Multi-gigaelectronvolt acceleration of positrons in a self-loaded plasma wakefield.

Electrical breakdown sets a limit on the kinetic energy that particles in a conventional radio-frequency accelerator can reach. New accelerator concepts must be developed to achieve higher energies and to make future particle colliders more compact and affordable. The plasma wakefield accelerator (PWFA) embodies one such concept, in which the electric field of a plasma wake excited by a bunch of charged particles (such as electrons) is used to accelerate a trailing bunch of particles. To apply plasma acceleration to electron-positron colliders, it is imperative that both the electrons and their antimatter counterpart, the positrons, are efficiently accelerated at high fields using plasmas. Although substantial progress has recently been reported on high-field, high-efficiency acceleration of electrons in a PWFA powered by an electron bunch, such an electron-driven wake is unsuitable for the acceleration and focusing of a positron bunch. Here we demonstrate a new regime of PWFAs where particles in the front of a single positron bunch transfer their energy to a substantial number of those in the rear of the same bunch by exciting a wakefield in the plasma. In the process, the accelerating field is altered--'self-loaded'--so that about a billion positrons gain five gigaelectronvolts of energy with a narrow energy spread over a distance of just 1.3 metres. They extract about 30 per cent of the wake's energy and form a spectrally distinct bunch with a root-mean-square energy spread as low as 1.8 per cent. This ability to transfer energy efficiently from the front to the rear within a single positron bunch makes the PWFA scheme very attractive as an energy booster to an electron-positron collider.

High-efficiency acceleration of an electron beam in a plasma wakefield accelerator.

High-efficiency acceleration of charged particle beams at high gradients of energy gain per unit length is necessary to achieve an affordable and compact high-energy collider. The plasma wakefield accelerator is one concept being developed for this purpose. In plasma wakefield acceleration, a charge-density wake with high accelerating fields is driven by the passage of an ultra-relativistic bunch of charged particles (the drive bunch) through a plasma. If a second bunch of relativistic electrons (the trailing bunch) with sufficient charge follows in the wake of the drive bunch at an appropriate distance, it can be efficiently accelerated to high energy. Previous experiments using just a single 42-gigaelectronvolt drive bunch have accelerated electrons with a continuous energy spectrum and a maximum energy of up to 85 gigaelectronvolts from the tail of the same bunch in less than a metre of plasma. However, the total charge of these accelerated electrons was insufficient to extract a substantial amount of energy from the wake. Here we report high-efficiency acceleration of a discrete trailing bunch of electrons that contains sufficient charge to extract a substantial amount of energy from the high-gradient, nonlinear plasma wakefield accelerator. Specifically, we show the acceleration of about 74 picocoulombs of charge contained in the core of the trailing bunch in an accelerating gradient of about 4.4 gigavolts per metre. These core particles gain about 1.6 gigaelectronvolts of energy per particle, with a final energy spread as low as 0.7 per cent (2.0 per cent on average), and an energy-transfer efficiency from the wake to the bunch that can exceed 30 per cent (17.7 per cent on average). This acceleration of a distinct bunch of electrons containing a substantial charge and having a small energy spread with both a high accelerating gradient and a high energy-transfer efficiency represents a milestone in the development of plasma wakefield acceleration into a compact and affordable accelerator technology.

Beam loading by distributed injection of electrons in a plasma wakefield accelerator.

We show through experiments and supporting simulations that propagation of a highly relativistic and dense electron bunch through a plasma can lead to distributed injection of electrons, which depletes the accelerating field, i.e., beam loads the wake. The source of the injected electrons is ionization of the second electron of rubidium (Rb II) within the wake. This injection of excess charge is large enough to severely beam load the wake, and thereby reduce the transformer ratio T. The reduction of the average T with increasing beam loading is quantified for the first time by measuring the ratio of peak energy gain and loss of electrons while changing the beam emittance. Simulations show that beam loading by Rb II electrons contributes to the reduction of the peak accelerating field from its weakly loaded value of 43 GV/m to a strongly loaded value of 26 GV/m.

High-field plasma acceleration in a high-ionization-potential gas.

Plasma accelerators driven by particle beams are a very promising future accelerator technology as they can sustain high accelerating fields over long distances with high energy efficiency. They rely on the excitation of a plasma wave in the wake of a drive beam. To generate the plasma, a neutral gas can be field-ionized by the head of the drive beam, in which case the distance of acceleration and energy gain can be strongly limited by head erosion. Here we overcome this limit and demonstrate that electrons in the tail of a drive beam can be accelerated by up to 27 GeV in a high-ionization-potential gas (argon), boosting their initial 20.35 GeV energy by 130%. Particle-in-cell simulations show that the argon plasma is sustaining very high electric fields, of ∼150 GV m(-1), over ∼20 cm. The results open new possibilities for the design of particle beam drivers and plasma sources.

Demonstration of a positron beam-driven hollow channel plasma wakefield accelerator.

Plasma wakefield accelerators have been used to accelerate electron and positron particle beams with gradients that are orders of magnitude larger than those achieved in conventional accelerators. In addition to being accelerated by the plasma wakefield, the beam particles also experience strong transverse forces that may disrupt the beam quality. Hollow plasma channels have been proposed as a technique for generating accelerating fields without transverse forces. Here we demonstrate a method for creating an extended hollow plasma channel and measure the wakefields created by an ultrarelativistic positron beam as it propagates through the channel. The plasma channel is created by directing a high-intensity laser pulse with a spatially modulated profile into lithium vapour, which results in an annular region of ionization. A peak decelerating field of 230 MeV m(-1) is inferred from changes in the beam energy spectrum, in good agreement with theory and particle-in-cell simulations.

Compression and genetic sequence analysis.

A novel approach to genetic sequence analysis is presented. This approach, based on compression of algorithms, has been launched simultaneously by Grumbach and Tahi, Milosavljevic and Rivals. To reduce the description of an object, a compression algorithm replaces some regularities in the description by special codes. Thus a compression algorithm can be applied to a sequence in order to study the presence of those regularities all over the sequence. This paper explains this ability, gives examples of compression algorithms already developed and mentions their applications. Finally, the theoretical foundations of the approach are presented in an overview of the algorithmic theory of information.

Traumatic tricuspid insufficiency. An underdiagnosed disease.

Twelve cases of traumatic tricuspid insufficiency (only three of which already have been published), collected in the same city, are reported. This figure strongly contrasts with the limited number of cases published to date in the literature. Emphasis is placed on the difficulty of diagnosis, the utility of echocardiography and Doppler ultrasound and therapeutic problems of this lesion.

Cardiomyoplasty does not preclude heart transplantation.

Stimulated skeletal muscle grafts have been proposed to improve left ventricle function in patients with severe myocardial failure. In 1 particular case reported here, however, the postoperative functional improvement was only transient and disabling heart failure recurred after 9 months in spite of a vigorous latissimus muscle contraction. Heart transplantation was proposed to this patient and performed successfully. Technically, the key to heart removal depends on the retrograde dissection of the ventricular cavities, starting from the right atrioventricular groove. The intraoperative observations confirmed the viability of the latissimus dorsi muscle, inefficient on a highly dilated cardiomyopathy. Histopathological examination of the latissimus dorsi muscles showed that the transformation process of the stimulated muscle was good. Thus, severe cardiac dilatation seems to be one of the limitations of cardiomyoplasty. Cardiomyoplasty, when it fails, does not preclude heart transplantation. The histochemical studies confirm the electrophysiologic principle of cardiomyoplasty in humans.

[Symptoms and natural history of cardiovascular diseases: obsolete?]. / Le symptôme et l'histoire naturelle des maladies cardiovasculaires: obsolètes?

The role of symptoms in establishing a diagnosis in cardiovascular diseases has decreased. Treatment privileges action and underestimates the potentially favourable spontaneous outcome of the treated condition. The object of this article is not to question the progress made in presymptomatic diagnosis of certain cardiovascular diseases or the benefits of treatment prescribed for some a- or paucisymptomatic patients: recent reports, particularly in asymptomatic aortic regurgitation, silent ischemia and subclinical left ventricular dysfunction of cardiomyopathy have confirmed the utility of therapy in these cases. However, the risks of misinterpreting a symptom and the natural history should be underlined: the "complaint" of the patient not properly assessed; cascades of "complementary investigations" of debatable utility and uncertain interpretation, when taken out of context; substitution of the medical therapeutic offer on demand of the patient and inadequate patient education concerning possible therapeutic approaches: the quest for intermediary objectives (anatomic, physiologic, biologic), which have not been shown to increase the quality or duration of life; the disproportion between the small number of validated therapies and the ever increasing range of interventional audacities in asymptomatic patients... It is important that general pathology and the natural history of cardiovascular diseases are taught again in France. With respect to symptoms, proper assessment no longer depends on "clinical judgment"; it should have a greater role in diagnosis of cardiovascular diseases by the understanding of the physiopathological mechanisms, Bayesian assessment of its predictive value, and accurate inclusion in multiparameter scores derived from recent large scale epidemiologic and therapeutic trials.

[Practical difficulties in drug prescription in chronic cardiac insufficiency]. / Difficultés pratiques de la prescription médicamenteuse dans l'insuffisance cardiaque chronique.

Many practical difficulties are encountered by physicians in the medical treatment of chronic cardiac failure. They are related to the choice of drug guided by therapeutic objectives: vasodilators and angiotensin converting enzyme inhibitors are no longer drugs of secondary intention reserved for chronic cardiac failure, but there is no information as to the place of these drugs in early stages of myocardial dysfunction before the appearance of the clinical signs of cardiac failure. Other difficulties are related to the use of many different drugs, to the many secondary effects, sometimes increased by drug interactions; these difficulties are accentuated by the multiplicity of the etiologies of cardiac failure, by the frequency of associated extracardiac disease and by the risks inherent to abnormal myocardial function. In addition, the prescriber must not forget that the drug is not everything in the treatment of cardiac failure and he must be able to recognise the pathology underlying the cardiac failure which may require specific therapy. Finally, the physician must decide the optimal timing for referring patients with very severe cardiac failure for transplantation.

[Risk of bacterial endocarditis after cardiac surgery]. / Risque d'endocardite infectieuse après intervention intracardiaque.

The risk of infective endocarditis (IE) after intracardiac surgery is dominated by the risk of IE on valvular prostheses. The reported prevalence of IE on prosthetic valves varies according to the chosen diagnostic criteria of IE and its timing. The risk of early IE is 0.4 to 1.3% and the linearized annual risk of late IE is about 0.5%. These values appear to be identical irrespective of the type and site of the prosthesis: the risk is higher in multiple valve replacement. In early IE, the commonest infecting organism is the staphylococcus: the bacteriological spectrum of late IE is the same as that of IE on native valves. The portal of entry is often detected in early IE but more rarely (50%) in late IE. The risk of IE in operated congenital heart disease is very low after surgery of left-to-right shunts or valvular stenosis; it is higher for patients with Tetralogy of Fallot and those with complex cyanotic disease, especially in cases with residual ventricular septal defects and with palliative surgery such as systemico-pulmonary anastomosis. The risk of IE on endocavitary catheters (pace-maker, defibrillator) after interventional cardiac procedures and after cardiac transplantation, seems to be very low. These results show that preventive measures against IE are only routinely required in prosthetic valve patients and after surgery of Tetralogy of Fallot and complex cyanotic cardiac disease.

[Tachycardia-dependent paroxysmal block in the bundle of His. Electrophysiological demonstration]. / Bloc intrahisien paroxystique dépendant de la tachycardie. Démonstration électrophysiologique

The authors report the case of a patient who presented with angina of effort followed by transient loss of consciousness or syncope. Clinical examination yielded little information. The electrocardiogram showed signs of an old antero-septal infarct, and a slightly prolonged PR interval at 0.22 s. A recording of the activity of the bundle of His showed a double H potential, whose two components were separated by an interval of 80 ms. This interval increased progressively under the influence of atrial stimulation. At a critical frequency of 125/mn, a complete block below the bundle of His was produced; this only reverted after slowing or cessation of stimulation. The same phenomenon occurred at rates between 125/mn and 150/mn. By contrast stimulation at 170/mn was followed by a 2:1 block below the bundle of His. It is to be presumed that the complete A-V block was occasioned by latent conduction of the impulse in the injured area. If the rate was increased yet again, the level of the block became displaced to the upper limit of this area. The absence of latent conduction then allowed the abnormal fibres to recover. The present study thus demonstrates the existance of a special type of paroxysmal A-V block: a bundle block which is dependant upon tachycardia.

[Should asymptomatic aortic valve disease be treated surgically?]. / Faut-il opérer un aortique asymptomatique?

The problems posed by asymptomatic aortic valve disease with regards to surgery differ according to whether the lesion is aortic stenosis or regurgitation. In stenotic lesions, even severe, the risk of spontaneous evolution is very small when the subject is totally asymptomatic. However, the quality of the results of surgery in asymptomatic or pauci-symptomatic patients and the increased difficulties in the very elderly are incentives not to differ surgery when the stenosis is severe, and especially when left ventricular function begins to degrade. In aortic regurgitation, the risk of progression is also low in asymptomatic patients but left ventricular dysfunction may develop before symptoms occur: this explains the necessity for regular clinical, echocardiographic and eventually radioisotopic examination of patients with severe aortic regurgitation. In dystrophic aortic regurgitation, an aneurysm of the sinuses of Valsalva and/or of the ascending aorta, or progressive fusiform dilatation of the ascending aorta are reasons for not postponing surgery for replacement of the aortic valve and the ascending aorta.

[Prognosis of surgically corrected chronic aortic insufficiency]. / Pronostic des insuffisances aortiques chroniques opérées.

It is difficult to determine the surgical indications of asymptomatic chronic aortic regurgitation (AR). This study was undertaken in 205 patients with pure AR, operated between 1970 and 1982: 136 patients were symptomatic (Classes III and IV of the NYHA, +/- cardiac failure +/- angina, mean age: 49.5 years). Sixty nine asymptomatic patients (Classes I and II of the NYHA without cardiac failure or angina, mean age: 42 years). The prognostic value of 58 variables was studied in these two groups. The mortality during the first postoperative month was 14% in the symptomatic patients and the 5 year survival rate was 68.7%; this was significantly lower in patients with ECG changes of systolic left greater than or equal to ventricular overload, with radiological cardiomegaly (cardiotolerance index 0.60) with calcific aortic valve disease, with raised arterio-venous difference and/or low cardiac output, and with a low ejection fraction. In the asymptomatic group, the hospital mortality was zero and the 5 year survival rate 86.8%. This was significantly decreased in patients with an increased PR interval and a low cardiac index. It is possible that the small number of patients did not demonstrate the predictive value of left ventricular function in asymptomatic patients (a hypothesis suggested by the causes of secondary mortality which were the same in both groups of patients).(ABSTRACT TRUNCATED AT 250 WORDS)

[Electrophysiologic basis of the anti-arrhythmic action of amiodarone in man]. / Bases électrophysiologiques de l'action antiarythmique de l'amiodarone chez l'homme

The aim of this study was to analyse the effect of an intra-atrial injection of 5 mg/kg of amiodarone on the heart rate, the A-V conducting pathways, and the right atrium and ventricle of 23 patients. By recording the potentials in the bundle of His, it was possible to measure the nodal A-V conduction time (A-H interval) and the conduction time below the bundle of His (H-V interval) during atrial pacemaking. The refractory periods were calculated by the stimulus-test method. The results were subjected to statistical analysis using Student's test. They may be summarised as follows: - slowing of the sinus rhythm; - depressed conduction in the A-V node, but without any effect on the speed of conduction infranodally; - an increase in the atrial effective refractory period; - persistant lengthening of both the effective and the functional refractory periods of the A-V node; - an increase in the relative refractory period in the His-Purkinje system, and in the effective refractory period of the ventricle. These findings throw light on the mechanism of the anti-arrhythmic action of amiodarone in man.

[Electrophysiologic effects of amiodarone in Wolf-Parkinson-White syndrome]. / Effets électrophysiologiques de l'amiodarone dans le syndrome de Wolff-Parkinson-White

Eight patients with WPW syndrome were catheterised and, during the course of this investigation, the electrophysiological effects of amiodarone were assessed. By registering the potentials of the bundle of His and by using the stimulus-test technique, we were able to measure the refractory periods of the atrium and of the normal and accessory conducting pathways both before and during the first 40 minutes after an intra-atrial injection of 5 mg/kg of amiodarone chlorhydrate. The action of the conduction time was also studied. In the five cases in which we were able to measure it, the effective refractory period of the abnormal pathway increased, which led in two instances to the temporary suppression of all pre-excitation. At the same time, it was repeatedly found that the refractory periods of the A-V node were increased: the effective refractory period in 3/3 cases, and the functional refractory period in 2/2 cases. The effective refractory period of the right atrium was increased in 5 cases, and did not change in the others. The intranodal conduction time (A-H- interval) was always increased after amiodarone. Finally, in three patients runs of reciprocal tachycardia could be initiated by premature atrial stimulation. In one case, this was no longer possible after amiodarone. In the other two cases, although the attacks could still be brought on, they were slower because of the lengthening of the A-H interval. These findings explain why amiodarone is effective in controlling the tachycardia of WPW syndrome.

[Study of the electrophysiological properties of bretylium tosylate in man]. / Etude des propriétés électrophysiologiques du tosylate de brétylium chez l'homme

The electrophysiological changes induced by bretylium tosylate have been studied in 22 patients during the 45 minutes following the intra-atrial injection of the drug at the dose of 5 mg/kg. The potential in the bundle of His was measured, and the stimulus-test method used to determine the refractory period. The results were subjected to Student's test. The sinus rate increased after bretylium. The A-V nodal conduction time (A-H interval) and infra hisian conduction time (HV interval), evaluated at the same imposed frequency, were not affected. The functional refractory period of the A-Vnode was decreased. The relative refractory period of the His-Purkinje system was increased. Finally, the right auricle showed an increase in its effective refractory period, while that of the ventricle remained unchanged. This study suggests that the electrophysiological effects of bretylium are due both to direct action at the cellular level and to sympathetic activity.

[Giant cell myocarditis. Attempt at interpretation, apropos of 3 cases]. / La myocardite à cellules géantes. Essai d'interprétation, à propos de 3 observations

Three cases of giant-cell myocarditis were observed, and 63 comparable cases were published in the literature. On the basis of this material, the significance of this disease was studied, which was considered for long to be of "granulomatous" origin, while it seems to be a peculiar histopathologic type of myocardialgeneration. The cause for this necrosis remains unknown; in spite of as complete as possible investigations in one of the three cases (in which death was preceded for labile recurring lung infiltrates during eleven months) no cause could be demonstrated.

[Electrophysiological study of 2 cases of ventricular pre-excitation compatible with the presence of Mahaim's fibers]. / Etude électrophysiologique de deux cas de préexcitation ventriculaire compatibles avec la présence de fibres de Mahaim

Report of two cases of ventricular pre-excitation in which the presence of Mahaim's fibres was suggested by the electro-physiological studies. In both cases, the short-circuit of the His bundle was responsible for the abnormally short HV interval. In a patient, atrial acceleration induced an increase of rh at first, related to an A-V delay without HV variation. For a cycle of 360 ms, HV lengthened suddenly from 15 to 40 ms while QRS changed configuration, which is in favour of a functional exclusion of an accessory pathway. Combination with the pre-excitation of a left bundle-branch block complicated, in this case, the problems of identification. The other case responded also to cardiac pacing by an increase of the A-V delay due mainly to intra-nodal conduction slowing. In the same time HV increases from 20 to 30 ms (for a cycle change from 480 to 315 ms) and the delta wave decreased progressively. For a higher rate there was a 2:1 A-V block proximal to H. The extrastimulus method produced initially the same results. On the contrary, when the coupling-interval decreased to 290 ms, HV increased suddenly from 25 to 45 ms, with disappearance of the pre-excitation pattern. At a still shorter interval, HV shortened again (25 ms) in parallel with a marked lengthening of the A-V nodal conduction times, thus revealing a "gap" phenomenon of the accessory pathway.

[Ventricular tachycardia due to premature supra-ventricular beats with a normal QRS complex. Analysis of a case]. / Tachycardie ventriculaire induite par des battements supra-ventriculaires prématurés à complexe QRS normal Analyse d'un cas

Report of attacks of ventricular tachycardia in a 39 year-old man, free from heart disease. Cardiac pacing showed that the tachycardia could be induced by premature supraventricular beats normally propagated to the ventricles, and by atrial acceleration. The same part played by the cardiac rate on triggering of attacks was demonstrated during ventricular pacing. Besides, the possibility to interrupt the tachycardia by one electrically induced ventricular beat, supports the hypothesis of a re-entry machanism. The QRS configuration during the arrythmia suggested a propagation of excitation starting from the posterior-inferior area of the left ventricle. However, the absence of anomalies of the supraventricular beats initiating the tachycardia excluded a circus movement resulting from a unidirectional block on the left anterior hemibranch. More probably there existed an area of micro-reentry, stimulating the left ventricle from the posterior branches of the His-Purkinje system.