Suppression of Alfvén Modes on the National Spherical Torus Experiment Upgrade with Outboard Beam Injection.

In this Letter we present data from experiments on the National Spherical Torus Experiment Upgrade, where it is shown for the first time that small amounts of high pitch-angle beam ions can strongly suppress the counterpropagating global Alfvén eigenmodes (GAE). GAE have been implicated in the redistribution of fast ions and modification of the electron power balance in previous experiments on NSTX. The ability to predict the stability of Alfvén modes, and developing methods to control them, is important for fusion reactors like the International Tokamak Experimental Reactor, which are heated by a large population of nonthermal, super-Alfvénic ions consisting of fusion generated α's and beam ions injected for current profile control. We present a qualitative interpretation of these observations using an analytic model of the Doppler-shifted ion-cyclotron resonance drive responsible for GAE instability which has an important dependence on k_{⊥}ρ_{L}. A quantitative analysis of this data with the hym stability code predicts both the frequencies and instability of the GAE prior to, and suppression of the GAE after the injection of high pitch-angle beam ions.

Diagnosis of Aortic Graft Infection: A Case Definition by the Management of Aortic Graft Infection Collaboration (MAGIC).

OBJECTIVE/BACKGROUND: The management of aortic graft infection (AGI) is highly complex and in the absence of a universally accepted case definition and evidence-based guidelines, clinical approaches and outcomes vary widely. The objective was to define precise criteria for diagnosing AGI. METHODS: A process of expert review and consensus, involving formal collaboration between vascular surgeons, infection specialists, and radiologists from several English National Health Service hospital Trusts with large vascular services (Management of Aortic Graft Infection Collaboration [MAGIC]), produced the definition. RESULTS: Diagnostic criteria from three categories were classified as major or minor. It is proposed that AGI should be suspected if a single major criterion or two or more minor criteria from different categories are present. AGI is diagnosed if there is one major plus any criterion (major or minor) from another category. (i) Clinical/surgical major criteria comprise intraoperative identification of pus around a graft and situations where direct communication between the prosthesis and a nonsterile site exists, including fistulae, exposed grafts in open wounds, and deployment of an endovascular stent-graft into an infected field (e.g., mycotic aneurysm); minor criteria are localized AGI features or fever ≥38°C, where AGI is the most likely cause. (ii) Radiological major criteria comprise increasing perigraft gas volume on serial computed tomography (CT) imaging or perigraft gas or fluid (≥7 weeks and ≥3 months, respectively) postimplantation; minor criteria include other CT features or evidence from alternative imaging techniques. (iii) Laboratory major criteria comprise isolation of microorganisms from percutaneous aspirates of perigraft fluid, explanted grafts, and other intraoperative specimens; minor criteria are positive blood cultures or elevated inflammatory indices with no alternative source. CONCLUSION: This AGI definition potentially offers a practical and consistent diagnostic standard, essential for comparing clinical management strategies, trial design, and developing evidence-based guidelines. It requires validation that is planned in a multicenter, clinical service database supported by the Vascular Society of Great Britain & Ireland.

Rotation and kinetic modifications of the tokamak ideal-wall pressure limit.

The impact of toroidal rotation, energetic ions, and drift-kinetic effects on the tokamak ideal wall mode stability limit is considered theoretically and compared to experiment for the first time. It is shown that high toroidal rotation can be an important destabilizing mechanism primarily through the angular velocity shear; non-Maxwellian fast ions can also be destabilizing, and drift-kinetic damping can potentially offset these destabilization mechanisms. These results are obtained using the unique parameter regime accessible in the spherical torus NSTX of high toroidal rotation speed relative to the thermal and Alfvén speeds and high kinetic pressure relative to the magnetic pressure. Inclusion of rotation and kinetic effects significantly improves agreement between measured and predicted ideal stability characteristics and may provide new insight into tearing mode triggering.

Pathology specific early outcome after thoracic endovascular aortic repair.

OBJECTIVES: Endovascular intervention is established for treatment of thoracic aortic dissection and aneurysm. The aim of this study was to compare the incidence of all-cause and aortic-related in-hospital mortality, stroke, spinal cord ischaemia, and major adverse event rate for patients undergoing thoracic aortic endovascular intervention to see if there is a pathology-specific effect. METHODS: Data were collected prospectively and analysed retrospectively for a cohort of 309 consecutive patients with either thoracic aortic dissection or aneurysm over a 14-year period. RESULTS: There were 209 men and 100 women with a median age of 72 years (interquartile range [IQR] 63-78 years). Aneurysm affected 62% (193/309) of patients and 37% (116/309) had complicated type B aortic dissection, of whom 43% (50/116) had acute and 57% (66/116) chronic presentations. In patients with aortic dissection compared to aneurysm, there was no significant difference in all-cause in-hospital mortality (6.9% vs. 8.3% respectively, p = 0.827, relative risk [RR] 0.83, 95% confidence interval [CI] 0.37-1.88), stroke (6.0% vs 6.2%, p = 1.00, RR 0.971, CI 0.39-2.39), spinal cord ischaemia (6.0% vs 6.2%, p = 1.00, RR 1.030, CI 0.42-2.54), or major adverse event rate (16.4% vs. 16.6%, p = 1.00, RR 0.988, CI 0.59-1.66). The rate of aortic related death was four times greater in the dissection than in the aneurysm group (4/8 = 50% vs 2/16 = 12.5%, p = 0.06, RR 6.99, CI 0.92-52.5) although this did not reach statistical significance. CONCLUSIONS: There was no difference in the incidence of in-hospital mortality, stroke, and spinal cord ischaemia between aneurysm and dissection. The higher rate of aortic related death in the dissection group may indicate the need to refine the clinical management of these patients, including procedural planning, endograft design, and operative technique.

High-harmonic fast-wave power flow along magnetic field lines in the scrape-off layer of NSTX.

A significant fraction of high-harmonic fast-wave (HHFW) power applied to NSTX can be lost to the scrape-off layer (SOL) and deposited in bright and hot spirals on the divertor rather than in the core plasma. We show that the HHFW power flows to these spirals along magnetic field lines passing through the SOL in front of the antenna, implying that the HHFW power couples across the entire width of the SOL rather than mostly at the antenna face. This result will help guide future efforts to understand and minimize these edge losses in order to maximize fast-wave heating and current drive.

Predictors of outcome after endovascular repair for chronic type B dissection.

OBJECTIVES: To assess the durability of endovascular repair (TEVAR) in chronic type B dissection (CD) and identify factors predictive of outcome. DESIGN: Retrospective analysis of a prospective database. MATERIALS: Patients undergoing TEVAR for CD at a tertiary referral centre 2000-2010. METHODS: Analysis of pre-operative characteristics, operative outcome, false lumen thrombosis, aortic diameter and survival. RESULTS: 58 consecutive patients were included (49 elective, 9 urgent, mean age 66 years). Mean aortic diameter was 6.4 cm (Standard deviation SD 1.3 cm). Three patients died perioperatively (5%, 1 urgent, 2 elective). Complications included retrograde type A dissection (n = 3), paraplegia (1), and transient ischaemic attack (1). Estimated survival (Kaplan-Meier) was 89% (1-year) and 64% (3-years). Forty-seven patients had mid-term imaging follow-up at mean 38 months. Reintervention rate was 15% at 1-year and 29% at 3-years. Aortic diameter decreased in 24, was stable in 15 and increased in 8. Mid-term survival was higher in patients with aortic remodelling (reduction of aortic diameter >0.5 cm; 3-year 89%) than without (54%; Log Rank p = 0.005). Remodelling occurred with extensive false lumen thrombosis. CONCLUSION: Satisfactory mid-term outcome after TEVAR for CD remains a challenge. Survival is associated with aortic remodelling, which is related to persistence of flow in the false lumen.

Total endovascular repair of thoracoabdominal aortic aneurysms.

BACKGROUND: Endovascular graft designs incorporating sidebranches, fenestrations and scallops offer a minimally-invasive alternative to open surgery and hybrid approaches for thoracoabdominal aortic aneurysms (TAAA). Our unit has offered total endovascular TAAA repair to selected higher-risk patients since 2008. We report the largest UK series to date of total endovascular TAAA repair. METHODS: Retrospective analysis of a prospectively-maintained operative database. RESULTS: 31 patients (21 male, 10 female) median age 71 years (range 58-84), with TAAA (12 Crawford type I, 13 type III, 6 type IV), median diameter 6.4 (4.3 (mycotic)- 9.9) underwent endovascular TAAA repair (total 48 sidebranches, 26 fenestrations, 13 scallops) between July 2008 and January 2011. Median operating time 225 min (65-540 min), X-ray screening time 58 min (4-212 min), contrast dose 175 ml (70-500 ml), blood loss 325 ml (100-400 ml). Median post-operative length of hospital stay 6 days (2-22 days). Three patients (3/31, 9.7%) died within 30 days of operation: multisystem organ failure (1) acute renal failure and paraplegia (1) and paraplegia (1). There were no other cases of in-hospital organ failure, paraplegia or major complications. The median change in pre-discharge from pre-operative renal function was 3.4% deterioration in eGFR (range: 32.7% deterioration to 73.0% improvement) One patient presented with late-onset paraparesis, a second developed acute renal failure 8 months after repair. One early high-pressure endoleak (type 3) required correction. Three patients had died by median follow-up 12 months (1-36), 2 from heart disease and one from haemopericardium secondary to acute dissection of the ascending aorta (the dissection did not involve, nor extend close to, the endovascular graft). CONCLUSIONS: Total endovascular repair of TAAA offers patients a minimally-invasive alternative to open surgery with early results at least comparable to those seen with open or hybrid surgical approaches.

Quality improvement framework for major amputation: are we getting it right?

INTRODUCTION: The quality improvement framework for major amputation was developed with the aim of improving outcomes and reducing the perioperartive mortality to less than 5% by 2015. The aim of the study was to assess our compliance with the framework guidelines and look for the reasons for non-compliance. METHOD: All major amputations performed between 2008 and 2010 were included. The following data were collected: presence of infection ± tissue loss, status of arterial supply, revascularisation attempts, time to surgery, type of amputation, morbidity and mortality. RESULTS: A total of 81 patients were included (42 BKAs, 39 AKAs). Ninety percentage had formal preoperative arterial investigations and 84% had an attempted revascularisation procedure. Patients who were transferred late from non-vascular units (n = 12) had a 30-day mortality of 50% whereas patients who presented directly to our unit had a 30-day mortality of 7.2%. The number of amputations has decreased over the last 3 years from 34 to 21 per year, coinciding with the doubling of crural revascularisation procedures performed (from 60 to 120 per year). Ten patients underwent a revision from BKA to AKA because of an inadequate profunda femoris artery (PFA), whereas all those with a healed BKA stump either had a good PFA or a named crural vessel. CONCLUSION: The overall number of amputations is decreasing from year to year. By doubling our crural revascularisation procedures we are saving more limbs. Thirty-day mortality is higher than expected, particularly in patients who present late. Expeditious referral may potentially improve the mortality rate among this group of patients.

Effect of collisionality on kinetic stability of the resistive wall mode.

The impact of collisionless, energy-independent, and energy-dependent collisionality models on the kinetic stability of the resistive wall mode is examined for high pressure plasmas in the National Spherical Torus Experiment. Future devices will have decreased collisionality, which previous stability models predict to be universally destabilizing. In contrast, in kinetic theory reduced ion-ion collisions are shown to lead to a significant stability increase when the plasma rotation frequency is in a stabilizing resonance with the ion precession drift frequency. When the plasma is in a reduced stability state with rotation in between resonances, collisionality will have little effect on stability.

Density gradient stabilization of electron temperature gradient driven turbulence in a spherical tokamak.

In this Letter we report the first clear experimental observation of density gradient stabilization of electron temperature gradient driven turbulence in a fusion plasma. It is observed that longer wavelength modes, k(⊥)ρ(s) ≲ 10, are most stabilized by density gradient, and the stabilization is accompanied by about a factor of 2 decrease in the plasma effective thermal diffusivity.

Suppression of electron temperature gradient turbulence via negative magnetic shear in NSTX.

Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible E×B shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced to intermittent bursts, while electron thermal diffusivity improves to below 0.1 electron gyro-Bohms.

Electromagnetic transport from microtearing mode turbulence.

This Letter presents nonlinear gyrokinetic simulations of microtearing mode turbulence. The simulations include collisional and electromagnetic effects and use experimental parameters from a high-ß discharge in the National Spherical Torus Experiment. The predicted electron thermal transport is comparable to that given by experimental analysis, and it is dominated by the electromagnetic contribution of electrons free-streaming along the resulting stochastic magnetic field line trajectories. Experimental values of flow shear can significantly reduce the predicted transport.

Continuous improvement of H-mode discharge performance with progressively increasing lithium coatings in the National Spherical Torus Experiment.

Lithium wall coatings have been shown to reduce recycling, improve energy confinement, and suppress edge localized modes in the National Spherical Torus Experiment. Here, we show that these effects depend continuously on the amount of predischarge lithium evaporation. We observed a nearly monotonic reduction in recycling, decrease in electron transport, and modification of the edge profiles and stability with increasing lithium. These correlations challenge basic expectations, given that even the smallest coatings exceeded that needed for a nominal thickness of the order of the implantation range.

Predictors of stroke and paraplegia in thoracic aortic endovascular intervention.

BACKGROUND: Endoluminal repair of thoracic aortic pathology has become established in clinical practice, but is associated with significant neurological complications. The aim of this study was to identify factors that were predictive of stroke and paraplegia. METHODS: Prospective data was collected for a cohort of 293 consecutive patients having thoracic aortic endovascular repair between August 1997 and September 2009. Patient and procedural characteristics were related to the incidence of stroke and paraplegia using multivariate logistic regression analysis. RESULTS: The median age was 68 years (18-87), there were 191 men and 102 women. Mortality was 5.1% for 195 elective and 13.4% for 98 urgent patients. Stroke affected 16 (5.5%) patients: 11 affected the anterior and 5 the posterior circulation. Coverage of the left subclavian artery with no revascularisation was the only significant factor predictive of stroke (OR 5.34 (1.42-20.40) P = 0.01). Paraplegia affected 16 patients (5.5%) but no independent risk factor was identified: 12 were identified perioperatively and 4 were delayed by up to 6 months. CONCLUSION: Covering the left subclavian artery without revascularisation increases the risk of stroke following endoluminal repair of thoracic pathology. Paraplegia appears to be more complex and no independent precipitating factor was identified.

Resistive wall mode instability at intermediate plasma rotation.

Experimental observation of resistive wall mode (RWM) instability in the National Spherical Torus Experiment (NSTX) at plasma rotation levels intermediate to the ion precession drift and ion bounce frequencies suggests that low critical rotation threshold models are insufficient. Kinetic modifications to the ideal stability criterion yield a more complex relationship between plasma rotation and RWM stability. Good agreement is found between an experimental RWM instability at intermediate plasma rotation and the RWM marginal point calculated with kinetic effects included, by the MISK code. By self-similarly scaling the experimental plasma rotation profile and the collisionality in the calculation, resonances of the mode with the precession drift and bounce frequencies are explored. Experimentally, RWMs go unstable when the plasma rotation is between the stabilizing precession drift and bounce resonances.

Triggered confinement enhancement and pedestal expansion in high-confinement-mode discharges in the national spherical torus experiment.

We report observation of a new high performance regime in discharges in the National Spherical Torus Experiment, where the H mode edge "pedestal" temperature doubles and the energy confinement increases by 50%. The spontaneous transition is triggered by a large edge-localized mode, either natural or externally triggered by 3D fields. The transport barrier grows inward from the edge, with a doubling of both the pedestal pressure width and the spatial extent of steep radial electric field shear. The dynamics suggest that 3D fields could be applied to reduce edge transport in fusion devices.

On demand triggering of edge localized instabilities using external nonaxisymmetric magnetic perturbations in toroidal plasmas.

The application of nonaxisymmetric magnetic fields is shown to destabilize edge-localized modes (ELMs) during otherwise ELM-free periods of discharges in the National Spherical Torus Experiment (NSTX). Profile analysis shows the applied fields increased the temperature and pressure gradients, decreasing edge stability. This robust effect was exploited for a new form of ELM control: the triggering of ELMs at will in high performance H mode plasmas enabled by lithium conditioning, yielding high time-averaged energy confinement with reduced core impurity density and radiated power.

Long-term surveillance with computed tomography after endovascular aneurysm repair may not be justified.

BACKGROUND: There is a common perception that a large number of secondary interventions are needed following endovascular aortic aneurysm repair. METHODS: Prospective data were collected for a cohort of 417 consecutive elective patients undergoing infrarenal aortic endograft repair between April 2000 and May 2008. The rate of secondary interventions, associated morbidity and need for reintervention following surveillance imaging were analysed. RESULTS: The male : female ratio was 11 : 1, median age 76 (range 40-93) years and median aneurysm diameter 6.1 (5.3-11) cm. The overall 30-day mortality rate was 1.7 per cent (seven of 417). Secondary interventions were performed in 31 patients (7.4 per cent), of which six (1.4 per cent) were detected by surveillance. Endoleaks requiring reintervention occurred in 12 patients (2.9 per cent; ten type I and two type III endoleaks). Limb ischaemia secondary to graft occlusion occurred in 17 patients (4.1 per cent); extra-anatomical bypass was needed in 15 patients (3.6 per cent) and the remaining two had an amputation. Graft explantation following late infection was required in two patients (0.5 per cent). CONCLUSION: Endoluminal repair of infrarenal aortic aneurysms can be performed with a low reintervention rate. The value of prolonged surveillance seems limited and current surveillance protocols may require revision.

Is endovascular repair of mycotic aortic aneurysms a durable treatment option?

OBJECTIVE: Endovascular repair for degenerative aortic aneurysms is well established, but its role in those with infective pathology remains controversial. This study aims to assess the durability of endovascular repair with a review of our midterm results. METHOD: A retrospective analysis of a prospectively maintained endovascular database (1998-2008) was conducted, which identified 673 consecutive patients with aortic aneurysms. RESULTS: Nineteen patients (2.8%) were identified with infected aortic aneurysms, in which there were a total of 23 separate aneurysms (16 thoracic and seven abdominal). Six patients (32%) presented with rupture. Eleven patients (58%) had received antibiotics preoperatively for a median duration of 11 days (1-54 days). Fifteen of the 19 (79%) had positive blood cultures, with Staphylococcus aureus being the most common organism. All 19 patients underwent endovascular repair. There were three Type I endoleaks (one requiring conversion to open repair) and two Type II endoleaks. One patient developed transient paraplegia, resolved by cerebrovascular fluid (CSF) drainage, and one patient had a stroke. The 30-day mortality was 11%, and survival at median follow-up of 20 months (0-83 months) was 73%. All eight deaths in the series were related to aneurysm. CONCLUSION: Endovascular treatment of infective aortic pathology provides an early survival benefit; however, concerns over on-going graft infection remain.

Successful endovascular repair of acute type B aortic dissection in undiagnosed Ehlers-Danlos syndrome type IV.

A 61-year-old man presented with an acute type B aortic dissection for which a stent-graft was introduced. He remains complication-free 4 years onwards and has since been diagnosed with Ehlers-Danlos syndrome type IV (EDS IV). His particular mutation is predicted to result in lesser levels of normal collagen and may explain his favourable outcome from endovascular intervention. Understanding the genotype-phenotype correlation may influence the choice of therapy offered to patients with EDS IV.