High-heat flux ball-pen probe head in ASDEX-Upgrade.

A new high heat flux ball-pen probe head installed on the midplane manipulator is currently being used in ASDEX-Upgrade (AUG). The probe was designed to withstand high heat fluxes making possible the investigation of the plasma edge under harsh conditions, such as low power H-mode. Composed of seven pins (four Langmuir probes, mounted in two Mach probe pairs, and three ball-pen probes), the new probe head allows us to measure several plasma parameters simultaneously and with high temporal resolution. A novel method to correct the sheath potential dynamically accounting for the total secondary electron emission is introduced together with applications to obtain the electron temperature and plasma potential profiles. The total secondary electron emission yield is obtained from particle in cell simulations in AUG condition and probe realistic impact angle with respect to the magnetic field. Finally, the probe capability to investigate turbulence around the separatrix of AUG is discussed.

Real-time feedback control of the impurity emission front in tokamak divertor plasmas.

In magnetic confinement thermonuclear fusion the exhaust of heat and particles from the core remains a major challenge. Heat and particles leaving the core are transported via open magnetic field lines to a region of the reactor wall, called the divertor. Unabated, the heat and particle fluxes may become intolerable and damage the divertor. Controlled 'plasma detachment', a regime characterized by both a large reduction in plasma pressure and temperature at the divertor target, is required to reduce fluxes onto the divertor. Here we report a systematic approach towards achieving this critical need through feedback control of impurity emission front locations and its experimental demonstration. Our approach comprises a combination of real-time plasma diagnostic utilization, dynamic characterization of the plasma in proximity to the divertor, and efficient, reliable offline feedback controller design.

Damage control surgery in perforated diverticulitis: ongoing peritonitis at second surgery predicts a worse outcome.

PURPOSE: Damage control strategy (DCS) is a two-staged procedure for the treatment of perforated diverticular disease complicated by generalized peritonitis. The aim of this retrospective multicenter cohort study was to evaluate the prognostic impact of an ongoing peritonitis at the time of second surgery. METHODS: Consecutive patients who underwent DCS for perforated diverticular disease of the sigmoid colon with generalized peritonitis at four surgical centers were included. Damage control strategy is a two-stage emergency procedure: limited resection of the diseased colonic segment, closure of oral and aboral colon, and application of a negative pressure assisted abdominal closure system at the initial surgery followed by second laparotomy 48 h later. Therein, decision for definite reconstruction (anastomosis or Hartmann's procedure (HP)) is made. An ongoing peritonitis at second surgery was defined as presence of visible fibrinous, purulent, or fecal peritoneal fluid. Microbiologic findings from peritoneal smear at first surgery were collected and analyzed. RESULTS: Between 5/2011 and 7/2017, 74 patients underwent a DCS for perforated diverticular disease complicated by generalized peritonitis (female: 40, male: 34). At second surgery, 55% presented with ongoing peritonitis (OP). Patients with OP had higher rate of organ failure (32 vs. 9%, p = 0.024), higher Mannheim Peritonitis Index (25.2 vs. 18.9; p = 0.001), and increased operation time (105 vs. 84 min., p = 0.008) at first surgery. An anastomosis was constructed in all patients with no OP (nOP) at second surgery as opposed to 71% in the OP group (p < 0.001). Complication rate (44 vs. 24%, p = 0.092), mortality (12 vs. 0%, p = 0.061), overall number of surgeries (3.4 vs. 2.4, p = 0.017), enterostomy rate (76 vs. 36%, p = 0.001), and length of hospital stay (25 vs. 18.8 days, p = 0.03) were all increased in OP group. OP at second surgery occurred significantly more often in patients with Enterococcus infection (81 vs. 44%, p = 0.005) and with fungal infection (100 vs. 49%, p = 0.007). In a multivariate analysis, Enterococcus infection was associated with increased morbidity (67 vs. 21%, p < 0.001), enterostomy rate (81 vs. 48%, p = 0.017), and anastomotic leakage (29 vs. 6%, p = 0.042), whereas fungal peritonitis was associated with an increased mortality (43 vs. 4%, p = 0.014). CONCLUSION: Ongoing peritonitis after DCS is a predictor of a worse outcome in patients with perforated diverticulitis. Enterococcal and fungal infections have a negative impact on occurrence of OP and overall outcome.

Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe.

The ball pen probe (BPP) technique is used successfully to make profile measurements of plasma potential, electron temperature, and radial electric field on the Mega Amp Spherical Tokamak. The potential profile measured by the BPP is shown to significantly differ from the floating potential both in polarity and profile shape. By combining the BPP potential and the floating potential, the electron temperature can be measured, which is compared with the Thomson scattering (TS) diagnostic. Excellent agreement between the two diagnostics is obtained when secondary electron emission is accounted for in the floating potential. From the BPP profile, an estimate of the radial electric field is extracted which is shown to be of the order ∼1 kV/m and increases with plasma current. Corrections to the BPP measurement, constrained by the TS comparison, introduce uncertainty into the ER measurements. The uncertainty is most significant in the electric field well inside the separatrix. The electric field is used to estimate toroidal and poloidal rotation velocities from E × B motion. This paper further demonstrates the ability of the ball pen probe to make valuable and important measurements in the boundary plasma of a tokamak.

On the reliability of scrape-off layer ion temperature measurements by retarding field analyzers.

The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measurements of ion temperature Ti in the tokamak scrape-off layer. In this paper we analyze some instrumental effects of the RFA and their influence on Ti measurements. It is shown that selective ion transmission through the RFA slit is responsible for an overestimation of Ti by less than 14%, even for a relatively thick slit plate. Therefore, thicker slit plates are preferable, since they reduce, e.g., the risk of melting during off-normal events, and the effect of positive space charge inside the cavity. The influence of the electron repelling grid, as well as misalignment of the slit with respect to the magnetic field on Ti measurements are negligible.