A method for operating a tokamak, and a tokamak

Abstract

This describes a method for operating a tokamak. The tokamak comprises a toroidal plasma chamber, poloidal and toroidal magnetic field coils configured to confine the plasma within the plasma chamber, a solenoid located at the center of the torus and containing a high-temperature superconducting material, and a current-driven system other than the solenoid, configured to ramp up the plasma current in the plasma confined within the plasma chamber. The plasma is started in the toroidal plasma chamber. The plasma current is ramped up in the plasma using the current-driven system until the plasma current reaches an operating point. At this point, the current in the solenoid is less than the solenoid's maximum operating current. After the plasma current reaches the operating point, the solenoid is used to control the plasma current, the plasma position, and / or the plasma shape. The plasma has an aspect ratio of less than 2.5.

Description

[Technical Field]

[0001] This invention relates to plasma control in a tokamak plasma chamber. [Background technology]

[0002] A tokamak is a device that uses a combination of toroidal and poloidal magnetic field coils to confine a plasma within a toroidal chamber using a high magnetic field. The magnetic field interacts with the electric current in the plasma ("plasma current"), confining the plasma within the toroidal chamber and preventing contact with the walls. Thus, tokamaks are particularly well-suited for confining high-energy plasmas, such as those required for nuclear fusion.

[0003] One specific tokamak design is the "spherical tokamak." Unlike conventional tokamaks, which have a ring-donut shape, this design is more like a hollowed-out apple, with a roughly spherical profile and a central column that is smaller than the radius of the plasma chamber. The ratio of the major radius of the plasma (i.e., the radius from the center of the central column to the geometric center of the plasma) to the minor radius (i.e., the radius of the plasma cross-section, the distance from the geometric center of the plasma to the outside of the plasma envelope on the equatorial plane of the tokamak) is called the "aspect ratio." In the case of a spherical tokamak, it is important to minimize this aspect ratio as much as possible. This is because the upper limit of the plasma energy that can be confined in a spherical tokamak strongly depends negatively on the aspect ratio. In other words, the lower the aspect ratio, the higher the plasma energy that can be confined with a given magnetic field, and conversely, the lower the magnetic field required to confine a given plasma energy. The minimum possible aspect ratio depends on engineering constraints, but the theoretical minimum of 1 is practically impossible due to the need to position magnetic coils to confine the plasma. Experimental spherical tokamaks for plasma confinement may have aspect ratios as low as 1.2 to 1.5, but larger spherical tokamaks suitable for sustained fusion reactions may not be able to achieve aspect ratios smaller than 1.9 to 2.0.

[0004] Since magnetic confinement in a tokamak is partially dependent on the plasma current, this plasma current needs to be initiated when the plasma is formed and maintained during tokamak operation. There are four main ways to drive the plasma current. Externally injected power ("external current drive") by using electromagnetic waves that resonate with plasma particles (RF current drive) and / or by the injection of neutral atoms (neutral particle beam current drive). • At high plasma pressures, the plasma tends to spontaneously drive a plasma current ("bootstrap current"). As plasma energy increases, the plasma attempts to expand. This expansion is resisted by compressing the plasma with the poloidal magnetic field of the tokamak, thereby driving the plasma current (perpendicular magnetic field drive). • Solenoids positioned in the central column of the tokamak are used to generate a time-varying magnetic flux, thereby driving the plasma current (solenoid drive).

[0005] An RF current-driven system typically consists of a high-power RF source, a transmission line for transporting RF power to the plasma, an antenna for radiating RF power into the plasma, and a matching circuit to match the impedance of the antenna (in the presence of plasma) to the impedance of the transmission line. RF electromagnetic waves are introduced into the plasma to propagate in the direction of the desired plasma current, and the electric field of the waves accelerates charged particles in the plasma. The specific design of the RF current-driven system can vary depending on the requirements of the tokamak, such as plasma density, temperature, and magnetic field configuration.

[0006] In neutral particle beam current drive, high-energy neutral atoms are injected into the plasma. Through collisions with electrons in the plasma, the neutral particle beam is ionized, generating an ionic current. Electrons attempt to cancel out the ionic current, but due to incomplete cancellation, a net plasma current remains.

[0007] Bootstrap current occurs when the plasma pressure is higher in the core than at the edges of the plasma envelope. This pressure difference spontaneously drives the toroidal current in the plasma. Tokamaks with a low aspect ratio (e.g., less than 2.5) during the main plasma pulse exhibit significantly higher bootstrap current (as a percentage of the total plasma current) than conventional tokamaks.

[0008] Vertical magnetic field driving occurs when the vertical magnetic field applied to the plasma is increased in response to an increase in plasma energy, in order to maintain plasma equilibrium. An increase in the vertical magnetic field results in an increase in the plasma current. This is typically done using outboard poloidal magnetic field coils, i.e., coils located outside the main radius of the plasma.

[0009] Solenoid driving relies on a change in the current within the solenoid in a predetermined direction. A suitable change in the solenoid flux linked to the plasma increases the plasma current. For example, to increase a positive plasma current, the solenoid must be reduced to a lower solenoid current (i.e., the change in flux is negative). While this is an efficient way to drive plasma current, it is considered suitable only for short plasma pulses because the length of the drivable pulse is limited by the maximum flux the solenoid can generate.

[0010] Introductory texts on these drive systems can be found in many standard references, such as the online version of the Encyclopedia Britannica (www.britannica.com / technology / fusion-reactor / Principles-of-magnetic-confinement). [Overview of the project]

[0011] According to a first embodiment, a method for operating a tokamak is provided. The tokamak comprises a toroidal plasma chamber, a poloidal magnetic field coil and a toroidal magnetic field coil configured to confine plasma within the plasma chamber, a solenoid located at the center of a torus and containing a high-temperature superconducting material, and a current drive system other than the solenoid, configured to ramp up the plasma current in the plasma confined within the plasma chamber. Plasma is started in the toroidal plasma chamber. The plasma current is ramped up in the plasma using the current drive system until the plasma current reaches an operating point. Here, when the plasma current reaches the operating point, the current in the solenoid is less than the maximum operating current of the solenoid. After the plasma current reaches the operating point, the solenoid is used to control the plasma current, the position of the plasma, and / or the shape of the plasma. The plasma has an aspect ratio of less than 2.5.

[0012] According to a second embodiment, a tokamak is provided. This tokamak comprises a toroidal plasma chamber, a poloidal field coil and a toroidal field coil, a solenoid, a current drive system other than the solenoid, and a controller. The poloidal field coil and the toroidal field coil are configured to confine the plasma within the plasma chamber. The solenoid is located at the center of the torus and contains a high-temperature superconducting material. The current drive system is configured to ramp up the plasma current in the plasma confined within the plasma chamber. The controller, Initiating plasma in a toroidal plasma chamber, A current-driven system is used to ramp up the plasma current until it reaches the operating point, wherein when the plasma current reaches the operating point, the current in the solenoid is less than the maximum operating current of the solenoid. After the plasma current reaches the operating point, use the solenoid. Controlling the plasma current, Controlling the position of the plasma, and / or Controlling the shape of the plasma, Maintaining the plasma at an aspect ratio of less than 2.5 and It was configured to perform the following actions.

[0013] Further embodiments are presented in claim 2 and subsequent claims. [Brief explanation of the drawing]

[0014] [Figure 1] This is an illustrative cross-sectional view of a tokamak. [Figure 2] Figure 1 is a flowchart showing how to operate the tokamak. [Figure 3] This graph shows the various characteristics of the tokamak in Figure 1 before, during, and after the plasma pulse. [Modes for carrying out the invention]

[0015] Plasma physics applications require the study of high-energy plasmas to investigate plasma properties and phenomena at temperatures associated with fusion operations. Current instruments have primarily focused on short-pulse operation (where the plasma is typically sustained for less than 10 seconds) or long-pulse operation at low plasma densities, plasma currents, and / or plasma temperatures. Increasingly, there is a growing need to study high-energy plasmas in long-pulse operation lasting at least several minutes, ideally several hours to several weeks, which is not achievable with current tokamak.

[0016] One of the main challenges in long - pulse operation is plasma control. In this aspect, it is necessary to maintain the plasma current near the desired operating point, which requires two types of control. First, it is necessary to continuously replenish the plasma current to cancel out the resistive losses in the plasma (this is partly driven by the plasma's own "bootstrap current", but generally some external input is required). Second, finer control is needed to maintain the plasma current at the operating point and prevent feedback loops that can cause plasma disruptions.

[0017] Put simply, when the plasma current decreases from the operating point, the plasma confinement time decreases. This causes a decrease in the plasma temperature, and hence a decrease in the plasma's bootstrap current, resulting in further reduction of the plasma current in a continuous negative feedback loop. Returning the plasma current to the desired operating point in such a situation is done by a solenoid. This is because the solenoid allows the plasma current to quickly return to the operating point (whereas other methods of ramping the plasma current, such as RF current drive, are generally too slow to cancel out the feedback loop).

[0018] The ability of a tokamak to control the plasma state using a solenoid depends on the ability to appropriately vary the current in the solenoid. If the magnitude of the current exceeds the solenoid's tolerance due to the required control input, that control input is impossible and this usually results in an undesirable decrease in the plasma current and / or movement of the plasma (unless alternative control methods are available). This is particularly a problem when the solenoid is made of a resistive (i.e., non - superconducting) material. In that case, the current in the solenoid cannot deviate from zero for long periods due to resistive heating. Thus, if multiple control inputs are required to push the current in the same direction (i.e., more positive or more negative), this results in the solenoid operating continuously at significant currents, which can in turn lead to excessive heat generation.

[0019] Solenoids can also be used to control the position and / or shape of the plasma. For example, solenoids can be used to control the inboard gap (i.e., the gap between the plasma and the inner boundary of the plasma chamber), the strong magnetic field side leg, triangularity, and / or the elongation of the plasma. Changing the current in the solenoid will change the magnetic field experienced by the plasma (e.g., the magnetic field experienced across the cross-section of the plasma), changing the plasma shape and / or plasma position. For example, decreasing the solenoid current (without adjusting other plasma control inputs) has the effect of changing the magnetic field experienced by the plasma, driving an increase in the plasma current and an increase in the inboard gap (depending on the geometric configuration and direction of the plasma current with respect to the solenoid current).

[0020] The ability to locally change and control the magnetic field is improved when the solenoid is divided into a plurality of independently controllable parts (segments) in the vertical direction (i.e., along the central axis of the solenoid). Each segment has its own current source. This improves the control of the plasma position and / or shape.

[0021] Using a poloidal magnetic field coil in combination with a solenoid can improve the simultaneous control of the plasma current, plasma shape, and plasma position. In embodiments where it is not desirable to change the plasma shape and / or position when using a solenoid to control the plasma current, this can be offset by using a poloidal magnetic field coil in a manner well known in the art. Further, when the solenoid is used to control the plasma shape and / or position, other current drive methods as described above may be used to maintain control of the plasma current.

[0022] One way to mitigate this is to use a superconducting solenoid. A superconducting solenoid is activated with a large positive current, ramped up to a moderate negative current when starting the plasma and ramping up the plasma current, and then held at that current, allowing for the ability to further manipulate the plasma using the leeway between the moderate negative current and the maximum current the solenoid can conduct. However, doing this is impractical for tokamaks with an aspect ratio of less than 2.5. This is because the solenoid would have to be large enough to allow some headroom in the current while ensuring that there is a sufficient change in the magnetic field to drive the plasma current during its initial ramp-up. This large solenoid requires considerable space in the central column, and its relatively low aspect ratio means that it is nearly impossible to achieve this without significant compromises in other components within the central column.

[0023] Instead, this disclosure proposes a hybrid approach in which a non-solenoid system is used instead of or in addition to a solenoid during the initial ramp-up of the plasma current. This solenoid is used for plasma control during the stable portion of the plasma pulse and is made from a superconducting material.

[0024] As schematically shown in the cross-sectional view of Figure 1, a tokamak is used comprising a toroidal plasma chamber 101, a poloidal magnetic field (PF) coil 102 and a toroidal magnetic field (TF) coil 103 for confining the plasma 104 within the chamber 101, a central solenoid 105, and a current drive system 106 other than the solenoid. The tokamak may further include a controller (not shown) configured to perform the methods described herein.

[0025] During tokamak operation (i.e., between pulses and after the plasma has reached the desired operating current), the plasma aspect ratio is less than 2.5, more preferably less than 2.3, and more preferably less than 2. Due to engineering limitations, the aspect ratio may be greater than 1.2, greater than 1.5, or greater than 1.9.

[0026] The current drive system may be, for example, the RF current drive system, the vertical magnetic field current drive system (including part of the PF coil), or the neutral particle injection current drive system, or any other suitable non-solenoid system for driving plasma current.

[0027] The central solenoid contains a high-temperature superconductor (HTS). This allows for a substantial current to flow for the duration of the plasma pulse without overheating due to resistance. High-temperature superconductors are particularly suitable here because they require less cooling in the central column to maintain the superconducting state, and their high current density allows for greater control of the plasma. Although the central solenoid is shown wound inside the TF coil, it may also be wound around the central column portion of the TF coil, i.e., between the TF coil and the plasma chamber.

[0028] A solenoid has a maximum operating current, i.e., the maximum current (positive or negative) it is designed to conduct. This may be set with an appropriate safety margin, for example, based on the resistive losses in non-superconducting components, by the capacity of the power supply, or by the critical current in superconducting components. The objective of the hybrid approach is to ensure that after the plasma current ramps up to the operating plasma current, there is a margin available between the solenoid current and the solenoid's maximum (positive or negative) operating current, which can be used to give control over the plasma current and / or plasma shape between pulses.

[0029] Figure 2 shows a flowchart of how to operate the tokamak in Figure 1, with the steps in the dotted boxes being optional.

[0030] In step S200, the solenoid is optionally initially charged to an initial current, as will be described in more detail later.

[0031] In step S201, the plasma is initiated by any suitable means known in the industry.

[0032] In step S202, after the plasma is started in the tokamak, the plasma current is ramped up to the operating plasma current using a (non-solenoid) current drive system. This may be done using the current drive system alone or in combination with current drive from a solenoid (S202a), but in either case, once the ramp-up is complete, the solenoid is left with a current smaller than (i.e., lower in magnitude than) the maximum operating current.

[0033] In step S203, the plasma current is then maintained as the operating plasma current for the pulse by a solenoid. This may include using a current drive system for coarse control (S203a, e.g., injecting a constant amount of power via current drive to counteract resistive losses) or using a solenoid for fine control (e.g., operating a solenoid in a feedback loop where the plasma current is monitored and, in response to detection of deviation from the operating point, the solenoid current is changed to counteract that deviation). In this scheme, the maximum operating current of the solenoid is unlikely to be a limiting factor on the length of the plasma pulse, because the deviation from the operating current after the effect of coarse control is likely to be, on average, roughly uniform in direction. Alternatively, the solenoid may be used for both coarse control (S203b) and fine control, but this would result in the solenoid reaching the maximum operating current sooner, because coarse control causes the solenoid current to increase more in the direction necessary to counteract resistive losses.

[0034] Before igniting the plasma, in step S200, the solenoid may be ramped up to an initial current. This current may be in the opposite direction to the current change required to raise the plasma current. That is, if the solenoid is used to control the plasma current during the ramp-up or pulse of the plasma current, the solenoid current first decreases to zero and then increases in the opposite direction. This allows for a larger change in the current within the solenoid during the ramp-up and pulse of the plasma current than if the current were zero at the start of the plasma.

[0035] The upper part of Figure 3 is a graph showing the contributions from the bootstrap current and external current drive, in addition to the plasma current, for an exemplary plasma pulse. The lower part of Figure 3 is a graph showing the currents in the solenoid and vertical magnetic field coil on the same time scale. In this example, the “current drive system” 106 in Figure 1 includes both external current drive (e.g., RF current drive) and vertical magnetic field drive.

[0036] In detail, during the initial period 301, the solenoid current ICS ranges from zero initial current to its maximum allowable positive current +I CS MAX It is charged to (S200) (in this example, the change from positive current to negative current ramps up the plasma current). At time 302, the plasma is started (S201), and thereafter, for the duration of period 303, the plasma current I p This is driven by an external (injection) current I EC , vertical magnetic field I VF This involves ramping the current to a negative current (S202) and operating the central solenoid with a moderate negative current (for example, the maximum negative operating current of the central solenoid - I CS MAX It ramps up when combined with ramping to (S202a) a negative 80%.

[0037] Figure 3 shows an illustration of the current in a tokamak with an aspect ratio of less than 2.5, so the bootstrap current IBS is the plasma current Ip is close to. In this example, during pulse 304, the external current drive I EC is maintained to cancel the resistive losses (S203a), and the vertical magnetic field is kept constant during the pulse. During the pulse, the central solenoid is used for fine tuning of the plasma (S203). When a negative deviation 311 of the plasma current from the operating point is detected, the solenoid current is driven negatively (312) to drive the plasma current positively to cancel the deviation 311. Instead, if the deviation is positive, the solenoid current is driven positively to drive the plasma current negatively.

[0038] Following the pulse, in period 305, the external current drive is gradually reduced, the vertical magnetic field is ramped back (returned) to zero, and the solenoid is ramped back to the initial current to safely extinguish the plasma without collapsing it by ramping down the plasma current. Finally, in period 306, following the extinction of the plasma, the solenoid is ramped down to zero current.

[0039] The overall effect is to allow the use of a solenoid for plasma control without requiring a large solenoid capable of handling a full startup of the plasma current while maintaining some energizing capacity as a reserve for use in plasma control.

Claims

1. A method for operating a tokamak, The aforementioned tokamak is Toroidal plasma chamber and A poloidal magnetic field coil and a toroidal magnetic field coil configured to confine the plasma within the plasma chamber, Located at the center of the torus, it contains a solenoid made of high-temperature superconducting material, A current drive system other than the solenoid, configured to ramp up the plasma current in the plasma confined within the plasma chamber. Equipped with, The aforementioned method, Initiating the plasma in the aforementioned toroidal plasma chamber, The current drive system is used to ramp up the plasma current in the plasma until the plasma current reaches an operating point, wherein when the plasma current reaches the operating point, the current in the solenoid is less than the maximum operating current of the solenoid. After the plasma current reaches the operating point, the solenoid is used to Controlling the plasma current, Controlling the position of the plasma, and / or Controlling the shape of the plasma and Includes, The method wherein the plasma has an aspect ratio of less than 2.

5.

2. The method according to claim 1, wherein the step of ramping up the plasma current until the plasma current reaches an operating point includes driving the plasma current using the current drive system and the solenoid.

3. The method according to claim 1, wherein the step of ramping up a plasma current until the plasma current reaches an operating point includes driving the plasma current using the current drive system, but does not include driving the plasma current using the solenoid.

4. The method according to any one of claims 1 to 3, wherein the plasma has an aspect ratio of 2.3 or less, more preferably 2 or less.

5. The method according to any one of claims 1 to 4, further comprising ramping the solenoid to an initial current before starting the plasma in the toroidal plasma chamber.

6. The method according to any one of claims 1 to 5, wherein the current drive system includes a radio frequency (RF) current drive system and / or a neutral particle injection system.

7. The method according to any one of claims 1 to 6, wherein the current drive system includes a subset of the poloidal magnetic field coils, the subset configured to drive the plasma current by increasing the magnetic field perpendicular to the plasma.

8. The method according to claim 7, wherein each poloidal field coil of the subset of poloidal field coils is located radially outward from the principal radius of the plasma.

9. Using the solenoid to maintain the plasma current at the operating point is Monitoring the aforementioned plasma current, To determine the fluctuation of the plasma current from the operating point, The current in the solenoid is changed to cancel out the aforementioned fluctuations. The method according to any one of claims 1 to 8, including the method described in any one of claims 1 to 8.

10. It was a tokamak, Toroidal plasma chamber and A poloidal magnetic field coil and a toroidal magnetic field coil configured to confine the plasma within the plasma chamber, Located at the center of the torus, it contains a solenoid made of high-temperature superconducting material, A current drive system other than the solenoid, configured to ramp up the plasma current in the plasma confined within the plasma chamber. Controller and Equipped with, The aforementioned controller, Initiating plasma in a toroidal plasma chamber, A current-driven system is used to ramp up the plasma current until it reaches the operating point, wherein when the plasma current reaches the operating point, the current in the solenoid is less than the maximum operating current of the solenoid. After the plasma current reaches the operating point, use the solenoid. Controlling the plasma current, Controlling the position of the plasma, and / or Controlling the shape of the plasma and Maintaining the plasma at an aspect ratio of less than 2.5 A tokamak, configured to perform a certain function.

11. The tokamak according to claim 10, wherein the controller is configured to maintain the plasma at an aspect ratio of less than 2.3, more preferably less than 2.

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