed from the discharge in an e-folding time of about half a second for neutral beam heated plasmas and in an e-folding time of about 1.5 s in an OH plasma. The exhaust efficiency of helium amounts to about 8% and is close to the one for deuterium. The fuelling efficiency for the injected helium is found to be in the range of 50–100%; the remaining part seems to be stored in the TEXTOR walls. An estimate of the surface density leads to a value of several times 1013 cm−2. This helium can easily be liberated in succeeding discharges and can be removed efficiently when ALT is pumping.
    Journal of Nuclear Materials 12/1990; 176:816-820. DOI:10.1016/0022-3115(90)90149-H · 1.87 Impact Factor

Publication Stats

643 Citations
83.65 Total Impact Points


  • 1990-2011
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Jülich, North Rhine-Westphalia, Germany
  • 1990-2000
    • University of California, Los Angeles
      • Department of Mechanical and Aerospace Engineering
      Los Angeles, California, United States