E I Pinzhenin

P A Bagryansky

V V Maximov

V V Prikhodko

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DD PRODUCT YIELD IN THE GDT CENTRAL CELL  
 
 
 
 
P. A. Bagryansky, V. V. Maximov, E. I. Pinzhenin, V. V. Prikhodko  
 
Budker Institute of Nuclear Physics, 630090, Novosibirsk, Russia  
Novosibirsk State University, 630090, Novosibirsk, Russia  
 
 
 
 
The report presents the recent results of experiments 
with deuterium neutral beam injection in the Gas 
Dynamic Trap (GDT) device. Neutron scintillation 
detector and single particle proton detector near fast ions 
mirror point allow  to control DD activity in each shot. A 
system for spatial measurements of fusion protons is 
prepared. DD reaction yield measurements are needed to 
compare with the predictions of the theory based on two-
body Coulomb collisions.  
 
 
I.  INTRODUCTION 
 
Experiments with plasma heated by deuterium beams 
are carried out in the Gas Dynamic Trap (GDT) to 
simulate the fusion reaction intensity in the project 14 
MeV neutron source based on GDT [1]. This project is 
developed for fusion materials testing. GDT is a long 
axially symmetric plasma confinement device with the 
high mirror ratio. The GDT plasma consists of two 
components. The first component is collisional target 
hydrogen or deuterium plasma with the electron 
temperature about 200 eV and density up to 5·1019 m-3 . 
This thermal plasma is confined in a collisional regime 
that is characterized by the condition that a mirror-to-
mirror distance considerably exceeds the effective ion 
mean free path of scattering into the loss cone. The 
second component is fast ions with mean energy of 
~10 keV and density up to 5·1019 m- 3. This component is 
produced by oblique neutral beam injection at the 
midplane of the device. These strongly anisotropic fast 
ions are confined in collisionless regime.  
According to the estimates which involve classical 
mechanisms only, the fast ion relaxation in the target 
plasma of GDT neutron source is to be dominated by 
electron drag and therefore, their pitch-angle distribution 
remains quite narrow. As a result, the fast ion density has 
strong maximum near the turning points where 
longitudinal velocities of fast ions are close to zero. In 
case of deuterium neutral beam injection this effect 
increases neutron yield within those regions, which house 
the test zones in the prospective GDT-based neutron 
source (GDT-NS) (in which the injection of deuterium 
and tritium neutral beams is suggested). This picked axial 
profile of the fusion reactions intensity is the 
distinguishing features of the projecting neutron source. 
There are two branches of the DD reaction:  
 
Usually 2.45 MeV neutron flux are measured, but 
3.02 MeV proton flux measurement has some advantages. 
The difference in the DD product fluxes is less than 2 %. 
The measurements can supplement each other. DD 
product yield measurements are used for comparison 
between the experimental results and the theoretical 
predictions. It is important to prove that fast ion relaxation 
and scattering are basically determined by two−body 
Coulomb collisions. Here we present the results of recent 
experiments with increased magnetic field and improved 
injection system. 
  
II. EXPERIMENTAL SETUP 
 
The GDT device (fig. 1) comprises the central cell 
and two end cells. The mirror-to-mirror distance is 7 m, 
magnetic field at midplane was increased to 0.33 T (for 
most experiments), and a typical mirror ratio is about 40. 
The deuterium target plasma was produced by the plasma 
gun located in one of the end cells. In this experiment 
eight new injectors with the focused beams are used. The 
duration of the beam injection was increased up to 5 ms, 
energies of the injected particles were increased up to 
25 keV, total beams power was about 5 MW. Neutral 
beam injectors were used for plasma heating and fast ions 
build-up. A material balance is supplied by extra gas 
injection near the magnetic plug with using gas-puffing 
system. MHD stability provides by differential rotation of 
edge plasma which is caused potential (300 V) between 
the plasma dump and limiter. Titanium evaporators 
provide vacuum condition.   
To detect the product yield of the DD synthesis near 
the turning point of fast ions was used two detectors. The 
first detector is organic scintillator combined with a photo 
multiplier tube (PMT). Using the PMT of the “fine mesh” 
type (Hamamatsu) enabled to locate the photo multiplier 
tube in a magnetic field as strong as 1.5 T. This PMT can 
operate when located at an angle of up to 30º with the 
magnetic field lines. This detector was outside the 
vacuum chamber and used for registration time evolution 
of 2.45 MeV neutron flux. The second detector is a 
special counter based on PIN diode with sensitive area 
0.5 cm2 and the current preamplifier. It was installed 
inside the vacuum vessel on radius 23 cm. Detector is 
sensitive to protons and insensitive to neutrons and 
magnetic field. The magnetic field in GDT forces on 
trajectory of proton, but the gyroradius of a 3 MeV proton 
in 0.66 T magnetic field is 38 cm and this effect easy to 
take into account. The entrance window was protected by 
the 10 µm aluminum foil. Single 3 MeV protons were 
registered as 35 ns pulses. The average amplitude of the 
pulse corresponded to the 4 µA diode current. The single-
particle counting regime permits to measure absolute 
3 MeV proton flux. The ADC824a used allows to record 
131 K points with the step 5 ns. One ADC allows to 
measure within 0.65 ms. Data of both detectors control 
DD product linear yield (number of DD reactions 
produced by 1 cm length of the plasma column) in 
maximum point during all shot. To calculate all DD 
product yield we used typical axial profile now [2].  
A new 3 MeV protons detector was prepared for more 
detailed longitudinal distribution measurements. The 
detector has PIN diode with sensitive area 3.1 cm2, 
current preamplifier and collimator to provide sufficient 
spatial resolution. It was installed inside the vacuum 
chamber. Design of detector allows shift it in four 
different positions in each port. The detector can be 
installed in one of the four upper GDT ports which 
required venting the device to air. So it is possible to get 
longitudinal distribution of proton flux in series of shots. 
The perpendicular scanning of the 3 MeV proton flux 
in the fast ion mirror point was produced by the detector 
based on scintillator and PMT [2]. 
 
 
Fig. 2. System of 2.45 MeV neutron and 3.02 MeV proton 
detectors.  
 
 
Fig. 3. Layout of 3 MeV proton detector for axial profile 
measurement.  
 
Other GDT diagnostics were also used in the DD 
experiments to provide the data for comparison to the 
numerical simulations discussed below. The trapped 
 
Fig. 1.The GDT layout. 
neutral beam power was derived from the neutral beam 
attenuation detector data. The energy contents in the fast 
ions and target plasma were measured by an array of 
diamagnetic loops. Thomson scattering system based on a 
Nd laser was used to measure the radial profiles of 
electron temperature and density. Dispersion 
interferometer was used of linear density measurements. 
 
III. EXPERIMENTAL RESULTS 
 
 
Fig. 4. Time evolution: 
a) Injected (Pinj) and trapped (Ptr)  NBI power 
b) Fast ion energy content 
c) Linear DD yield near fast ion the mirror point.  
 
The trapped NBI power, fast ion energy content and 
DD reaction yield were controlled in each shot now. It 
was significant for optimization of the operating regime at 
GDT. Only fast ion energy content was not enough 
because of  effects of the radial fast ion centre distribution 
and energy distribution. Typical oscillograms are 
presented in fig. 4. The maximal DD reaction yield 
presented on the fig.4(c) was measured by detector by 
3 MeV proton detector installed at the position near fast 
ion mirror point (Z=173 cm). 
The growth of neutral beam injection power, 
increasing the magnetic field and the optimization of the 
operating regime at GDT has led to the significant 
enhancement of DD reaction products flux. Near the 
mirror point the maximum linear DD reaction yield was 
1.6·109 s-1 cm-1, maximum neutron flux was 3·1011 s-1. 
GDT generates 109 neutrons per shot in experiments with 
deuterium injection and deuterium target plasma. This 
calculation used the previous axial distribution DD 
reaction yield [2]. The detailed axial distribution for new 
regime will be measured later. 
The result of perpendicular scanning of the 3 MeV 
proton flux in the fast ion mirror point is presented in 
fig. 5.  
 
Fig. 5. Perpendicular profile of DD reaction yield in the 
fast ion mirror point.  
 
IV. CONCLUSIONS 
 
Analysis of DD reaction yield and diamagnetism of 
fast ions in the experiments with deuterium and hydrogen 
target plasma shows that fast ions collisions are defined 
with DD reactions. Fusion reactions between fast 
component and target plasma give low contribution into 
total number of reactions in comparison with previous 
experiments.  
Presented results and detailed axial distribution will 
be compared with results of numerical simulation. 
 
ACKNOWLEDGMENTS 
 
The work was supported by P1580 и P969 state 
contract with Federal Education Agency (FCP ”Kadry”). 
 
REFERENCES 
 
1. A. A. IVANOV, I. A. KOTELNIKOV, 
E. P. KRUGLYKOV et al. In Proc. Of  XVII 
Symp. on Fusion Technology, Rome, Italy, v.2 
(1992) 1394.  
2. V. V. MAXIMOV, A. V. ANIKEEV et al. Nucl. 
Fusion 44 (2004), pp. 542-547.  
 


DD PRODUCT YIELD IN THE GDT CENTRAL CELL

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DD PRODUCT YIELD IN THE GDT CENTRAL CELL

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