Max-Planck-Institut für Kernphysik Heidelberg

Structure and dynamics of few electron ions in an EBIT

Priv.-Doz. Dr. José Ramon Crespo López-Urrutia

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The Heidelberg Electron Beam Ion Trap

HCI | EBIT | Cooling the ions | Laboratory astrophysics | Extracting the ions

Highly charged ions

At very high temperatures, atoms loose many electrons. Their nuclear positive charge is not neutralized anymore by the electronic shell. They become highly charged ions (HCI). This form of matter is very rare on earth, but highly charged ions are very common in the universe, because stars and other massive bodies are in most cases at extreme temperatures. Those elements of the periodic table heavier than hydrogen are born under nuclear fusion conditions at extreme temperatures. Supernova explosions are believed to be the main mechanism for the production of elements heavier as iron Fe, Z=26). As new nuclei are formed, they are "bare" or "naked", and may remain so for extended periods of time. Their radiation emission is strongly affected by the ionic charge state. As their environment cools down, ions gradually recombine by capturing free electrons, eventually becoming neutral atoms. But other energetic processes, as those occurring in active galactic nuclei, stellar cores and coronae, accretions disks and shocks can cause again ionization up to very high stages. Since heavy atoms tend to emit more radiation than light atoms, and highly charged ions even more so, the radiation from these "exotic" ions is very strong. As an example, the most intense line in the solar corona is produced by 13-fold ionized iron, Fe13+. The radiation produced by HCIs at different energy ranges of the electromagnetic spectrum can be used to understand ("diagnose") such astrophysical processes.

In laboratories, producing such ions is not an easy task. Large nuclear fusion plasma experiments, like tokamaks, produce them as material from the vessel's wall evaporates and diffuses into the million-degree hot core of the plasma discharges. Very powerful lasers can also ionize matter to such charge states during brief periods of time, typically few nanoseconds. But analyzing the results obtained under such transient conditions is very difficult, because the composition, temperature and geometry of the plasma change too quickly. To obtain a well defined sample of highly charged ions, large accelerators can be used. They can bring low-charge ions to velocities which are a substantial fraction of the speed of light. This relativistic ion beam is then shot through very thin foils (beam-foil method), which let the small nucleus go through but "catch" the outer electrons because their interact more strongly with the foil's electrons. A very fast moving highly charged ion remains, which is than brought to a so-called storage ring, a large facility where the ions are kept moving in closed circular path several tens of meters diameter by using magnetic fields. In such an storage ring, experiments with the stored ions can be carried out (reference to GSI and MPI-K). Currently, only one accelerator facility in the world (GSI) can produce and store "naked" uranium, also U92+, which is the "holy grail" in the field.

The electron beam ion trap (EBIT)

The electron beam ion trap is the main alternative approach, if you want to study very heavy elements with only a few electrons left. It is a much smaller device which takes only a few square meter floor space. Essentially, in an EBIT ions are confined by a combination of electric and magnetic fields in the center of the so-called trap. They are kept "floating" in an extremely good vacuum, and cannot escape a small region of say 0.1 mm diameter by 40 mm length. These ions are the target of a powerful, highly focused electron beam. A superconducting magnet creates a very high magnetic field compressing the beam electrons together. The current density of the beam can reach several thousands of amps per square centimeter, and would instantaneously evaporate any metallic wire. Vacuum, however, does not get hot as the beam passes through it. But the ions, constantly bombarded by the beam, are stripped more and more of their outer electrons. As long as the kinetic energy of the beam electrons is higher than the binding energy of a given bound electron, collisions will eventually free it. In this way, all ions trapped are brought to a charge state defined by the parameter beam energy. Keeping the ions trapped during the whole process, which can take anything from milliseconds to a few seconds (if you want "naked" uranium") requires a strong holding force. The electron beam kicks the ions in all possible directions, making them "hot". They would like to leave the trap. To avoid that, the trap is made as deep as possible. The trick is to use the negative charge of the electrons in the beam. Putting so many electrons together, even as they keep moving, produces a so called "space charge". This negative charge attracts the positive ions into the beam, at least in the radial direction. Now, the ions can leave along the beam axis. But this motion can be constrained by using three electrodes, the drift tubes, where two of them are positive (repelling the ions) on both sides of the trap, and the central drift tube, negative, that attracts the ions. The electron beam fulfills two tasks in an EBIT: ionizing and trapping. That is ultimately the reason for its success in achieving the highest charge states. SuperEBIT, at the Lawrence Livermore National Laboratory, showed 1992 that "naked" uranium U92+ ions could be produced and stored in a small laboratory. The Heidelberg EBIT, developed at the Freiburg University from 1998 to 2001, is designed to reach similar charge states. The current maximum beam energy of 100 keV does not yet allow to produce those ions, but lets us work comfortably with ions such as helium-like mercury (Hg78+) or hydrogenic barium (Ba55+), and to strip all electrons from elements like krypton (Kr36+). We are currently working in conditioning the device to run stably at the necessary operation voltages.


Principle of operation of an electron beam ion trap


Section through the Heidelberg EBIT

Cooling the ions in the trap

Due to the continuous collisions with the beam electrons, the trapped ions are heated up. Eventually, their random thermal energy would suffice to bring them over the potential well and allow them to leave the trap. This process can happen very quickly, even before the ions are sufficiently stripped. So a mechanism to cool the ions has to be found. This is achieved by mixing light atoms(pink) with the heavier elements(blue) one wants to ionize. The lighter ions share the thermal energy with the heavier ones, but are not so tightly bound to the trapping potential because of their smaller ionic charge, which factors in with the potential difference. Neon can get only tenfold ionized, since it only has ten electrons to loose, whereas under the same conditions iron, for instance, can be 26-fold ionized, and xenon (Z=54) may be 44-fold ionized. So the light ions "evaporate" from the trap, leaving behind the heavier ions, and cooling them in the process. Neon is very appropriate to cool anything from Fe (Z=26) to U (Z=92). A single tenfold ionized Ne removes an energy equivalent to 10 times the trapping potential from the ion ensemble. This is roughly equivalent to give an additional second of trapping time to one of the heavy ions. By steadily injecting small amounts of Ne as an atomic beam, the heavy ions can be kept trapped essentially forever. This is very important, because in some cases more than 20 seconds are necessary to reach the desired charge state. An additional benefit of the cooling is that it reduces the "Doppler broadening" of the radiation coming from the trap. When the ions move randomly at high speed as they bounce in the trap between the walls, they spoil the resolution of the measurements. More precise determination of the wavelengths are possible if the ions are cooled. So, in an EBIT you produce ions which are typical for environments at many million degrees temperature, but you can kept them relatively cold to observe them.


Evaporative cooling of trapped ions

Laboratory astrophysics

Once the ions are produced and trapped, the next task is to study them. For this purpose, we use instruments which are similar in their purpose and sensitivity to those an astronomer will point to a far galaxy. The first information atoms and ions deliver is radiation. The radiation spectrum emitted by an ion allows to understand its electronic structure, the excitation processes, as well as the temperature and density of a given element or ion. HCIs are very strong x-ray emitters, but produce visible and ultraviolet radiation as well. So, different types of spectrometers are aimed at the trapped ions, depending of the wavelength of the radiation under study. Since the number of trapped ions is quite small (1000000 as a rule-of-thumb), and despite of the optimal sensitivity of the instrumentation, observations may need several hours or days to collect the desired signal. But since we know beforehand which element's ions are in the trap, their ionization stage, their temperature and so on, the data obtained can be assigned quickly. This just the opposite case to the "real" astrophysics, where you see some signal through your telescope and try to identify its origin. Unfortunately, the theoretical models used sometimes in astrophysics to figure out what goes on with HCIs are in many cases just rough approximations because there is very little data to compare them with. So it is very convenient to know what you have in your trap and measure very precisely whatever radiation comes out of it. So, with an EBIT you can reproduce processes happening in stars and supernovae in the scale of a small laboratory.

Extracting the ions

In the trap we can study the internal electronic structure of the ion, and its interactions with photons and electrons, the processes leading to the emission and absorption of radiation. However, for certain experiments, having the ions trapped is not useful. So we let the ions leave the trap an lead them through vacuum beam lines to other experimental apparatuses. There, for instance, the HCIs interact with other ions or atoms in collisions. During a collision processes, both the projectile and the target are strongly distorted for a very short time, and they may change their structure permanently as well. They can break apart, or exchange electrons, their trajectories are modified. They can exchange energy in various amounts. But collisions occur in extremely short periods of time, because ions and atoms are small (0.0000000001 m) and almost always fast (at room temperature 1000 m/s, in the Sun's corona 100000 m/s). So those two particles will pass by in one femtosecond (10-15), and those are "slow" collisions. Fast collisions need only an attosecond (10-18), thousand times less, to happen. The shortest laser pulses nowadays still last a few femtoseconds.

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